ALBERT

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 176〈/p〉 〈p〉Author(s): Hoheok Kim, Tatsuki Yamamoto, Yushi Sato, Junya Inoue〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We investigate the viability of establishing low-cost surrogate structure-property (S–P) linkages by introducing a Bayesian model selection method to extend the Materials Knowledge Systems (MKS) homogenization framework, which employs the n-point spatial correlation function, principal component analysis, and regression techniques. In particular, we place emphasis not only on choosing the important structural features but also on interpreting their implications for the property under consideration. First, the yield strengths of synthetic microstructures with various morphological characteristics are estimated by physics-based crystal plasticity simulation. Then, the dimension-reduced microstructural features are revealed by a combination of 2-point spatial correlations and principal component analysis. The Bayesian model selection method is further applied to establish a microstructure-to-yield-strength surrogate model. Finally, the model is validated with an independent dataset and its constituent features are interpreted with a morphology reconstruction based on a Monte Carlo algorithm. The method is found to be capable of interpreting the key microstructural features as well as modeling the mechanical response of a dual-phase metallic composite in consideration of the diverse microstructural factors.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304367-fx1.jpg" width="274" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 176〈/p〉 〈p〉Author(s): Denise C. Ford, David Hicks, Corey Oses, Cormac Toher, Stefano Curtarolo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Metallic glasses are excellent candidates for biomedical implant applications due to their inherent strength and corrosion resistance. However, use of metallic glasses in structural applications is limited because bulk dimensions are challenging to achieve. Glass-forming ability (GFA) varies strongly with alloy composition and becomes more difficult to predict as the number of chemical species in a system increases. Here, we present a theoretical model — implemented in the AFLOW framework — for predicting GFA based on the competition between crystalline phases. The model is applied to biologically relevant binary and ternary systems. Elastic properties of Ca- and Mg-based systems are estimated for use in biodegradable orthopedic support applications. Alloys based on Ag〈sub〉0.33〈/sub〉Mg〈sub〉0.67〈/sub〉, Cu〈sub〉0.5〈/sub〉Mg〈sub〉0.5〈/sub〉, Cu〈sub〉0.37〈/sub〉Mg〈sub〉0.63〈/sub〉, and Cu〈sub〉0.25〈/sub〉Mg〈sub〉0.5〈/sub〉Zn〈sub〉0.25〈/sub〉, and in the Ag-Ca-Mg and Ag-Mg-Zn systems, are recommended for further study.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304380-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 176〈/p〉 〈p〉Author(s): Manuel J. Pfeifenberger, Vladica Nikolić, Stanislav Žák, Anton Hohenwarter, Reinhard Pippan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The brittleness of tungsten at room temperature represents a severe challenge particularly for structural applications. Tungsten composites, consisting of foils or wires, overcome this low ductility by utilizing the remarkable mechanical properties of ultrafine grained tungsten materials. A comprehensive understanding of the fracture behaviour of these ultrafine grained tungsten materials is therefore essential for a further development of high performance structural composites. However, the dimensions of specimens used for classical fracture toughness experiments are not applicable to test all important crack growth directions in the case of thin foils and wires, especially, in the direction of the presumably lowest fracture toughness, which is along their characteristically elongated microstructure. Femtosecond laser processing allows to fabricate micro single leg bending specimens, which enable to properly evaluate the fracture toughness in this orientation. The fracture toughness value at crack initiation found for the foil is 2.4  〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mtext〉MPa〈/mtext〉〈msqrt〉〈mtext〉m〈/mtext〉〈/msqrt〉〈/math〉, whereas for the wire a value of 5.3  〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mtext〉MPa〈/mtext〉〈msqrt〉〈mtext〉m〈/mtext〉〈/msqrt〉〈/math〉 was determined. In both cases the results are significantly below the values reported for other orientations. This strongly anisotropic fracture behaviour is responsible for the reduced brittle to ductile transition temperature and the delamination induced toughening for crack orientations perpendicular to the elongated ultrafine grained structure. The distinct difference of the fracture toughness at crack initiation and the R-curve between wire and foil specimens could be primarily explained by the morphologies of the fracture surfaces, exhibiting significantly different roughnesses of the evolving crack paths.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304252-fx1.jpg" width="497" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 176〈/p〉 〈p〉Author(s): Peter Müllner〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Recent reports on highly mobile type II twin boundaries challenge the established understanding of deformation twinning and motivate this study. We consider the motion of twin boundaries through the nucleation and growth of disconnection loops and develop a mechanism-based model for twin boundary motion in the framework of isotropic linear elasticity. While such mechanisms are well established for type I and compound twins, we demonstrate based on the elastic properties of crystals that type II twin boundaries propagate in a similar way. Nucleation of a type I twinning disconnection loop occurs in a discrete manner. In contrast, nucleation of a type II twinning disconnection loop occurs gradually with increasing Burgers vector. The gradual nucleation of a type II disconnection loop accounts for the higher mobility of type II twin boundaries compared with type I twin boundaries. We consider the homogeneous nucleation of a disconnection loop, which is adequate for twinning in shape memory alloys with a low-symmetry crystal lattice. For the magnetic shape memory alloy Ni–Mn-Ga, the model predicts twinning stresses of 0.33 MPa for type II twinning and 4.7 MPa for type I twinning. Over a wide temperature range, the twinning stress depends on temperature only through the temperature dependence of the elastic constants, in agreement with experimental results.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304343-fx1.jpg" width="374" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 3 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Cuncai Fan, Qiang Li, Jie Ding, Yanxiang Liang, Zhongxia Shang, Jin Li, Ruizhe Su, Jaehun Cho, Di Chen, Yongqiang Wang, Jian Wang, Haiyan Wang, Xinghang Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉There are increasing studies that show nanotwinned (NT) metals have enhanced radiation tolerance. However, the mechanical deformability of irradiated nanotwinned metals is a largely under explored subject. Here we investigate the mechanical properties of He ion irradiated nanotwinned Cu with preexisting nanovoids. In comparison with coarse-grained Cu, nanovoid nanotwinned (NV-NT) Cu exhibits prominently improved radiation tolerance. Furthermore, 〈em〉in situ〈/em〉 micropillar compression tests show that the irradiated NV-NT Cu has an ultrahigh yield strength of ∼ 1.6 GPa with significant plasticity. Post radiation analyses show that twin boundaries are decorated with He bubbles and thick stacking faults. These stacking fault modified twin boundaries introduce significant strengthening in NT Cu. This study provides further insight into the design of high-strength, advanced radiation tolerant nanostructured materials for nuclear reactor applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304331-fx1.jpg" width="307" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 176〈/p〉 〈p〉Author(s): Y. Kobayashi, J. Takahashi, K. Kawakami〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The influence of a pre-deformation with a true strain of 0.5 on the precipitation behavior during isothermal aging at 580 °C in ferritic steel containing 0.03C-0.1Ti-0.20Mn–3Al (mass %) was investigated. Atom probe tomography (APT) analysis revealed that titanium carbide (TiC) precipitates much earlier and more finely in pre-deformed steel than in steel without a pre-deformation. It was found that the precipitation sites of TiC are not only located on the dislocations but are also distributed homogeneously in a matrix in pre-deformed steel. In steel without a pre-deformation, coarse cementite first precipitates during the early stage of aging, and the cementite then dissolves owing to the subsequent precipitation of TiC. Meanwhile, in pre-deformed steel, cementite has difficulty precipitating, and carbon atoms are considered to segregate to high-density dislocations during the early stage of aging prior to the precipitation of TiC. A kinetic model that explains the difference between the precipitation behaviors of steel with and without a pre-deformation is proposed. Moreover, the difference observed between TiC particle strengthening in steel with and without a pre-deformation is discussed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930429X-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Chunguang Shen, Chenchong Wang, Xiaolu Wei, Yong Li, Sybrand van der Zwaag, Wei Xu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉With the development of the materials genome philosophy and data mining methodologies, machine learning (ML) has been widely applied for discovering new materials in various systems including high-end steels with improved performance. Although recently, some attempts have been made to incorporate physical features in the ML process, its effects have not been demonstrated and systematically analysed nor experimentally validated with prototype alloys. To address this issue, a physical metallurgy (PM) -guided ML model was developed, wherein intermediate parameters were generated based on original inputs and PM principles, e.g., equilibrium volume fraction (〈em〉V〈/em〉〈sub〉〈em〉f〈/em〉〈/sub〉) and driving force (〈em〉D〈/em〉〈sub〉〈em〉f〈/em〉〈/sub〉) for precipitation, and these were added to the original dataset vectors as extra dimensions to participate in and guide the ML process. As a result, the ML process becomes more robust when dealing with small datasets by improving the data quality and enriching data information. Therefore, a new material design method is proposed combining PM-guided ML regression, ML classifier and a genetic algorithm (GA). The model was successfully applied to the design of advanced ultrahigh-strength stainless steels using only a small database extracted from the literature. The proposed prototype alloy with a leaner chemistry but better mechanical properties has been produced experimentally and an excellent agreement was obtained for the predicted optimal parameter settings and the final properties. In addition, the present work also clearly demonstrated that implementation of PM parameters can improve the design accuracy and efficiency by eliminating intermediate solutions not obeying PM principles in the ML process. Furthermore, various important factors influencing the generalizability of the ML model are discussed in detail.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305452-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 20 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Hao Chen, Valery Levitas, Liming Xiong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Molecular dynamics (MD) simulations of the amorphous band nucleation and growth ahead of the tip of a shuffle 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msup〉〈mrow〉〈mn〉60〈/mn〉〈/mrow〉〈mrow〉〈mi〉o〈/mi〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 dislocation pileup at different grain boundaries (GBs) in diamond-cubic (dc) silicon (Si) bicrystal under shear are performed. Amorphization initiates when the local resolved shear stress reaches approximately the same value required for amorphization in a perfect single crystal (8.6-9.3GPa) for the same amorphization plane. Since the local stresses at the tip of a dislocation pileup increase when the number of dislocations in the pileup is increased, the critical applied shear stress 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈msub〉〈mrow〉〈mi〉τ〈/mi〉〈/mrow〉〈mrow〉〈mi〉a〈/mi〉〈mi〉p〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 for the formation of an amorphous shear band significantly decreases with the dislocation accumulation at the GBs. In particular, when the number of the dislocations in a pileup increases from 3 to 8, the critical shear stress drops from 4.7GPa to 1.6GPa for both the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉9〈/mn〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉19〈/mn〉〈/mrow〉〈/math〉 GBs and from 4.6GPa to 2.1GPa for the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉 GB, respectively. After the formation of steps and disordered embryos at the GBs, the atomistic mechanisms responsible for the subsequent amorphous shear band formations near different GBs are found to distinct from each other. For a high-angle GB, such as 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉, an amorphous band propagates through the crystalline phase along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si6.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉112〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane. For the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉9〈/mn〉〈/mrow〉〈/math〉 GB, partial dislocations forming a stacking fault precede the formation of an amorphous band along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉110〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane. For the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉19〈/mn〉〈/mrow〉〈/math〉 GB, the one-layer stacking fault along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane transforms into an interesting intermediate phase: a two-layer band with the atomic bonds being aligned along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane (i.e., rotated by 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si9.svg"〉〈mrow〉〈msup〉〈mrow〉〈mn〉30〈/mn〉〈/mrow〉〈mrow〉〈mi〉o〈/mi〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 with respect to the atomic bonds outside the band). This intermediate phase transforms to the amorphous band along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane under a further shearing. The obtained results represent an atomic-level confirmation of the effectiveness of dislocation pileup at the nucleation site for various strain-induced phase transformations (PTs), and exhibit some limitations.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930535X-fx1.jpg" width="218" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): K. Sofinowski, M. Šmíd, I. Kuběna, S. Vivès, N. Casati, S. Godet, H. Van Swygenhoven〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A multi-phase Ti–6Al–4V prepared by electron beam melting and thermal post treatments has been shown to exhibit increased strength and ductility over standard wrought or hot isostatic pressed Ti–6Al–4V. The mechanical improvements are due to a prolonged, continuous work hardening effect not commonly observed in Ti alloys. 〈em〉In situ〈/em〉 x-ray diffraction and high resolution digital image correlation are used to examine the strain partitioning between the phases during tensile loading with post-mortem electron microscopy to characterize the deformation behavior in each phase. Specimens heat treated between 850 and 980 °C were tested and the effect of annealing temperature on the micromechanical response is discussed. It is shown that the work hardening is the result of composite load-sharing behavior between three mechanically distinct microstructures: large 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mi〉α〈/mi〉〈/mrow〉〈/math〉 lamellae and a martensitic region of fine acicular 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mi〉α〈/mi〉〈mo〉'〈/mo〉〈/mrow〉〈/math〉 and a third phase not previously reported in this alloy.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930549X-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Charlette M. Grigorian, Timothy J. Rupert〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu–Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (〉95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability comparable to that of the binary Cu-Zr alloy, and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Despite carbide formation and growth in these alloys during milling and annealing, the thermal stability of the ternary alloys is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that ternary alloy compositions have thicker boundary films compared to the binary alloys with similar global dopant concentrations. While it is not required for amorphous complexion formation, this work shows that having at least three elements present at the interface can lead to thicker grain boundary films, which is expected to maximize the previously reported toughening effect.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305439-fx1.jpg" width="400" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Hangfeng Zhang, Bin Yang, Haixue Yan, Isaac Abrahams〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Switchable ferroelectric/antiferroelectric ceramics are of significant interest for high power energy storage applications. Grain size control of this switching is an interesting approach to controlling polarization and hence dielectric properties. However, the use of this approach in technologically relevant ceramics is hindered by difficulty in fabricating dense ceramics with small grain sizes. Here an intermediate polar ferroelectric phase (〈em〉P〈/em〉2〈sub〉1〈/sub〉〈em〉ma〈/em〉) has been isolated in dense bulk sodium niobate ceramics by grain size control through spark plasma sintering methods. Our findings, supported by XRD, DSC, P-E (I-E) loops and dielectric characterization, provide evidence that the phase transition from the antiferroelectric (AFE) R-phase, in space group 〈em〉Pnmm〈/em〉, above 300 °C, to the AFE P-phase, in space group 〈em〉Pbma〈/em〉, at room temperature, always involves the polar intermediate 〈em〉P〈/em〉2〈sub〉1〈/sub〉〈em〉ma〈/em〉 phase and that the 〈em〉P〈/em〉2〈sub〉1〈/sub〉〈em〉ma → Pbma〈/em〉 transition can be suppressed by reducing grain size.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305506-fx1.jpg" width="296" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): M.J. Konstantinović, I. Uytdenhouwen, G. Bonny, N. Castin, L. Malerba, P. Efsing〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermal stability and the structure of solute-vacancy clusters formed by neutron irradiation are studied by means of positron annihilation spectroscopy and hardness measurements of post-irradiation annealed reactor pressure vessel steels with high and low Ni contents. Two distinct recovery stages were observed and assigned to (a) the dissolution of vacancy clusters at about 650 K, and (b) the dissolution of solute-vacancy clusters at about 750 K. In steels with high Ni content, hardening mainly recovers during the second stage. Atomistic and coarse grain models suggest that during this stage, the removal of vacancies from vacancy-solute clusters leads to complete cluster dissolution, which indicates that solute clusters are radiation induced.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305403-fx1.jpg" width="280" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Hui Chen, Qingsong Wei, Yingjie Zhang, Fan Chen, Yusheng Shi, Wentao Yan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The packing density of the powder layer plays a key role in the final quality of the parts fabricated via powder-bed-based (PBB) additive manufacturing. This paper presents a combined experimental and computational modeling study on the scraping type of powder-spreading process, in order to understand the fundamental mechanisms of the packing of the powder layer. The deposition mechanisms at the particulate scale, including particle contact stress and particle velocity, are investigated, using the discrete element method, while the macro-scale packing density is validated by experiments. It is found that there is a stress-dip at the bottom of powder pile scraped by the rake. This stress-dip makes the powder particles uniformly deposited. Three kinds of deposition mechanisms dominating the powder-spreading process are identified: cohesion effect, wall effect, and percolation effect. The cohesion effect, which leads to particle agglomerations and thus reduces the packing density, becomes stronger with the decrease of particle size. The wall effect, which leads to more vacancies in the powder layer, becomes stronger with the decrease of layer thickness or the increase of particle size. The percolation effect exists in bimodal powder particles, which leads to particle segregation within the powder layer and thus reduces the packing density. The three kinds of deposition mechanisms compete with each other during the powder-spreading process and make combined effects on the packing density of the powder layer.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305427-fx1.jpg" width="264" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Qi-Nan Han, Shao-Shi Rui, Wenhui Qiu, Xianfeng Ma, Yue Su, Haitao Cui, Hongjian Zhang, Huiji Shi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effect of crystal orientation on fretting fatigue induced crack initiation and dislocation distribution is studied by in-situ SEM observation and electron back-scattered diffraction (EBSD) in this paper. Cracks and slip lines are observed in the fretting contact area of Ni-based single-crystal (NBSX) superalloys. The in-situ SEM observation captures different crack and slip line behaviors under different crystal orientations. The EBSD analysis results show obvious misorientation and orientation deviation in the fretting contact area. For both crystal orientations, the geometrically necessary dislocation (GND) density distributions in the contact area are obtained by using Hough-based EBSD methods. The peak position of grain reference orientation deviation (GROD) and GND density matches with the fretting fatigue crack formation position. EBSD analysis shows that the dislocation density distribution on each slip system is closely related to the crack initiation direction. The direction of slip system with the maximum dislocation density agrees with the crack initiation direction obtained by in-situ observation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305476-fx1.jpg" width="448" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Jinghao Xu, Hans Gruber, Dunyong Deng, Ru Lin Peng, Johan J. Moverare〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10〈sup〉−5〈/sup〉/s, 1 × 10〈sup〉−6〈/sup〉/s, and 1 × 10〈sup〉−7〈/sup〉/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305464-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Garth C. Egan, Tae Wook Heo, Amit Samanta, Geoffrey H. Campbell〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report a novel mechanism for explosive crystallization in amorphous germanium (a-Ge), which operates through liquid-mediated nucleation occurring under extreme thermal gradient conditions. The crystallization kinetics of sputter-deposited films with thicknesses ranging from 30 to 150 nm were characterized using 〈em〉in situ〈/em〉 movie-mode dynamic transmission electron microscopy (MM-DTEM). After localized heating from a short laser pulse, explosive liquid phase nucleation (LPN) was observed to occur during the early stage (〈2 μs) of crystallization in the thicker (〉50 nm) films deposited on silicon nitride substrates. The crystallization front propagated at ∼12–15 m/s and produced nanocrystalline microstructure with ∼50 nm grains. A mechanism involving the existence of a relatively thick (〉100 nm) transient liquid layer and a high nucleation rate is proposed to explain the behavior. The key thermodynamic and kinetic features as well as the feasibility of the mechanism are further explored by employing parametric and systematic phase-field modeling and simulations.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305385-fx1.jpg" width="418" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Le Van Lich, Minh-Tien Le, Tinh Quoc Bui, Thanh-Tung Nguyen, Takahiro Shimada, Takayuki Kitamura, Trong-Giang Nguyen, Van-Hai Dinh〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A reversal of polarization vortexlike domains in ferroelectric nanostructures plays important roles for next generations of electronic nanodevices. However, a direct switching of the polarization vortexlike domains in ferroelectrics is a nontrivial task since the toroidal moment is conjugated to a curled electric field rather than a homogeneous one. This work is dedicated to developing an approach to directly switch the toroidal ordering under an irrotational (homogeneous) electric field with the use of compositionally graded ferroelectric (cgFE) nanodots. The variation in material compositions induces an additionally broken spatial inversion symmetry at a scale beyond unit-cell level, giving rise to a formation of asymmetric flux-closure domain (FCD) in a cgFE nanodot. More interestingly, such an asymmetric character facilitates to a switch of FCD by an irrotational electric field. In particular, the rotation of polarization can be directly switched from counter-clockwise to clockwise rotations and vice versa without a formation of intermediate domain structures during the switching process. This switching behavior is distinguished from that in homogeneous counterparts. We further demonstrate that the variation in material compositions tailors the distributions of electrostatic and total free energies in the cgFE nanodot that can control the annihilation/initiation process of FCD under irrotational electric field, providing fundamental reason for the direct switching of the toroidal moment. Another interesting issue is found that both the amplitude and frequency of applied electric field strongly affect the switching behavior of FCD in cgFE nanodot.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305373-fx1.jpg" width="459" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Dipak Kumar Khatua, Anupam Mishra, Naveen Kumar, Gobinda Das Adhikary, Uma Shankar, Bhaskar Majumdar, Rajeev Ranjan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Driven by environmental concerns and governmental directives, a sustained research effort in the last decade and half has led to the development of lead-free alternatives which can potentially replace the commercial lead-based piezoceramics in niche applications. Na〈sub〉0.5〈/sub〉Bi〈sub〉0.5〈/sub〉TiO〈sub〉3〈/sub〉 (NBT)-based lead-free piezoceramics have found acceptance as promising lead-free transducers in high power ultrasonic devices. An issue of concern however is the low depolarization temperature which limits the device's tolerance for temperature rise during operation. While several strategies have been reported to improve thermal depolarization in NBT-based piezoceramics, there is a lack of consensus regarding the most fundamental factor/mechanism which enhances the depolarization temperature. In this paper we unravel a coupled microstructural-structural mechanism which controls the thermal depolarization in NBT-based piezoceramics. First, we demonstrate the phenomenon of a considerable increase in the depolarization temperature, without significantly losing the piezoelectric property in unmodified NBT by increasing the grain size. We then establish a grain size controlled structural mechanism and demonstrate that the rise in depolarization temperature is primarily associated with the bigger grains allowing relatively large lattice distortion to develop in the poling stabilized long range ferroelectric phase. We reconfirmed the validity of this mechanism in the model morphotropic phase boundary (MPB) composition 0.94Na〈sub〉0.5〈/sub〉Bi〈sub〉0.5〈/sub〉TiO〈sub〉3〈/sub〉-0.06BaTiO〈sub〉3〈/sub〉. For the sake of generalization, we demonstrate that the same mechanism is operative in another lead-based relaxor-ferroelectric system 0.62PbTiO〈sub〉3〈/sub〉-0.38Bi(Ni〈sub〉0.5〈/sub〉Hf〈sub〉0.5〈/sub〉)O〈sub〉3〈/sub〉. Our study provides the fundamental structural basis for understanding thermal depolarization delay in relaxor ferroelectric based piezoceramics.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305348-fx1.jpg" width="240" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Hao Sun, Shaohua Fu, Chichi Chen, Zhirui Wang, Chandra Veer Singh〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Nickel carbonyl vapor deposition (CVD) is a high-efficiency process used to produce nickel shell molds with high yield strength, reasonable ductility, and strong corrosion resistance. Such advantageous properties arise from the nanocrystals and nanotwins inside CVD nickel. However, the nanotwins do not persist at high temperatures, transforming into dislocation cells after 40-min annealing at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mn〉800〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mo〉°C〈/mo〉〈/mrow〉〈/math〉. Using experimental examinations and computational simulations, we investigated the kinetics of the annealing-induced detwinning in CVD nickel. TEM examinations showed that detwinning is realized by incoherent twin boundary (ITB) migration; meanwhile, plentiful dislocations are generated from coherent twin boundaries (CTBs). Our theoretical analysis revealed that these dislocations are necessary for the formation of the ITBs. Next, using molecular dynamics simulations, we found that the dislocations nucleated from CTBs during annealing are intrinsic grain boundary dislocations (IGBDs). Driven by the internal stress intensified by grain growth in the nanocrystalline regime, the IGBDs can separate from CTBs due to creep at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mn〉800〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mo〉°C〈/mo〉〈/mrow〉〈/math〉, resulting in a higher dislocation density inside the twin lamella than that of the outside. These dislocations can trigger the formation of ITBs. Overall, unlike grain growth, stress is necessary for detwinning, so a monolithic nanotwin structure should be more stable than the nanotwins inside a nanocrystalline matrix.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305208-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Zefeng Yu, Chenyu Zhang, Paul M. Voyles, Lingfeng He, Xiang Liu, Kelly Nygren, Adrien Couet〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Proton irradiation induced Nb redistribution in Zr-xNb alloys (x = 0.4, 0.5, 1.0 wt%) has been investigated using scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM/EDS). Zr-xNb alloys are mainly composed of Zr matrix, native Zr–Nb–Fe phases, and β-Nb precipitates. After 2 MeV proton irradiation at 350 °C, a decrease of Nb content in native precipitates, as well as irradiation-induced precipitation of Nb-rich platelets (135 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mo〉±〈/mo〉〈/mrow〉〈/math〉 69 nm long and 27 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mo〉±〈/mo〉〈/mrow〉〈/math〉 12 nm wide) were found. Nb-rich platelets and Zr matrix form the Burgers orientation relationship, [〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mn〉1〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉]//[〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mn〉2〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉0〈/mn〉〈/mrow〉〈/math〉] and (011)//(0002). The platelets were found to be mostly coherent with the matrix with a few dislocations near the ends of the precipitate. The coherent strain field has been measured in the matrix and platelets by the 4D-STEM technique. The growth of Nb-rich platelets is mainly driven by coherency and dislocation-induced strain fields. Irradiation may both enhance the diffusion and induce segregation of interstitial Nb to the ends of the irradiation induced platelets, further facilitating their growth.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305221-fx1.jpg" width="250" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Gi-Dong Sim, Kelvin Y. Xie, Kevin J. Hemker, Jaafar A. El-Awady〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Here, an experimental study utilizing 〈em〉in-situ〈/em〉 scanning electron microscopy (SEM) micro-compression testing and post-mortem transmission electron microscopy (TEM) imaging is presented to quantify the effect of temperature on the transition in deformation modes in twin-oriented Mg single crystals. Single crystal micropillars were fabricated using FIB milling, then tested by 〈em〉in-situ〈/em〉 SEM micro-compression from 20 °C to 225 °C. It is observed that plasticity in the deformed Mg microcrystals at temperatures at and below 100 °C is governed by 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mn〉10〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉2〈/mn〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈/math〉 extension twinning. However, an anomalous increase of the flow stresses is observed at 100 °C, which is likely due to paucity of dislocation sources that are required to promote twin boundary migration. At 150 °C and above, extension twinning is suppressed and a continuous plastic flow and strain softening induced by prismatic dislocation mediated plasticity is observed. By comparing the current results with those from bulk scale studies for other hexagonal-closed-pack single crystals (e.g. titanium (Ti) and zirconium (Zr)), a general trend for the effect of temperature on the transition in deformation modes in HCP materials is proposed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305245-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): X.C. Tang, C. Li, H.Y. Li, X.H. Xiao, L. Lu, X.H. Yao, S.N. Luo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A special spallation morphology in bulk metallic glass, named as the “cup-cone” structure, is of particular interest since it manifests a unique “ductile–brittle” transition. To gain insights into the underlying mechanism for the formation of a cup-cone structure, we conduct planar impact experiments at various impact velocities, as well as finite element method analysis. Spall strength increases with increasing impact velocity. Scanning electron microscopy and X-ray computed tomography are performed on postmortem samples to characterize cup-cone structures; their average size and spacing decrease as impact velocity increases, and they dominate fracture morphology at high impact velocities. Cups and cones are generally distributed on the side away from and on the side closer to the target free surface, respectively. The initial nucleation sites of voids become the conical vertices of cup-cones, and the subsequent nucleation sites form along the conical surface and coalesce into the cracks and fracture surfaces.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305087-fx1.jpg" width="287" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Satoshi Okamoto, Kazunori Miyazawa, Takahiro Yomogita, Nobuaki Kikuchi, Osamu Kitakami, Kentaro Toyoki, David Billington, Yoshinori Kotani, Tetsuya Nakamura, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono, Yukio Takada, Takashi Sato, Yuji Kaneko, Akira Kato〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305063-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Sumeet Mishra, Manasij Yadava, Kaustubh N. Kulkarni, N.P. Gurao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A new methodology for analyzing strain hardening behavior of face centered cubic materials based on transition from restricted glide/single slip to multiple slip has been developed. The proposed modification considers strain dependence of orientation factor spanning between lower bound iso-stress Sachs model and upper bound iso-strain Taylor model. The modifications are suitably incorporated in the classical two internal variable model to develop a new slip activity based strain hardening model. The proposed model is shown to be performing better than the existing one parameter forest strengthening model and two internal variable model in predicting strain hardening behavior in the presence of wide range of microstructural features such as solutes, semi-coherent and incoherent precipitates, grain sizeand twins. Experimental validation of the proposed concept of transition in slip behavior is shown in terms of evolution of dislocation density and character from X-ray diffraction and surface roughness, slip lines and micro-texture from in-situ electron back scatter diffraction tests.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930504X-fx1.jpg" width="257" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): B. Christiaen, C. Domain, L. Thuinet, A. Ambard, A. Legris〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The growth of zirconium alloys under irradiation is a phenomenon experimentally identified and associated with the development beyond a threshold dose of dislocation loops with vacancy character having a Burgers vector with a component parallel to the c axis. In this work, by combining atomic simulations (DFT and empirical potential) and continuous modeling, we show that prismatic stacking fault pyramids or bipyramids whose base rests on the basal plane of the hcp structure are likely precursors to the formation of ‹c› vacancy loops. In other words, these would not be formed by progressive accretion of vacancies but rather by collapse of the pyramids or bipyramids beyond a certain size. This mechanism could explain the fact that the ‹c› vacancy loops are never observed below a size of the order of 10 nm and their appearance at high fluence.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304707-fx1.jpg" width="389" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): P.E. Seiler, H.C. Tankasala, N.A. Fleck〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Additive manufacture and rapid prototyping are versatile methods for the generation of lattice materials for applications in the creep regime. However, these techniques introduce defects that can degrade the macroscopic creep strength. In the present study, the uniaxial tensile response of two-dimensional PMMA lattices is measured in the visco-plastic regime: tests are performed at 100 °C which is slightly below the glass transition temperature 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msub〉〈mrow〉〈mi〉T〈/mi〉〈/mrow〉〈mrow〉〈mtext〉g〈/mtext〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 of PMMA. Both 〈em〉as-manufactured〈/em〉 defects (Plateau borders and strut thickness variation) and 〈em〉as-designed〈/em〉 defects (missing cell walls, solid inclusions, and randomly perturbed joints) are introduced. The dispersion in macroscopic strength is measured for relative densities in the range of 0.07–0.19. It is observed that initial failure of the lattice is diffuse in nature: struts fail at a number of uncorrelated locations, followed by the development of a single macroscopic crack transverse to the loading direction. In contrast, the same PMMA lattice fails in a correlated, brittle manner at room temperature. An FE study is performed to gain insight into the diffuse failure mode and the role played by 〈em〉as-manufactured〈/em〉 defects, including the dispersion in tensile strength of individual struts of the lattice. A high damage tolerance to 〈em〉as-designed〈/em〉 defects is observed experimentally: there is negligible knock-down in strength due to the removal of cell walls or to the presence of solid inclusions. These findings aid the design and manufacture of damage tolerant lattices in the creep regime.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Elastic-brittle versus visco-plastic failure ofPMMA lattices.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305026-fx1.jpg" width="229" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 177〈/p〉 〈p〉Author(s): Christopher A. Schuh〈/p〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): N. Almirall, P.B. Wells, T. Yamamoto, K. Wilford, T. Williams, N. Riddle, G.R. Odette〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPV-type steels, with an unusually wide range of dissolved Mn (0.06–1.34 at.%) and Ni (0.19–3.50 at.%) contents, were irradiated at ≈ 290 °C to ≈ 1.4 × 10〈sup〉20〈/sup〉 n/cm〈sup〉2〈/sup〉 at an accelerated test reactor flux of ≈3.6 × 10〈sup〉12〈/sup〉 n/cm〈sup〉2〈/sup〉-s (E 〉 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with ≈ Ni〈sup〉1.6〈/sup〉Mn〈sup〉0.8〈/sup〉. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305397-fx1.jpg" width="415" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 14 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Bar Danino, Gil Gur-Arieh, Doron Shilo, Dan Mordehai〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ferroic materials typically exhibit a microstructure that contains twins or domains separated by twin boundaries (walls). The deformation of these materials is governed by twin boundary motion under mechanical/electrical/magnetic driving force. The Landau-Ginzburg model is a widely accepted phenomenological model used to describe twin boundary properties. However, it is incapable of describing energy barriers for motion due to the lack of atomistic description. In this work, we present a model interatomic potential for studying the relations between the lattice barrier for twin boundary motion and measurable material properties. The interatomic potential emulates the continuum Landau-Ginzburg model and reproduces known results of twin boundary thickness and energy as a function of the model parameters. An atomic model system is constructed, with a single twin boundary separating crystals of different orientations and we employ the Nudged Elastic Band method to calculate the energy barriers for the motion of twin boundaries with different thicknesses under different externally-applied shear stresses. The results are summarized in a closed-form expression relating the energy barriers with material properties and the external loading. The energy barrier function extends the Landau-Ginzburg model and allows treating the motion of twin boundary as a thermally activated process.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305282-fx1.jpg" width="446" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Hongping Li, Mitsuhiro Saito, Chunlin Chen, Kazutoshi Inoue, Kazuto Akagi, Yuichi Ikuhara〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Metal/oxide heterointerfaces are ubiquitous in functional materials, and their microstructures frequently govern the macroscopic properties. It has been believed that the interfacial interactions are very weak at incoherent interfaces with large mismatches. Combining atomic-resolution scanning transmission electron microscopy with density functional theory calculations, we investigated the interaction and bonding reconstruction at Pd/ZnO{0001} interfaces, which have large mismatches. Molecular beam epitaxy was employed to grow Pd films on clean Zn-terminated ZnO(0001) and O-terminated ZnO(000〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉) polarized surfaces. Atomically sharp Zn-terminated interfaces formed on both substrates, and the large lattice misfits between them were not strongly accommodated, suggesting the formation of incoherent regions. The interfacial atoms were located almost at bulk lattice points in the stoichiometric Zn-terminated Pd(111)/ZnO(0001) structure, whereas the interfacial Pd and Zn atoms underwent relatively large relaxations on the interfacial plane in the nonstoichiometric Zn-terminated Pd(111)/ZnO(〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mn〉000〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/mrow〉〈/math〉) interface. Effective Pd–Zn chemical bonds were formed across both interfaces, but the bonding mechanisms were quite different, depending on the local atomic geometry. The Pd–Zn bonds exhibited site-dependent characteristics and gradually transitioned from covalent to ionic at the Pd(111)/ZnO(0001) interface, whereas most of Pd–Zn bonds exhibited strong covalent behavior at the Pd/ZnO(〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mn〉000〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/mrow〉〈/math〉) interface. The adhesive energies indicated that the Zn-terminated Pd/ZnO(〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mn〉000〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/mrow〉〈/math〉) interface is energetically preferable to the Zn-terminated Pd/ZnO(0001) interface. Thus, the interfacial interaction can be strong and direct metal–metal interactions can play a critical role in metal/oxide heterointerfaces with large mismatches, opening up a new avenue for understanding the origins of interface-related issues.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305300-fx1.jpg" width="344" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Keita Nomoto, Hiroshi Sugimoto, Xiang-Yuan Cui, Anna V. Ceguerra, Minoru Fujii, Simon P. Ringer〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Boron (B) and phosphorous (P) co-doped colloidal silicon nanocrystals (Si NCs) have unique size-dependent optical properties, which lead to potential applications in optoelectronic and biomedical applications. However, the microstructure of the B and P co-doped colloidal Si NCs – in particular, the exact location of the dopant atoms in real space, has not been studied. A lack of understanding of this underlying question limits our ability to better control sample fabrication, as well as our ability to further develop the optical properties. To study the microstructure, a process enabling atom probe tomography (APT) of colloidal Si NCs was developed. A dispersion of colloidal Si NCs in a SiO〈sub〉2〈/sub〉 sol-gel solution and a low temperature curing are demonstrated as the key sample preparation steps. Our APT results demonstrate that a B-rich region exists at the surface of the Si NCs, while P atoms are distributed within the Si NCs. First principles density functional theory calculations of a Si NC embedded in SiO〈sub〉2〈/sub〉 matrix reveal that P atoms, which always prefer to reside inside a Si NC, significantly influence the distribution of B atoms. Specifically, P atoms lower the B diffusion barrier at Si/SiO〈sub〉2〈/sub〉 interface and stabilize B atoms to reside within individual Si NCs. We propose that the B-modified surface changes the chemical properties of the Si NCs by (i) offering chemical resistance to attack by HF and (ii) enabling dispersibility in solution without aggregation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305233-fx1.jpg" width="205" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Arthur S. Nishikawa, Goro Miyamoto, Tadashi Furuhara, André P. Tschiptschin, Hélio Goldenstein〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305038-fx1.jpg" width="346" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): P. Tozman, Y.K. Takahashi, H. Sepehri-Amin, D. Ogawa, S. Hirosawa, K. Hono〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Zr is one of the essential elements to stabilize the ThMn〈sub〉12〈/sub〉 structure in rare earth (R) transition metal (M) hard magnetic compounds, RM〈sub〉12〈/sub〉. In this work, the effects of Zr on the intrinsic hard magnetic properties of (Sm〈sub〉1-x〈/sub〉 Zr〈sub〉x〈/sub〉)(Fe〈sub〉0.8〈/sub〉Co〈sub〉0.2〈/sub〉)〈sub〉12〈/sub〉 compounds are investigated using epitaxially grown thin films. The increase of Zr substitution for Sm from 〈em〉x〈/em〉 = 0 to 0.26 for (Sm〈sub〉1-x〈/sub〉 Zr〈sub〉x〈/sub〉)(Fe〈sub〉0.8〈/sub〉Co〈sub〉0.2〈/sub〉)〈sub〉12〈/sub〉 increases saturation magnetization (μ〈sub〉0〈/sub〉〈em〉M〈/em〉〈sub〉s〈/sub〉) from 1.78 T to 1.90 T, the highest value reported for hard magnetic compounds. The largest μ〈sub〉0〈/sub〉〈em〉H〈/em〉〈sub〉a〈/sub〉 and 〈em〉T〈/em〉〈sub〉〈em〉c〈/em〉〈/sub〉 for Zr-doped samples were found to be 9.8 T and 671 K for 〈em〉x〈/em〉 = 0.18 which is superior to those for Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B. Sm-rich Sm〈sub〉1.30〈/sub〉Zr〈sub〉0.27〈/sub〉(Fe〈sub〉0.8〈/sub〉Co〈sub〉0.2〈/sub〉)〈sub〉12,〈/sub〉 obtained as sub-μm thick films, has remanence, μ〈sub〉0〈/sub〉〈em〉M〈/em〉〈sub〉r〈/sub〉 of 1 T, which appears to be useful for near-field applications such as micro-electro-machines and magnetic recording media if microstructure can be optimized to obtain a sufficient coercivity.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305051-fx1.jpg" width="441" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): X. Lu, D. Wang, D. Wan, Z.B. Zhang, N. Kheradmand, A. Barnoush〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The susceptibility of age-hardened nickel-based Alloy 718 to hydrogen embrittlement was studied by the controlled electrochemical charging combined with slow strain-rate tensile tests (SSRT) and advanced characterization techniques. We proposed some novel ideas of explaining hydrogen embrittlement mechanisms of the studied material in regard to two cracking morphologies: transgranular and intergranular cracking. It is for the first time to report that electrochemical charging alone could cause slip lines, surface and subsurface cracks on nickel-based superalloys. The formation of pre-damages was discussed by calculating the hydrogen concentration gradient and the internal stress generated during cathodic charging. Pre-damages were proved to result in transgranular cracks and lead to the evident reduction of mechanical properties. In addition, the STRONG (Slip Transfer Resistance of Neighbouring Grains) model was used to analyze the dependence of hydrogen-assisted intergranular cracking on the microscopic incompatibility of the grain boundaries. The results show that in the presence of hydrogen, grain boundaries with a lower dislocation slip transmission are more prone to cracking during loading and vice versa.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305324-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): Amin Nozariasbmarz, Mahshid Hosseini, Daryoosh Vashaee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report that microwave radiation can decompose continuous solid-solution materials into their constituent phases – a process that is thermodynamically unfavorable at equilibrium. A detailed analysis of the interaction of the electromagnetic wave with the material showed that a strong ponderomotive force preferentially separates the constituent phases via an enhanced mass transport process amplified particularly near the interfaces. The proof of concept experiments showed that the material, whether it is a solid-solution of two elements, e.g. (Si〈sub〉1-x〈/sub〉Ge〈sub〉x〈/sub〉), or two compounds, e.g. (Bi〈sub〉2〈/sub〉Te〈sub〉3〈/sub〉)〈sub〉1-x〈/sub〉(Sb〈sub〉2〈/sub〉Te〈sub〉3〈/sub〉)〈sub〉x〈/sub〉, decomposes into the constituent phases when radiated by a polarized microwave field. The dissolution happens in the bulk of the material and even below the melting point. The degree of decomposition can be controlled by radiation parameters to produce structures composed of gradient phases of the solid-solution. This offers a novel and facile method for synthesizing gradient composite and complex structures for application in thermoelectricity as well as fabrication of core-shell structures for catalysts and biomedical applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305294-fx1.jpg" width="306" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Anh Tran, Hoang Tran〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Microstructure reconstruction problems are usually limited to the representation with finitely many number of phases, e.g. binary and ternary. However, images of microstructure obtained through experimental, for example, using microscope, are often represented as a RGB or grayscale image. Because the phase-based representation is discrete, more rigid, and provides less flexibility in modeling the microstructure, as compared to RGB or grayscale image, there is a loss of information in the conversion. In this paper, a microstructure reconstruction method, which produces images at the fidelity of experimental microscopy, i.e. RGB or grayscale image, is proposed without introducing any physics-based microstructure descriptor. Furthermore, the image texture is preserved and the microstructure image is represented with continuous variables (as in RGB or grayscale images), instead of binary or categorical variables, which results in a high-fidelity image of microstructure reconstruction. The advantage of the proposed method is its quality of reconstruction, which can be applied to any other binary or multiphase 2D microstructure. The proposed method can be thought of as a subsampling approach to expand the microstructure dataset, while preserving its image texture. Moreover, the size of the reconstructed image is more flexible, compared to other machine learning microstructure reconstruction method, where the size must be fixed beforehand. In addition, the proposed method is capable of joining the microstructure images taken at different locations to reconstruct a larger microstructure image. A significant advantage of the proposed method is to remedy the data scarcity problem in materials science, where experimental data is scare and hard to obtain. The proposed method can also be applied to generate statistically equivalent microstructures, which has a strong implication in microstructure-related uncertainty quantification applications. The proposed microstructure reconstruction method is demonstrated with the UltraHigh Carbon Steel micrograph DataBase (UHCSDB).〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305178-fx1.jpg" width="322" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): O.I. Gorbatov, A.Yu Stroev, Yu.N. Gornostyrev, P.A. Korzhavyi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al–Cu, Al–Mg, Al–Zn, and Al–Si alloys, we carry out cluster expansions of 〈em〉ab initio〈/em〉 calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930521X-fx1.jpg" width="361" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 179〈/p〉 〈p〉Author(s): T. Glechner, R. Hahn, T. Wojcik, D. Holec, S. Kolozsvári, H. Zaid, S. Kodambaka, P.H. Mayrhofer, H. Riedl〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Using a combination of density functional theory calculations and nanomechanical testing of sputter-deposited, 110-oriented Ta〈sub〉0.47〈/sub〉C〈sub〉0.34〈/sub〉N〈sub〉0.19〈/sub〉 thin films, we show that non-metal alloying – substituting C with N atoms – in TaC results in a super-hard material with enhanced ductility. Based on the calculated elastic constants, with Pugh and Pettifor criteria for ductile character, we predict that stoichiometric and sub-stoichiometric Ta-C-N alloys are more ductile than Ta-C compounds. From nanoindentation of the as-deposited coating, we measure hardness of 43 ± 1.4 GPa. 〈em〉In situ〈/em〉 scanning electron microscopy (SEM) based micro-compression of cylindrical pillars, prepared via focused ion beam milling of the coating, revealed that Ta-C-N alloys are ductile and undergo plastic deformation with a yield strength of 17 ± 1.4 GPa. The post-compression SEM images of the pillars show {111} 〈〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mn〉01〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/mrow〉〈/math〉〉 as the active slip system operating during compression. Additional 〈em〉in situ〈/em〉 SEM based cantilever tests suggest that the Ta-C-N films exhibit superior fracture toughness compared to Ta-C coatings. Our results provide a new perspective on the role of alloying on the mechanical behavior of ultra-high temperature compounds such as transition-metal carbides.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305257-fx1.jpg" width="490" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Zhen Zhang, Ningbo Liao, Hongming Zhou, Wei Xue〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The combination of silicon and carbon layers exhibits superior lithium capacity and rate performance; however, the corresponding lithiation mechanism on the atomic-scale is not clear. In this work, the impact of the carbon layer on the electrochemical performance of silicon-carbon film systems as the lithium anode is investigated by a combination of experiments and first principles calculations. Experimental results show that the sample with the thickest carbon layer presents the smallest first cycle discharge capacities (2814 mAhg〈sup〉−1〈/sup〉); however, this sample also results in the largest capacity retentions after 100 cycles (69%) and the rate capability test (48.4%). Based on first principles calculations, the average length of the Li–Si bond near the silicon-carbon interface is significantly shorter than that in silicon, indicating an irreversible capacity loss. The structure with the largest carbon layer thickness corresponds to enhanced reversible capacity, electronic conductivity and lithium diffusion coefficient, which is consistent with experimental results. Our calculations provide a deeper understanding of irreversible capacity loss and how the primary nanostructure contributes to superior rate performance for silicon-carbon film anode materials.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305191-fx1.jpg" width="393" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): J. Narayan, A. Bhaumik, A. Haque〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report pseudo-topotactic growth of single-crystal diamond fibers by nanosecond laser melting of amorphous carbon nanofibers (CNFs) and crystalline multi-wall carbon nanotubes (MWCNTs). A rapid laser melting in a super undercooled state and subsequent quenching convert the tips of CNFs and MWCNTs into phase-pure 〈110〉 nanodiamonds along the growth directions. Subsequent laser pluses melt regions below 〈110〉 nanodiamonds that provide seeds for epitaxial growth. By repeating this process, the length of 〈110〉 nanodiamond fibers can be increased, as each pulse results in ∼50 nm nanodiamond region, depending upon the initial size of CNFs and MWCTs. This conversion process can be carried at ambient temperature and pressure in air. The epitaxial nature of 〈110〉 nanodiamond fibers has been confirmed by systematic electron-back-scatter-diffraction studies along the fiber in high-resolution scanning electron microscopy, and high-resolution TEM imaging and diffraction. The nature of C–C bonding characteristics was studied by high-resolution electron-energy-loss spectroscopy to establish the formation of diamond phase by the characteristic peak at 292 eV for sp〈sup〉3〈/sup〉 bonding (σ〈sup〉∗〈/sup〉), and absence of 284 eV peak for sp〈sup〉2〈/sup〉 (π〈sup〉∗〈/sup〉) graphitic bonding. The characteristic diamond Raman peak at 1332 cm〈sup〉−1〈/sup〉 is found to downshift to 1321 cm〈sup〉−1〈/sup〉 because of phonon confinement in nanodiamonds associated with nanofibers. These nanodiamond structures can be doped with both n- and p-type dopants with concentrations far higher than thermodynamic solubility limit due to solute trapping during quenching from the liquid phase. Thus, these nanodiamond structures provide ideal platform for nanosensing, computing and communication, including efficient field emitting devices.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930518X-fx1.jpg" width="237" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 8 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Diwakar P. Naragani, Paul A. Shade, Peter Kenesei, Hemant Sharma, Michael D. Sangid〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The small fatigue crack (SFC) growth regime in polycrystalline alloys is complex due to the heterogeneity in the local micromechanical fields, which result in high variability in crack propagation directions and growth rates. In this study, we employ a suite of techniques, based on high-energy synchrotron-based X-ray experiments that allow us to track a nucleated crack, propagating through the bulk of a Ni-based superalloy specimen during cyclic loading. Absorption contrast tomography is used to resolve the intricate 3D crack morphology and spatial position of the crack front. Initial near-field high-energy X-ray diffraction microscopy (HEDM) is used for high-resolution characterization of the grain structure, elucidating grain orientations, shapes, and boundaries. Cyclic loading is periodically interrupted to conduct far-field HEDM to determine the centroid position, average orientation, and average lattice strain tensor for each grain within the volume of interest. Reciprocal space analysis is used to further examine the deformation state of grains that plasticize in the vicinity of the crack. Analysis of the local micromechanical state in the grains ahead of the crack front is used to rationalize the advancing small crack path and growth rate. Specifically, the most active slip system in a grain, determined by the maximum resolved shear stress, aligns with the crack growth direction; and the degree of microplasticity ahead of the crack tip helps to identify directions for potential occurrences of crack arrest or propagation. The findings suggest that both the slip system level stresses and microplasticity events within grains are necessary to get a complete description of the SFC progression. Further, this detailed dataset, produced by a suite of X-ray characterization techniques, can provide the necessary validation, at the appropriate length-scale, for SFC models.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305075-fx1.jpg" width="270" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Sam Bakhtiari, Jefferson Zhe Liu, Yinong Liu, Hong Yang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ti〈sub〉50〈/sub〉Ni〈sub〉50-x〈/sub〉Cu〈sub〉x〈/sub〉 alloys are observed to exhibit multiple martensitic transformations from B2 to an orthorhombic B19 and a monoclinic B19′ phase. In addition, DFT calculations have predicted a B19ʺ phase with a higher monoclinic angle as the thermodynamically stable ground state. This study investigated the effects of Cu content and shear stress on the monoclinic angles, phase stabilities of the various martensites, the minimum energy pathways, and the relative total energies among the phases in this pseudo-equiatomic Ti(Ni〈sub〉50-x〈/sub〉Cu〈sub〉x〈/sub〉) system. A new monoclinic phase (B19〈sub〉M〈/sub〉) with a monoclinic angle lower than that of B19′ was found at above a critical Cu content. This confirms the formation of an intermediate phase in the martensitic transformation sequence of the pseudo-equiatomic Ti(Ni〈sub〉50-x〈/sub〉Cu〈sub〉x〈/sub〉) system but contradicts the crystal structure of the experimentally observed phase. It was found that the monoclinic angles of both B19〈sub〉M〈/sub〉 and B19ʺ decrease with increasing the magnitude of an opposing shear stress to their monoclinic distortion. At above certain critical values of the opposing shear stress, the B19〈sub〉M〈/sub〉 and B19ʺ phases destabilise and transform to lower monoclinic angle phases. In addition, the evidence suggests that the experimentally observed monoclinic B19′ phase is in fact a distorted B19ʺ with a reduced monoclinic angle under an opposing shear stress. With the same argument, the experimentally reported B19 phase is a metastable phase formed under the effect of an opposing shear stress to the monoclinic distortion of B19〈sub〉M〈/sub〉.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304987-fx1.jpg" width="254" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 30 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Manon Bonvalet-Rolland, Thomas Philippe, John Ågren〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Nucleation kinetics in a multicomponent supersaturated solid solution is examined. Attachment rate of atoms to a nucleus of a size close to the critical one is determined combining a thermodynamic extremum principle and the Fokker-Planck equation. Two limiting cases are examined; when bulk diffusion controls the nucleation kinetics and when the process is limited by the interfacial mobility. The mixed regime is also treated. Moreover, the growth law in multicomponent alloys is derived in the general case, when both mechanisms are considered. Additionally, the attachment rate is derived, in the classical framework, from a new macroscopic growth equations and the fundamental role of the interfacial mobility is examined. These new general expressions, for the attachment rates and the growth laws, determined either applying the thermodynamic extremum principle or derived from the classical formalism are found to be consistent.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301740-fx1.jpg" width="397" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 29 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Wei Chen, Shuo Cao, Wenjuan Kou, Jinyu Zhang, Yue Wang, You Zha, Yan Pan, Qingmiao Hu, Qiaoyan Sun, Jun Sun〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The ductile-to-brittle transition is commonly observed in metastable 〈em〉β〈/em〉-titanium (Ti) alloys containing 〈em〉ω〈/em〉-precipitates, while the fundamental understanding on 〈em〉ω〈/em〉-embrittlement hitherto remains elusive. In this work, the prototypical Ti-20wt.% Mo metastable 〈em〉β〈/em〉-Ti alloy has been systematically investigated by coupling experiments and first-principles calculation to eliminate this puzzle. It is shown that the structural evolution of 〈em〉ω〈/em〉-phase controls the deformation mechanism transition of twinning-to-slip in Ti-Mo alloy, being the origin of ductile-to-brittle transition of this alloy. The initial trigonal 〈em〉ω〈/em〉-structure continuously collapses to hexagonal 〈em〉ω〈/em〉-structure (structural collapse) whilst Mo-atoms is rejected out concurrently (stoichiometric varieties), both leading to hardening of 〈em〉ω〈/em〉-precipitates. This self-hardening of 〈em〉ω〈/em〉-precipitates was further rationalized in terms of the enhanced propensity for a covalent character of the atomic bond demonstrated by the electronic density of states (DOS) from first-principles calculation. Specifically, the self-hardening behavior of 〈em〉ω〈/em〉-precipitates promotes dislocation slip on isolated planes in lieu of correlative slip on successive planes inside 〈em〉ω1〈/em〉-variant, while dislocations are completely blocked ahead 〈em〉ω2/ω3/ω4-〈/em〉variants. This in turn renders the transition from deformation twinning that contributes to great macro-plasticity to ordinary dislocation slip that contributes to localized deformation bands in the present Ti-Mo alloy.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301776-fx1.jpg" width="377" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Elizaveta Y. Plotnikov, Zugang Mao, Sung-Il Baik, Mehmet Yildirim, Yongsheng Li, Daniel Cecchetti, Ronald D. Noebe, Georges Martin, David N. Seidman〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The temporal evolution of ordered γ’(L1〈sub〉2〈/sub〉)-precipitates precipitating in a disordered γ(f.c.c.) matrix is studied in extensive detail for a Ni-12.5 Al at.% alloy aged at 823 K (550 〈sup〉o〈/sup〉C), for times ranging from 0.08 to 4096 h. Three-dimensional atom-probe tomography (3-D APT) results are compared to monovacancy-mediated lattice-kinetic Monte Carlo (LKMC〈sub〉1〈/sub〉) simulations on a rigid lattice, which include monovacancy-solute binding energies through 4〈sup〉th〈/sup〉 nearest-neighbor distances, for the same mean composition and aging temperature. The temporal evolution of the measured values of the mean radius, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mrow〉〈mo〉〈〈/mo〉〈mrow〉〈mi〉R〈/mi〉〈mrow〉〈mo〉(〈/mo〉〈mi〉t〈/mi〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈/mrow〉〈/math〉, number density, aluminum supersaturations, and volume fraction of the γ’(L1〈sub〉2〈/sub〉)-precipitates are compared to the predictions of a modified version of the Lifshitz-Slyozov diffusion-limited coarsening model due to Calderon, Voorhees et al. The resulting experimental rate constants are used to calculate the Gibbs interfacial free-energy between the γ(f.c.c.)- and γ’(L1〈sub〉2〈/sub〉)-phases, which enter the model〈em〉,〈/em〉 using data from two thermodynamic databases, and its value is compared to all exiting values. The diffusion coefficient for coarsening is calculated utilizing the same rate-constants and compared to all archival diffusivities, 〈em〉not determined from coarsening experiments, and it is demonstrated to be the inter-diffusivity,〈/em〉 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mi〉D〈/mi〉〈mo〉˜〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈em〉, of Ni and Al.〈/em〉 The monovacancy-mediated LKMC〈sub〉1〈/sub〉 simulation results are in good agreement with our 3-D APT data. The compositional interfacial width, for the {100}-interface, between the γ(f.c.c.)- and γ’(L1〈sub〉2〈/sub〉)-phases, decreases continuously with increasing aging time and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mrow〉〈mo〉〈〈/mo〉〈mrow〉〈mi〉R〈/mi〉〈mrow〉〈mo〉(〈/mo〉〈mi〉t〈/mi〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈/mrow〉〈/math〉, both for the 3-D APT results and the monovacancy-mediated LKMC〈sub〉1〈/sub〉 simulations, in disagreement with an 〈em〉ansatz〈/em〉 intrinsic to the trans-interface diffusion-controlled coarsening model, which assumes the exact opposite trend for binary alloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301594-fx1.jpg" width="255" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Chenxi Wang, Xiaoming Yang, Zujian Wang, Chao He, Xifa Long〈/p〉 〈div xml:lang="en"〉 〈h5〉ABSTRACT〈/h5〉 〈div〉〈p〉Switching behavior is a general feature in ferroelectrics. The related fatigue effects influenced by defect dipoles in ferroelectrics are still controversial that is focused on the positive and negative effects of oxygen vacancies. Here, we report the polarization switching behavior of acceptor-doped ceramics using the first-order reversal curve (FORC) approach, especially for the abnormal self-rejuvenation effect and the enhanced fatigue endurance in acceptor-doped ceramics. The reversible and irreversible components under electric field in the ceramics were distinguished by the FORC distribution of ideal “hysteron”. The abnormal self-rejuvenation behavior stemmed from dispersive response of hysteron for undoped samples while from the redistribution of defect dipoles for acceptor-doped samples. The self-rejuvenation was induced mainly by the irreversible component. For the fatigue effect, the pinning of domain walls was not the main reason. The re-annealing treatment for a fatigued sample weakened the interactions between the spontaneous polarizations and defect dipoles, but enhanced the dispersion of coercive field. The enhancement of fatigue endurance came from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution existed in undoped ceramic with weak fatigue endurance. Our study shed new light on the interactions between spontaneous polarization and defect dipoles under repetitive AC electric field.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301764-fx1.jpg" width="491" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 165〈/p〉 〈p〉Author(s): Rafael Herschberg, Chu-Chun Fu, Maylise Nastar, Frédéric Soisson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The diffusion of C in Fe〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Cr solid solutions is modelled and compared to experimental data. A set of binding energies and migration barriers for C diffusion in different local chemical environments are first calculated using density functional theory. A pair interaction model is developed in order to reproduce these data and predict the migration barriers in other environments. The diffusion model is then implemented in a kinetic Monte Carlo method to simulate tracer diffusion experiments, using a standard procedure, and internal friction experiments, using a novel method. Simulations of internal friction show a unique Snoek peak in the whole concentration range, between pure iron and pure chromium. The average migration barrier for C diffusion in Fe〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Cr alloys is found to increase progressively with the Cr concentration, with a small rate below 6 %Cr. In Cr-rich alloys, the effective migration barrier for C diffusion is found to be larger in tracer diffusion than in the internal friction simulations. We conclude that the effective migration barrier extracted from tracer diffusion is closely related to trapping effects of C atoms in Fe-rich local environments, whereas the migration barrier associated with internal friction is mainly controlled by the migration barriers of the most probable configurations, as it is clearly shown in the Cr-rich domain.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Tracer diffusion and Internal Friction experiments were simulated via AKMC and compared with the available experimental data. A new model for the Internal Friction was developed and it was also compared with a previous model found in the experimental literature.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309017-fx1.jpg" width="426" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Sinan S. Faouri, Ali Mostaed, Julian S. Dean, Dawei Wang, Derek C. Sinclair, Shiyu Zhang, William G. Whittow, Yiannis Vardaxoglou, Ian M. Reaney〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ceramics-ceramic composites in series (1-〈em〉x〈/em〉)Li〈sub〉2〈/sub〉MoO〈sub〉4〈/sub〉-〈em〉x〈/em〉BaFe〈sub〉12〈/sub〉O〈sub〉19〈/sub〉 (LMO-BF12, 0.00 ≤ 〈em〉x〈/em〉 ≤ 0.15) have been cold sintered at 120 °C and their structure and properties characterized. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that compositions were dual phase and had a dense microstructure. Composites in the 〈em〉x〈/em〉BF12-(1-〈em〉x〈/em〉)LMO (0.0 ≤ 〈em〉x〈/em〉 ≤ 0.15) series resonated at MW frequencies (∼6 GHz) with 5.6≤〈em〉ε〈/em〉〈sub〉〈em〉r〈/em〉〈/sub〉 ≤ 5.8 and 〈em〉Qf〈/em〉 = 16,000–22,000 GHz, despite the black colour of compositions with 〈em〉x〈/em〉 〉 0. The permeability of the composites was measured in the X band (∼8 GHz) and showed an increase from 0.94 (〈em〉x〈/em〉 = 0.05) to 1.02 (〈em〉x〈/em〉 = 0.15). Finite element modelling revealed that the volume fraction of BF12 dictates the conductivity of the material, with a percolation threshold at 10 vol% BF12 but changes in 〈em〉ε〈/em〉〈sub〉〈em〉r〈/em〉〈/sub〉 as a function of 〈em〉x〈/em〉 were readily explained using a series mixing model. In summary, these composites are considered suitable for the fabrication of dual mode or enhanced bandwidth microstrip patch antennas.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310085-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): X.F. Kong, I.J. Beyerlein, Z.R. Liu, B.N. Yao, D. Legut, T.C. Germann, R.F. Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Low-energy structures of bimetal interfaces commonly occur in nature, yet higher energy forms, made by deviations in the interface plane, are also likely. While these variants may occur less frequently, they can still play an important role in the response of the material under deformation, by acting as preferential sites for defect formation and boundary motion. Here, using atomic-scale simulation and interface defect theory and considering two bimetal systems, Cu/Ag and Cu/Nb, we show that high-energy interfaces can achieve a local, low-energy state by forming atomic-scale serrations and that the predicted size and location of the serrations are consistent with experimental observation. For several distinct strain states, we reveal that interfaces with atomic-scale serrations bear both higher barriers for dislocation nucleation and higher resistances to interfacial shear compared to their planar interface variants. This desirable combination is not a characteristic of the more commonly studied low-energy interfaces, which typically possess a high nucleation barrier and low sliding resistance. We explain, through an analysis of the misfit dislocation structure, that the serrations alter the number of dislocations emitted, change the favorable slip systems, and alleviate the stress concentrations generated by misfit dislocations. An interface engineering strategy is then proposed for designing atomically serrated interfaces to improve mechanical strength and hinder localization and creep of metallic nanomaterials.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310024-fx1.jpg" width="265" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Zijiang Yang, Yuksel C. Yabansu, Dipendra Jha, Wei-keng Liao, Alok N. Choudhary, Surya R. Kalidindi, Ankit Agrawal〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Data-driven methods are attracting growing attention in the field of materials science. In particular, it is now becoming clear that machine learning approaches offer a unique avenue for successfully mining practically useful process-structure-property (PSP) linkages from a variety of materials data. Most previous efforts in this direction have relied on feature design (i.e., the identification of the salient features of the material microstructure to be included in the PSP linkages). However due to the rich complexity of features in most heterogeneous materials systems, it has been difficult to identify a set of consistent features that are transferable from one material system to another. With flexible architecture and remarkable learning capability, the emergent deep learning approaches offer a new path forward that circumvents the feature design step. In this work, we demonstrate the implementation of a deep learning feature-engineering-free approach to the prediction of the microscale elastic strain field in a given three-dimensional voxel-based microstructure of a high-contrast two-phase composite. The results show that deep learning approaches can implicitly learn salient information about local neighborhood details, and significantly outperform state-of-the-art methods.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309960-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Paraskevas Kontis, Aleksander Kostka, Dierk Raabe, Baptiste Gault〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni〈sub〉3〈/sub〉Ta as measured by atom probe tomography and their structure is consistent with the D0〈sub〉24〈/sub〉 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M〈sub〉5〈/sub〉B〈sub〉3〈/sub〉 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541830990X-fx1.jpg" width="498" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Jinming Guo, María Jazmin Duarte, Yong Zhang, Andrea Bachmaier, Christoph Gammer, Gerhard Dehm, Reinhard Pippan, Zaoli Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309911-fx1.jpg" width="369" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Daniel Gaertner, Katrin Abrahams, Josua Kottke, Vladimir A. Esin, Ingo Steinbach, Gerhard Wilde, Sergiy V. Divinski〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The diffusion kinetics in a CoCrFeMnNi high entropy alloy is investigated by a combined radiotracer–interdiffusion experiment applied to a pseudo-binary Co〈sub〉15〈/sub〉Cr〈sub〉20〈/sub〉Fe〈sub〉20〈/sub〉Mn〈sub〉20〈/sub〉Ni〈sub〉25〈/sub〉/Co〈sub〉25〈/sub〉Cr〈sub〉20〈/sub〉Fe〈sub〉20〈/sub〉Mn〈sub〉20〈/sub〉Ni〈sub〉15〈/sub〉 couple. As a result, the composition-dependent tracer diffusion coefficients of Co, Cr, Fe and Mn are determined. The elements are characterized by significantly different diffusion rates, with Mn being the fastest element and Co being the slowest one. The elements having originally equiatomic concentration through the diffusion couple are found to reveal up-hill diffusion, especially Cr and Mn. The atomic mobility of Co seems to follow a S-shaped concentration dependence along the diffusion path. The experimentally measured kinetic data are checked against the existing CALPHAD-type databases.〈/p〉 〈p〉In order to ensure a consistent treatment of tracer and chemical diffusion a generalized symmetrized continuum approach for multi-component interdiffusion is proposed. Both, tracer and chemical diffusion concentration profiles are simulated and compared to the measurements. By using the measured tracer diffusion coefficients the chemical profiles can be described, almost perfectly, including up-hill diffusion.〈/p〉 〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309832-fx1.jpg" width="273" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): T. Müller, M.W. Kapp, A. Bachmaier, P. Felfer, R. Pippan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A new ultrahigh-strength structure is created by severe plastic deformation of a martensitic 0.1 wt.-% C steel using high pressure torsion (HPT) at room temperature. Tensile tests reveal an ultimate tensile strength of 2.4 ± 0.1 GPa after an equivalent strain of ε〈sub〉vM〈/sub〉 = 7.5 – to our knowledge the highest tensile strength ever reported for a carbon steel with such low carbon content. During HPT, a lamellar microstructure is formed with decreasing lamellar spacing for increasing plastic strain. Micropillar compression tests give crucial insights into the mechanical properties, which are correlated to the deformation behavior of this material. Strong similarities compared to HPT-treated pearlitic steel are found in spite of the large differences concerning both carbon content and phase composition. The possibilities and limits of strength maximization in carbon steels are evaluated and discussed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309704-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): David Kleiven, Olve L. Ødegård, Kari Laasonen, Jaakko Akola〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Cluster Expansion formalism based on Density Functional Theory (DFT) simulation data has been applied for Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Mg alloys with high accuracy (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mo〉∼〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉 meV/atom). The atomistic simulations are used to model the Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Mg phase diagram, phase boundaries and the initial solute clustering at different compositions and temperatures. The obtained free energies of formation for the FCC, HCP and 〈em〉γ〈/em〉-phase are in accordance with the experimental phase diagram. The calculations demonstrate the formation of Guinier-Preston (GP) zones of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mtext〉Al〈/mtext〉〈/mrow〉〈mrow〉〈mn〉3〈/mn〉〈/mrow〉〈/msub〉〈mtext〉Mg〈/mtext〉〈/mrow〉〈/math〉 (L〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 phase) within the Al matrix under varying conditions. The computed transition temperatures where the ordered structures dissolve are approximately 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.gif" overflow="scroll"〉〈mrow〉〈mn〉50〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mtext〉K〈/mtext〉〈/mrow〉〈/math〉 higher than experimental data. The free energy barriers associated with the formation of GP-zones increase as the solute (Mg) concentrations are reduced and the temperature is increased.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310012-fx1.jpg" width="411" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Cun-hong Yin, Yi-long Liang, Yu Liang, Wei Li, Ming Yang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dry sliding wear tests of steel disks of various compositions were performed with consideration of the effects of microstructure, shear strain and shear strain rate using a ball-on-disk tester with a tungsten carbide ball under ambient atmosphere and room temperature. We found a severe–mild wear transition phenomenon, which we called self-lubrication, and categorized friction behavior into two regimes: a high-friction regime and a low-friction regime. However, self-lubrication occurs only at specific strain levels and rates related to materials with martensite and pearlite structures. We characterized the microstructures of the worn surface, subsurface and matrix and found a thin self-lubricating layer composed of nano-oxide particles with sizes in the range 6–20 nm attached to the surface. The formation of these nano-oxide particles is controlled by lamellar structure (martensite or pearlite) nano-lamination, oxidation and solid-state amorphization during dry sliding. The high density of geometrically necessary dislocations and defects induced by plastic deformation under wear shear strain levels and rates is important in the formation of nano-lamellar microstructures and amorphous oxide. These results both clarify nano-lamellar microstructure solid-state amorphization and oxidation behavior and reveal the mechanism of self-lubricating layer formation observed in this work.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310000-fx1.jpg" width="487" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Siddharth Gupta, Adele Moatti, Anagh Bhaumik, Ritesh Sachan, Jagdish Narayan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Localized charge injection by formation of vacancies provides an attractive platform for the development of multifunctional nanomaterials with direct implications in spintronics. However, further progress in spintronics critically depends on a deeper understanding of polaronic interactions between the localized charge states. This report is focused on TiN metallic system, which exhibits Pauli paramagnetism due to the absence of unpaired localized spin states. Here, nitrogen vacancies (V〈sub〉N〈/sub〉) are used as a variable to tune the magnetic properties of epitaxial TiN thin films by thermal annealing in high-vacuum and N〈sub〉2〈/sub〉 environment. Systematic introduction of V〈sub〉N〈/sub〉 generates robust magnetic ordering in vacuum-annealed TiN〈sub〉1-x〈/sub〉 films, with Curie temperature (〈em〉T〈/em〉〈sub〉〈em〉C〈/em〉〈/sub〉) ∼700 K, and saturation magnetization (〈em〉M〈/em〉〈sub〉〈em〉s〈/em〉〈/sub〉) at absolute zero of 13.6 emu g〈sup〉−1〈/sup〉. The signature spin-glass behavior below the irreversibility temperature (〈em〉T〈/em〉〈sub〉〈em〉ir〈/em〉〈/sub〉 ∼40 K) indicates the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling interactions between the unpaired localized spin-states. Through spatially resolved electron energy-loss spectroscopy, we have determined the generation of unpaired localized spins at Ti〈sup〉+2〈/sup〉 polarons with ∼12 ± 2 at.% V〈sub〉N〈/sub〉 in TiN〈sub〉1-x〈/sub〉 films. Such a large concentration of V〈sub〉N〈/sub〉 results in increased spin stiffness and high 〈em〉T〈/em〉〈sub〉〈em〉C〈/em〉〈/sub〉. These findings open a definitive pathway for tuning the magnetic nature of metallic materials for spintronic applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309923-fx1.jpg" width="422" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Jiang Yi, Yanlin Jia, Yuyuan Zhao, Zhu Xiao, Kejian He, Qi Wang, Mingpu Wang, Zhou Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Cu〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Ni〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Si alloys have been widely applied in electronic and electrical industries. The precipitation behavior of some of the Cu〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Ni〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Si alloys is still not well understood. In this study, the precipitation behavior of the Cu-3.0Ni-0.72Si alloy aged at 600 °C for different times was investigated by transmission electron microscopy, atom probe tomography and phenomenological theory of precipitation crystallography. A new orientation relationship (OR) between the precipitates and the Cu matrix was found in the over-aged condition and a coarsening mechanism of the metastable precipitates was put forward. The two- and three-dimension invariant line theories were successfully applied in interpreting the evolution of the ORs and the morphologies in the Cu/δ-Ni〈sub〉2〈/sub〉Si (Cu/δ) system. At the early stage of aging, the fine metastable δ’-(Cu, Ni)〈sub〉2〈/sub〉Si precipitates are coherent with the Cu matrix, with a quasi-Bain OR of (110)〈sub〉Cu〈/sub〉||(100)〈sub〉δ′〈/sub〉 and [001]〈sub〉Cu〈/sub〉||[001]〈sub〉δ′〈/sub〉, and four pairs of parallel conjugate planes: (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉1)〈sub〉Cu〈/sub〉||(〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si93.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉3〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉01)〈sub〉δ′〈/sub〉, (111)〈sub〉Cu〈/sub〉||(301)〈sub〉δ′〈/sub〉, (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉11)〈sub〉Cu〈/sub〉||(021)〈sub〉δ′〈/sub〉, and (1〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉1)〈sub〉Cu〈/sub〉||(0〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si92.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉1)〈sub〉δ’〈/sub〉. The precipitates have a δ-Ni〈sub〉2〈/sub〉Si structure, with some Ni atoms substituted by Cu atoms. During growth, the core region of the metastable δ’-(Cu,Ni)〈sub〉2〈/sub〉Si precipitate transforms into stable δ-Ni〈sub〉2〈/sub〉Si, with a quasi-NW OR of (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉11)〈sub〉Cu〈/sub〉||(021)〈sub〉δ〈/sub〉 and [110]〈sub〉Cu〈/sub〉||[100]〈sub〉δ〈/sub〉, while a layer of metastable δ’- (Cu, Ni)〈sub〉2〈/sub〉Si still exists around the core. With prolonging aging time, the δ-Ni〈sub〉2〈/sub〉Si precipitates with the OR of (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉11)〈sub〉Cu〈/sub〉||(021)〈sub〉δ〈/sub〉 and [110]〈sub〉Cu〈/sub〉||[100]〈sub〉δ〈/sub〉 grow two-dimensionally to form a plate-like shape, while those with the OR of (111)〈sub〉Cu〈/sub〉||(301)〈sub〉δ〈/sub〉 and [〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si91.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉10]〈sub〉Cu〈/sub〉||[010]〈sub〉δ〈/sub〉 grow one-dimensionally to form a fiber-like shape.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309984-fx1.jpg" width="441" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Luchan Zhang, Yang Xiang, Jian Han, David J. Srolovitz〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉High-entropy alloys (HEAs), i.e., single-phase, (nearly) equiatomic multicomponent, metallic materials, are associated with novel mechanical properties, such as high strength, fracture resistance etc. In this paper, a stochastic Peierls-Nabarro (PN) model is proposed to understand how random site occupancy affects intrinsic strength. The stochastic PN model accounts for the randomness in the composition, characterized by both the standard deviation of the perturbation in the interplanar potential and the correlation length within the spatial compositional distribution. The model presented includes the effects of non-uniform compositional distribution both in the direction of dislocation glide and along a dislocation line to predict overall dislocation glide resistance. The model predicts the intrinsic strength of HEAs as a function of the standard deviation and the correlation length of the randomness. We find that, in most of the parameter space, the compositional randomness in an HEA gives rise to an intrinsic strength that far exceeds that of any of the pure metals from which the HEA is composed. This approach provides a fundamental explanation to the origin of the high strength of HEAs.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309820-fx1.jpg" width="266" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): I.R. Souza Filho, A. Kwiatkowski da Silva, M.J.R. Sandim, D. Ponge, B. Gault, H.R.Z. Sandim, D. Raabe〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and 〈em〉in-situ〈/em〉 magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (T〈sub〉Curie〈/sub〉) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the T〈sub〉Curie〈/sub〉 of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309972-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Jin Hyeok Choi, Min Chul Jo, Hyungsoo Lee, Alireza Zargaran, Taejin Song, Seok Su Sohn, Nack J. Kim, Sunghak Lee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Austenitic high-Mn steels have been nominated as desirable ultra-high-strength cold-rolled steels whose mechanical properties are greatly improved by powerful deformation mechanisms of transformation- and twinning-induced plasticity (TRIP and TWIP). In this study, an austenitic high-Mn TRIP steel was suggested to achieve a good strength-ductility balance, and 1–2 wt.% Cu was added as an element for increasing stacking fault energy (SFE) as well as an austenite stabilizer to exploit a transition from TRIP to TWIP. The non-Cu-added steel showed the highest yield and tensile strengths (502 MPa and 1137 MPa, respectively) and the lowest elongation (34.6%) with a serrated flow. Yield and tensile strengths decreased with increasing Cu content, while the elongation was the highest in the 1%-Cu-added steel. TRIP and TWIP mechanisms showed good agreements with calculated SFEs in consideration of (Mn,Cu)-segregated bands. In the non-Cu-added steel, the TRIP occurred step by step as localized deformation bands passed through the specimen gage section to activate the serrated flow, which were reduced (or improved) by the transition from TRIP to TWIP with increasing Cu content. In the 1%-Cu-added steel, overall tensile properties were improved (yield strength; 461 MPa, tensile strength; 1093 MPa, elongation; 65.1%) as both TRIP and TWIP were well homogenized to produce synergic effects.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309959-fx1.jpg" width="449" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 165〈/p〉 〈p〉Author(s): Peng Li, Yu Huan, Weiwei Yang, Fangyuan Zhu, Xiaolong Li, Xingmin Zhang, Bo Shen, Jiwei Zhai〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Lead-free piezoelectric ceramics are urgently needed in the field of electromechanical conversion devices due to the restriction on the use of lead-based ceramics. In this study, the polymorphic phase boundary (PPB) were tuned by incorporating different concentration of (Bi〈sub〉0.5〈/sub〉K〈sub〉0.5〈/sub〉)HfO〈sub〉3〈/sub〉 into the matrix (K〈sub〉0.5〈/sub〉Na〈sub〉0.5〈/sub〉)(Nb〈sub〉0.965〈/sub〉Sb〈sub〉0.035〈/sub〉)O〈sub〉3〈/sub〉〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉CaZrO〈sub〉3〈/sub〉, and the 〈00〈em〉l〈/em〉〉〈sub〉c〈/sub〉 crystallographic texture was realized by templated grain growth method. The maximal 〈em〉d〈/em〉〈sub〉33〈/sub〉 (∼550 pC/N) and 〈em〉k〈/em〉〈sub〉p〈/sub〉 (∼72%) were achieved in the 〈00〈em〉l〈/em〉〉〈sub〉c〈/sub〉 textured ceramics with composition around rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary. It is proposed that the enhanced piezoelectricity should be ascribed to several combined effects, which primarily contain the R-O-T phase boundary facilitating polarization rotation, the crystallographic orientation induced intrinsic piezoelectric anisotropy, electric-field-induced lattice distortion and phase transitions, and NaNbO〈sub〉3〈/sub〉 seed-crystal-driven nanodomain structures. This work provides an effective solution to enhance piezoelectric properties by simultaneous tailoring polymorphic phase boundary and using crystallographic texture in potassium-sodium niobate based piezoelectric ceramics. We believe that the simple solution and design principle can also be applied to other piezoelectric ceramic systems, no matter lead-based or lead-free.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Superior piezoelectric properties (〈em〉d〈/em〉〈sub〉33〈/sub〉∼550 pC/N, 〈em〉k〈/em〉〈sub〉p〈/sub〉∼72%) were achieved by simultaneous tailoring polymorphic phase boundary and using crystallographic texture in lead-free KNN-based piezoelectric ceramics. The excellent piezoelectric properties benefit mainly from crystallographic orientation induced intrinsic piezoelectric anisotropy, high polarization efficiency, electric-field-induced lattice distortion and NaNbO〈sub〉3〈/sub〉 seed-crystal-driven nanodomain structure. These piezoelectric properties in conjunction with high Curie temperature (〈em〉T〈/em〉〈sub〉c〈/sub〉∼256 °C) in KNN-based ceramics are comparable to that of commercial PZT ceramics.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309662-fx1.jpg" width="223" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Hamzeh Kashani, Mingwei Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dealloyed nanoporous metals are emerging as a new class of structural and functional materials for a wide range of applications. Nevertheless, the dealloying process usually leads to significant volume shrinkage, large internal stresses, surface oxidation, stress-corrosion cracking and thereby extensive fabrication flaws. This is particularly serious for nanoporous non-noble metals because of their high affinity with oxygen and resulting serious oxidation. Therefore, dealloyed nanoporous metals usually have poor mechanical performances, particularly, under tension which is highly sensitive to flaws. Consequently, high tensile strength has not been achieved from technically-important and economic nanoporous transition metals, such as Ni and Cu. In this study we report flaw-free nanoporous Ni fabricated by utilizing an ultrafine grained precursor alloy, high-temperature dealloying and post-dealloying annealing. The resulting nanoporous Ni shows an excellent tensile strength, which is one order of magnitude higher than all reported tensile strengths of dealloyed nanoporous metals. The strong and ductile nanoporous Ni developed in this study can be scaled up for large-scale structural and functional applications where tensile properties are required.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310073-fx1.jpg" width="363" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Luis A. Marqués, María Aboy, Manuel Ruiz, Iván Santos, Pedro López, Lourdes Pelaz〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉By using classical molecular dynamics simulations and a novel technique to identify defects based on the calculation of atomic strain, we have elucidated the detailed mechanisms leading to the anomalous generation and growth of {001} loops found after ultra-fast laser annealing of ion-implanted Si. We show that the building block of the {001} loops is the very stable Arai tetra-interstitial [N. Arai, S. Takeda, M. Kohyama, Phys. Rev. Lett. 78, 4265 (1997)], but their growth is kinetically prevented within conventional Ostwald ripening mechanisms under standard processing conditions. However, our simulations predict that at temperatures close to the Si melting point, Arai tetra-interstitials directly nucleate at the boundaries of fast diffusing self-interstitial agglomerates, which merge by a coalescence mechanism reaching large sizes in the nanosecond timescale. We demonstrate that the crystallization of such agglomerates into {001} loops and their subsequent growth is mediated by the tensile and compressive strain fields that develop concurrently around the loops. We also show that further annealing produces the unfaulting of {001} loops into perfect dislocations. Besides, from the simulations we have fully characterized the {001} loops, determining their atomic structure, interstitial density and formation energy.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418310036-fx1.jpg" width="369" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Jose F. Gómez-Cortés, María L. Nó, Isabel Ruíz-Larrea, Tomasz Breczewski, Angel López-Echarri, Christopher A. Schuh, Jose M. San Juan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Micro and nano pillars of Copper-based shape memory alloys (SMAs) with feature sizes between about 2 μm and 250 nm are known to exhibit ultra-high mechanical damping due to the nucleation and motion of stress-induced martensite interfaces during superelastic straining. While this behavior could be extremely useful to protect micro electro-mechanical systems (MEMS) against vibrations in aggressive environments, a fundamental question must yet be answered in order to envisage further applications, namely, whether this damping is reproducible and stable over long times and many cycles, or whether the damping is a signal of accumulating damage that could compromise long-term usage. In the present paper this crucial question is answered; we show that micropillar arrays of Cu-Al-Ni SMAs exhibit a completely recoverable and reproducible superelastic response, with an ultra-high damping loss factor η 〉 0.1, or even higher for sub-micrometer pillars, η 〉 0.2, even after thousands of cycles (〉5000) and after long times spanning more than four years. Furthermore, the first high-frequency tests on such nanoscale SMAs show that their superelastic response is very fast and relevant to ultra-high damping even at frequencies as high as 1000 Hz. This paves the way for the design of micro/nano dampers, based on SMAs, to improve the reliability of MEMS in noisy environments.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309947-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): G. Rogl, K. Yubuta, V.V. Romaka, H. Michor, E. Schafler, A. Grytsiv, E. Bauer, P. Rogl〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉With a small gap in the density of states and a substantially semiconducting behavior half Heusler alloys have drawn attention as thermoelectric materials. For this study we have selected Hf-free compounds, Ti〈sub〉0.5〈/sub〉Zr〈sub〉0.5〈/sub〉NiSn, Ti〈sub〉0.5〈/sub〉Zr〈sub〉0.5〈/sub〉NiSn (with a densification aid (DA)) and Ti〈sub〉0.5〈/sub〉Zr〈sub〉0.5〈/sub〉NiSn〈sub〉0.98〈/sub〉Sb〈sub〉0.02〈/sub〉 as well their parent alloys TiNiSn and ZrNiSn as cheap thermoelectrics. Electrical resistivity, thermal conductivity and specific heat were evaluated below room temperature (4.2–300 K) in order to get insight into the mechanism of transport properties. SEM and TEM investigations as well as DFT (density functional theory) calculations accompany this research. The fine-grained epitaxial microstructure with a large number of dislocations warrants a low thermal conductivity at ultralow values (∼30 mW/cmK at 300 K) at a narrow band gap with a sufficiently high density of states at the Femi level. High order of components mixing strongly affects the stability of the solid solutions by the configuration entropy term, which causes a shrinkage of the miscibility gap. For the electronic density of states (DOS) the split Zr band and impurity Ni band induce a significant reduction of the effective energy gap and thus explain n-type of conductivity of the compounds and solid solutions studied.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309935-fx1.jpg" width="480" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 166〈/p〉 〈p〉Author(s): Chu Lun Alex Leung, Sebastian Marussi, Michael Towrie, Robert C. Atwood, Philip J. Withers, Peter D. Lee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Understanding defect formation during laser additive manufacturing (LAM) of virgin, stored, and reused powders is crucial for the production of high quality additively manufactured parts. We investigate the effects of powder oxidation on the molten pool dynamics and defect formation during LAM. We compare virgin and oxidised Invar 36 powder under overhang and layer-by-layer build conditions using 〈em〉in situ〈/em〉 and 〈em〉operando〈/em〉 X-ray Imaging. The oxygen content of the oxidised powder was found to be 〈em〉ca.〈/em〉 6 times greater (0.343 wt.%) than the virgin powder (0.057 wt.%). During LAM, the powder oxide is entrained into the molten pool, altering the Marangoni convection from an inward centrifugal to an outward centripetal flow. We hypothesise that the oxide promotes pore nucleation, stabilisation, and growth. We observe that spatter occurs more frequently under overhang conditions compared to layer-by-layer conditions. Droplet spatter can be formed by indirect laser-driven gas expansion and by the laser-induced metal vapour at the melt surface. Under layer-by-layer build conditions, laser re-melting reduces the pore size distribution and number density either by promoting gas release from keyholing or by inducing liquid flow, partially or completely filling pre-existing pores. We also observe that pores residing at the track surface can burst during laser re-melting, resulting in either formation of droplet spatter and an open pore or healing of the pore via Marangoni flow. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in LAM.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Using a laser additive manufacturing process replicator with 〈em〉in situ〈/em〉 and 〈em〉operando〈/em〉 X-ray imaging (a) permits capturing the formation of (b) porosity and (c) spatter during laser-matter interaction. In addition, we performed post mortem X-ray computed tomography analysis (d) reveal two types of pores inside the melt track: (i) open pores and (ii) closed pores.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645418309698-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Yuanbo T. Tang, Phani Karamched, Junliang Liu, Jack C. Haley, Roger C. Reed, Angus J. Wilkinson〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The serration of grain boundaries in Inconel 600 caused by heat treatment is studied systematically. A new method based on Fourier transforms is used to analyse the multiple wave-like character of the serrated grain boundaries. A new metric – the serration index – is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and non-serrated boundaries. By considering the variation of the serration index with processing parameters, a causal relationship between degree of serration and solution treatment/cooling rate is elucidated. Processing maps for the degree of serration are presented. Two distinct formation mechanisms arise which rely upon grain boundary interaction with carbides: (i) Zener-type dragging which hinders grain boundary migration and (ii) a faceted carbide growth-induced serration.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306275-fx1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Ruixuan Song, Yu Zhao, Weili Li, Yang Yu, Jie Sheng, Ze Li, Yulei Zhang, Hetian Xia, Wei-Dong Fei〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Low temperature stability has limited the applications at elevated temperature for ABO〈sub〉3〈/sub〉-type lead-free ceramics. And multi-grade resonances of piezoelectric ceramics are hardly obtained although the resonances are very important in some applications such as filter and resonator. High piezoelectric properties and large mechanical quality factors with multi-grade resonances can be obtained in (Li〈sup〉+〈/sup〉-La〈sup〉3+〈/sup〉) co-doped BaTiO〈sub〉3〈/sub〉 ceramics, and excellent temperature stabilities are achieved in the ceramics. It has been shown that the thermal treatments both with and without electric field at 200 °C improve the piezoelectric properties and thermal stability further. Large piezoelectric constant (272 pC N〈sup〉−1〈/sup〉) and huge mechanical quality factor (2010) was obtained after thermal-electrical treatment, which is caused by Li〈sup〉+〈/sup〉-La〈sup〉3+〈/sup〉 ionic pair aligning along the external electric filed during thermal-electrical treatment. Moreover, the present study provides an effective method to design combination properties with high temperature stability for ABO〈sub〉3〈/sub〉 perovskite ferroelectric ceramics.〈/p〉〈/div〉 〈h5〉Graphical Abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306354-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 180〈/p〉 〈p〉Author(s): Chika Izawa, Stefan Wagner, Martin Deutges, Mauro Martín, Sebastian Weber, Richard Pargeter, Thorsten Michler, Haru-Hisa Uchida, Ryota Gemma, Astrid Pundt〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values.〈/p〉 〈p〉For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances.〈/p〉 〈p〉A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance.〈/p〉 〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306093-fx1.jpg" width="483" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Q.C. Sherman, P.W. Voorhees, L.D. Marks〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the classical theories of oxidation of metals it is assumed that the interface between the oxide and metal is in thermodynamic equilibrium. However, in many cases this is not true, the oxide grows too fast or the fluxes through the interface are too large for local interfacial equilibrium to exist, leading to nonequilibrium solute capture. We present a thermodynamic analysis using both an available database as well as density functional theory calculations of the thermodynamic conditions for this during the oxidation of Ni-Cr alloys. The analysis indicates that nickel atoms can be captured in the rocksalt or corundum crystallographies for a very wide range of compositions, consistent with recent experimental observations. The density functional theory analysis also provides information about the electronic structure of these oxides which is important to understand their properties, and also indicates that interpretation of spectroscopic data is not simple as mixed valence states as well as Cr〈sup〉4+〈/sup〉 can occur under oxidizing conditions. We point out that across at least the first transition row of elements the thermodynamic conditions for nonequilibrium solute capture can easily be met.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉illustration of a moving oxidation front and how the velocity of the interface connects to equilibrium or nonequilibrium formation of the oxide 〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306330-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Yu-Hao Li, Hong-Bo Zhou, Linyun Liang, Ning Gao, Huiqiu Deng, Fei Gao, Gang Lu, Guang-Hong Lu〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mechanical responses of tungsten (W) and its alloys are strongly controlled by the properties of 1/2〈111〉 screw dislocations. Rhenium (Re), as a typical alloying and transmutation element in W, can substantially modify the properties of the dislocations, thus the plasticity of the materials. In this study, we investigate the interaction of Re and Re clusters with the screw dislocations in W by first-principles calculations in combination with theoretical models. Specifically, we propose two competing and Re-distribution dependent mechanisms, i.e. “ductilizing effect” and “hardening effect”; both are crucial to the mechanical properties of W. For the ductilizing effect, dispersed Re atoms weaken the surrounding interatomic interaction and reduce the shear resistance, thus facilitating the motion of the dislocation. In contrast, for the hardening effect, Re clusters formed by aggregated Re atoms due to irradiation can increase the Peierls stress and energy, thus hindering the motion of the dislocations. The proposed mechanisms shed light on the experimental observations that there is a Re-induced transition from ductilizing to hardening due to irradiation. The current work provides a theoretical guidance to the development of W-based future fusion materials in search of ductilizing alloying elements.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306251-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Christian Ebner, Jagannathan Rajagopalan, Christina Lekka, Christian Rentenberger〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Rejuvenation of an amorphous TiAl thin film under external tensile stress by high energy electron irradiation is observed via in-situ transmission electron microscopy (TEM). Electron beam (e-beam) irradiation results in a characteristic change of the elastic properties over time, as measured by the atomic-level elastic strain contained in the TEM diffraction pattern. Specifically, a time dependent increase/decrease of elastic strain is observed along the tensile direction, the saturation value of which correlates linearly with the preceding stress increment/decrement but shows little dependence on the e-beam condition. The low sensitivity of the saturation value to the e-beam condition indicates that the elastic strain change is induced by the structural transitions of a population (dependent on stress increment/decrement) of unstable atomic configurations to local, elastically soft areas. Classical molecular dynamics (MD) simulations including high energy electron scattering events are performed under tensile load to obtain insights into the structural modification that leads to time dependent changes in elastic strain under irradiation. The simulations reveal a change in quantities that are characteristic of structural rejuvenation, with a reduction of the local shear modulus manifesting as time dependent increase in the atomic-level elastic strain at fixed external stress. This link to the experimental data is confirmed by tracking elliptic distortions of simulated diffraction patterns calculated from MD configurations. The presented findings are highly relevant for experimental characterization of amorphous materials using TEM and give a new perspective on local structural modifications by electron irradiation.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306238-fx1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): A. Borroto, S. Bruyère, S. Migot, J.F. Pierson, T. Gries, F. Mücklich, D. Horwat〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Controlling surface morphology is a key issue for obtaining functional materials with surface-based properties. In this paper, we explore the possibility of using the self-separation of phases as a way of controlling the surface morphology features. We demonstrate using X-ray diffraction and transmission electron microscopy that a competitive self-separation of a nanocrystalline and an amorphous phases occurs in co-sputtered Zr-Mo thin films with a Mo content of 60 at%, corresponding to a composition intermediate to those necessary to form single-phased amorphous and nanocrystalline films. The dependence of the residual stress with the thickness at the biphased composition is discussed in terms of the morphology evolution and a possible mechanism for the self-separation of phases is presented. We show that the self-separation of phases as presented here is not limited to Zr-Mo alloys and can be extended to other systems. By changing the film thickness, it is possible to change the surface morphology of the films at the biphasic composition, due to the competitive growth of the nanocrystalline phase in the amorphous phase. In this way, it was possible to control the surface roughness and, because of this, tuning the film reflectance at a determined wavelength. The occurrence of an interference pattern in the reflectance spectra was discussed and associated to the presence of two different height levels at the film surface.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306263-fx1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 14 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Ryo Yamada, Michael R. von Spakovsky, William T. Reynolds〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The kinetics of ordering and concurrent ordering and phase separation are analyzed with an equation of motion initially developed to account for dissipative processes in quantum systems. A simplified energy eigenstructure, or pseudo-eigenstructure, is constructed from a static concentration wave method to describe the configuration-dependent energy in a binary alloy. This pseudo-eigenstructure is used in conjunction with an equation of motion that follows steepest entropy ascent to calculate the kinetic path that leads to ordering and phase separation in a series of hypothetical alloys. By adjusting the thermodynamic solution parameters, it is demonstrated that the model can predict: (〈em〉a〈/em〉) the stable equilibrium state, (〈em〉b〈/em〉) the unique thermodynamic path and kinetics of continuous or discontinuous ordering, and (〈em〉c〈/em〉) the kinetics of concurrent processes involving simultaneous ordering and phase separation.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306548-fx1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 10 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 181〈/p〉 〈p〉Author(s): Y.Q. Guo, S.H. Zhang, I.J. Beyerlein, D. Legut, S.L. Shang, Z.K. Liu, R.F. Zhang〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Zn-based and Mg-based alloys have been considered highly promising biodegradable materials for cardiovascular stent applications due to their excellent biocompatibility and moderate in vitro degradation rates. However, their strength is too poor for use in cardiovascular stents. The strength of these metals can be related to the sizes of the dislocation cores and the threshold stresses needed to activate slip, i.e., the Peierls stress. Using density functional theory (DFT) and an 〈em〉ab initio〈/em〉-informed semi-discrete Peierls–Nabarro model, we investigate the coupled effect of the solute element and mechanical straining on the stacking fault energy, basal dislocation core structures and Peierls stresses in both Zn-based and Mg-based alloys. We consider several biocompatible solute elements, Li, Al, Mn, Fe, Cu, Mg and Zn, in the same atomic concentrations. The combined analysis here suggests some elements, like Fe, can potentially enhance strength in both Zn-based and Mg-based alloys, while other elements, like Li, can lead to opposing effects in Zn and Mg. We show that the effect of solute strengthening and longitudinal straining on SFEs is much stronger for the Zn-based alloys than for the Mg-based alloys. DFT investigations on electronic structure and bond lengths reveal a coupled chemical-mechanical effect of solute and strain on electronic polarization, charge transfer, and bonding strength, which can explain the weak mechanical effect on Zn-based alloys and the variable strengthening effect among these solutes. These findings can provide critical information needed in solute selection in Zn-based and Mg-based alloy design for biomedical applications.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306524-fx1.jpg" width="246" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Huan Ma, Alla S. Sologubenko, Max Döbeli, Kay Sanvito, Alex Heusi, Kaj Pletscher, Ralph Spolenak〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉With high resistance against interdiffusion, electromigration, creep and fatigue, single-crystal films of refractory metals are highly desired for applications in opto- and micro-electronic devices. Their fabrication is nevertheless very challenging, primarily due to their high melting points. Requiring no external thermal input, ion-irradiation presents an alternative route for single-crystal films by converting from their polycrystalline counterparts through irradiation-induced selective grain growth. The incomplete poly- to single-crystal conversion is however a common problem which limits the application of this method. Here we use refractory W thin films, the element with the highest melting point, as a model system. We systematically investigate the influences of film microstructure (texture and grain size) and irradiation temperature (77 K and room temperature) on the evolution of selective grain growth under 4.5 MeV Au〈sup〉+〈/sup〉 ion-irradiation. We find that the driving force for selective grain growth can be increased by narrowing the texture spread of the films, refining the grain size and decreasing the irradiation temperature. Following this guidance, a complete conversion of a polycrystalline W film into a single crystal is achieved. This study therefore provides insights into the mechanism of selective grain growth and proves that it is an effective technique for microstructure engineering in thin film materials. The concepts can be applied to all crystalline metal films.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930881X-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Bo-In Park, Jin-Sung Park, Seunggun Yu, So-Hye Cho, Ji Young Byun, Jihun Oh, Seung Yong Lee〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hollow/porous structured SnO〈sub〉2〈/sub〉 nanoparticles were synthesized by simple oxidation of dense metal chalcogenide precursors via nanoscale Kirkendall diffusion effect. First, tin chalcogenide (SnS, SnSe) nanoparticles were synthesized by mechanochemical method, which is considered a facile, scalable, and eco-friendly process. Hollow/porous-walled SnO〈sub〉2〈/sub〉 nanoparticles were synthesized by simple oxidation of the prepared Sn chalcogenide precursors, for which the transformation mechanism was verified in detail. Nanoscale Kirkendall diffusion process was thoroughly investigated by morphological, crystallographic, and elemental analyses performed at various oxidation temperatures and times. To examine the morphological effect of hollow/porous-walled SnO〈sub〉2〈/sub〉 nanoparticles on the electrochemical performance, the synthesized nanoparticles were applied as anode material in a lithium-ion battery. Anode material showed highly improved electrochemical properties compared to its dense counterpart, with 83% capacity retention from the second cycle at the 400〈sup〉th〈/sup〉 cycle and capacity of 302 mA h g〈sup〉−1〈/sup〉 at a high current density of 30 A g〈sup〉−1〈/sup〉.〈/p〉〈/div〉 〈h5〉Graphical abstract (Table of contents)〈/h5〉 〈div〉〈p〉Sn-chalcogenides as precursors are synthesized by facile mechanochemical method. Sn-chalcogenides are oxidized under air atmosphere in a controlled manner. Finally, peculiar hollow/porous-walled SnO〈sub〉2〈/sub〉 particles are formed via nanoscale Kirkendall diffusion effect. When the SnO〈sub〉2〈/sub〉 particles are used as anode, the structural benefits result in improvement of electrochemical properties of lithium-ion battery.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930878X-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 30 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Yi-Hou Xiang, Ling-Zhi Liu, Jun-Chao Shao, Hai-Jun Jin〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The fact that ligament strength is strongly affected by size has become an obstacle to understanding the relationship between structural topology and mechanical response of dealloyed nanoporous metals. Herein we studied the mechanical properties of porous Fe〈sub〉0.80〈/sub〉Cr〈sub〉0.20〈/sub〉 prepared by liquid metal dealloying. The ligament diameters of these samples are stabilized at  ∼ 4 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si48.svg"〉〈mi〉μ〈/mi〉〈/math〉m, so that the ligament strength is constant in all samples. The variation of strength (or flow stress) and Young’s modulus with relative density, on a log–log scale, is nonlinear. Both properties decrease more steeply with decreasing relative density at lower relative density. These results are similar to the observations in nanoporous gold prepared by (electro)chemical dealloying but deviate from Gibson–Ashby (G-A) scaling laws. However, the strength of the porous Fe〈sub〉0.80〈/sub〉Cr〈sub〉0.20〈/sub〉 plotted against the Young’s modulus on a log–log scale exhibits a linear relation in the full range, with a slope of  ∼ 3/4 that matches perfectly with the standard G-A prediction. This confirms the significant role played by “dangling ligaments” in the deformation of dealloyed porous materials: Coarsening-induced pinch-off of some ligaments is responsible for the anomalously low strength and stiffness of dealloyed porous materials; the load-bearing network remains self-similar, and its mechanical response follows the standard G-A scaling laws, despite the fact that the porous material itself may not do so. Our study confirms that, for dealloyed porous materials, the G-A scaling relations are valid if the apparent relative density or, alternatively, genus-density-related prefactors are introduced.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308857-fx1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 27 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): D. Guedes, L. Cupertino Malheiros, A. Oudriss, S. Cohendoz, J. Bouhattate, J. Creus, F. Thébault, M. Piette, X. Feaugas〈/p〉 〈h5〉ABSTRACT〈/h5〉 〈div〉〈p〉The design of an electrochemical permeation device on a tensile machine has allowed to control the hydrogen flux and to isolate the effects of trapped and mobile hydrogen on the hydrogen embrittlement of a martensitic steel. Based on a local approach of fracture, tensile tests on several notched specimens were completed in order to investigate the impact of hydrostatic stress, equivalent plastic strain, hydrogen concentration and flux on the damage processes. Analysis of the fracture surfaces revealed that trapped hydrogen favors ductile fracture, enhancing nucleation and growth of voids by reducing the interface energy between precipitates/inclusions and matrix. Mobile hydrogen leads to quasi-cleavage along the substructure (lath and/or blocks) boundaries at mainly the {101} planes. For both mechanisms, the mutual interaction between hydrogen and dislocations (drag process increasing hydrogen diffusion and hydrogen favoring dislocations mobility) has a large contribution to the hydrogen embrittlement of the martensitic steel.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308845-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 23 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Qizhong Wang, Cristina Ramírez, Connor S. Watts, Oscar Borrero-López, Angel L. Ortiz, Brian W. Sheldon, Nitin P. Padture〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉There is growing interest in using 2D graphene-related reinforcements to toughen brittle ceramics in nanocomposites. However, there is a lack of fundamental understanding of the toughening mechanisms and microstructural effects in such nanocomposites. To address this paucity, fully-dense nanocomposites of aluminum oxide (Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉) matrix and reduced graphene-oxide (rGO) reinforcements (∼5 vol%) of different average-thicknesses and orientations are fabricated and characterized. The interactions between stably propagating cracks and rGO in the Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉/rGO nanocomposites are observed 〈em〉in situ〈/em〉 inside a scanning electron microscope (SEM). Toughening by pullout of thick rGO in the crack-tip wake in the cross-section orientation is found to be the most effective, which is consistent with the highest fracture toughness (〈em〉K〈/em〉〈sub〉IC〈/sub〉∼6.7 MPa.m〈sup〉0.5〈/sup〉) measured in those Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉/rGO nanocomposites. Interestingly, upon unloading and reloading, the intact rGO crack-bridges appear to crinkle and uncrinkle without a remnant crease, respectively, which is a unique deformation property of multi-layer graphene-like materials. This points to a possible new cyclic-fatigue resistance mechanism in those nanocomposites. Sliding-wear properties of the Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉/rGO nanocomposites are also studied, where the hardness and microstructural heterogeneities are found to play dominant roles. The results from this study have implications for the creation of high-toughness, fatigue-resistant, and wear-resistant graphene-reinforced ceramic nanocomposites of the future.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308742-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Keiji Shiga, Kensaku Maeda, Haruhiko Morito, Kozo Fujiwara〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The top surface of a crystal–melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary formation on the rate of grain growth parallel to the 〈111〉 direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal–melt interface in the direction parallel to the 〈111〉B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal–melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the {111}A plane of GaSb is almost ten times of that on the {111}B plane, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the 〈111〉A direction would be significantly lower than that in the 〈111〉B direction. These results indicate different growth rates in opposite 〈111〉 directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308675-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 23 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Cole D. Fincher, Daniela Ojeda, Yuwei Zhang, George M. Pharr, Matt Pharr〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Despite renewed interest in lithium metal anodes, unstable electrodeposition of Li during operation has obstructed progress in practical battery applications. While deformation mechanics likely play a key role in Li's mechanical stability as an anode material, reports of Li's mechanical properties vary widely, perhaps due to variations in testing procedures. Through bulk tensile testing and nanoindentation, we provide a comprehensive assessment of the strain-rate and length-scale dependent mechanical properties of Li in its most commonly used form: high purity commercial foil. We find that bulk Li exhibits a yield strength between 0.57 and 1.26 MPa for strain rates from 5E-4 s〈sup〉−1〈/sup〉 to 5E-1 s〈sup〉−1〈/sup〉. For indentation tests with target  〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mover accent="true"〉〈mi〉P〈/mi〉〈mo〉˙〈/mo〉〈/mover〉〈mo linebreak="goodbreak"〉/〈/mo〉〈mi〉P〈/mi〉〈/mrow〉〈/math〉 = 0.05 s〈sup〉−1〈/sup〉, the hardness decreases precipitously from nearly 43 MPa to 7.5 MPa as the indentation depth increases from 250 nm to 10 μm. The plastic properties measured from bulk and nanoindentation testing exhibit strong strain-rate dependencies, with stress exponents of n = 6.55 and 6.9, respectively. We implement finite element analysis to relate the indentation depth to length scales of relevance in battery applications. Overall, the results presented herein may provide important guidance in designing Li anode architectures and charging conditions to mitigate unstable growth of Li during electrochemical cycling.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308754-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 23 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Raiyan Seede, David Shoukr, Bing Zhang, Austin Whitt, Sean Gibbons, Philip Flater, Alaa Elwany, Raymundo Arroyave, Ibrahim Karaman〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Martensitic steels have gained renewed interest recently for their use in automotive, aerospace, and defense applications due to their ultrahigh yield strengths and reasonable ductility. A recently discovered low alloy martensitic steel, AF9628, has been shown to exhibit strengths greater than 1.5GPa with more than 10% tensile ductility, due to the formation of ε-carbide phase. In an effort to produce high strength parts with a high degree of control over geometry, the work herein presents the effects of selective laser melting (SLM) parameters on the microstructure and mechanical properties of this new steel. An optimization framework to determine the process parameters for building porosity-free parts is introduced. This framework utilizes the computationally inexpensive Eager-Tsai model, calibrated with single track experiments, to predict the melt pool geometry. A geometric criterion for determining maximum allowable hatch spacing is also developed in order to avoid lack of fusion induced porosity in the as-printed parts. Using this framework, fully dense samples were successfully fabricated over a wide range of process parameters, allowing the construction of an SLM processing map for AF9628. The as-printed samples displayed tensile strengths of up to 1.4GPa, the highest reported to date for any 3D printed alloy, with up to 11% elongation. The demonstrated flexibility in process parameter selection, while maintaining full density, opens up the possibility of local microstructural refinement and parameter optimization for improved mechanical properties in as-printed parts. The process optimization framework introduced here is expected to allow successful printing of new materials in an accelerated fashion.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419308766-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 182〈/p〉 〈p〉Author(s): Qingfeng Wu, Zhijun Wang, Xiaobing Hu, Tao Zheng, Zhongsheng Yang, Feng He, Junjie Li, Jincheng Wang〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Eutectics in high entropy alloys (HEAs) have shown excellent properties and promising applications. With empirical rules, various of eutectic high entropy alloys (EHEAs) have been proposed. The current design strategies shed light on the formation of eutectics in HEAs, but they are incapable of confirming multiple variables quantitatively in the selection of a specific system. In the present study, the eutectic formation in the multi-principal element systems is uncovered via data mining with machine learning (ML), where the critical elements and strongly associated elements were discovered. Taking the Al–Co–Cr–Fe–Ni system as an example, Al is confirmed to be the critical element for the eutectic formation and Cr is the strongly associated element with Al, Ni, Co, Fe and minor additions with comparably large solid solubility can be considered overall. With these understandings, a three-step approach can be summarized for designing EHEAs in a given system. Within the designed EHEAs, properties can be tested for optimization of application orientated design. The findings can not only accelerate the exploitation of EHEAs with better performance but also provide new ideas for designing compositionally complex alloys.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419307050-fx1.jpg" width="301" alt="Image, graphical abstract" title="Image, graphical abstract"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Matheus A. Tunes, Robert W. Harrison, Stephen E. Donnelly, Philip D. Edmondson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉M〈sub〉〈em〉n〈/em〉+1〈/sub〉AX〈sub〉〈em〉n〈/em〉〈/sub〉 phases, or simply MAX phases, are unique nanolayered materials that have been attracting the attention of the nuclear materials community worldwide due to the recent reports of superior radiation resistance compared to conventional ceramics. However, the knowledge and understanding of their response to neutron irradiation is fairly limited, in particular at high temperatures where MAX phases are expected to have high thermodynamic phase stability. In this paper, a complete and extensive study of neutron-irradiation effects at high temperatures on Ti-based MAX phases is presented. The MAX phases Ti〈sub〉3〈/sub〉SiC〈sub〉2〈/sub〉 and Ti〈sub〉2〈/sub〉AlC were irradiated at 1273 K in the High-Flux Isotope Reactor located at the Oak Ridge National Laboratory up to 10 displacement per atom (dpa). Post-irradiation characterisation was performed within a Transmission Electron Microscope on both irradiated and pristine samples. Upon increasing the dose from 2 to 10 dpa, the areal density of black-spots in the Ti〈sub〉2〈/sub〉AlC was observed to significantly increase while in the Ti〈sub〉3〈/sub〉SiC〈sub〉2〈/sub〉, disordered dislocation networks were observed. Regarding the Ti〈sub〉3〈/sub〉SiC〈sub〉2〈/sub〉, black-spot damage was observed to be concentrated within secondary phases, but absent in the matrix. Dislocation lines and loops were observed at both 2 and 10 dpa. The dislocation loops were identified to be of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mo〉〈〈/mo〉〈/mrow〉〈/math〉a〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈/math〉 type. At 2 dpa, stacking faults were observed in both materials, but were absent at 10 dpa. Cavities have also been observed, although no relationship with between size and dose was obtained. Finally, at 10 dpa, both MAX phases exhibited evidences of phase decomposition and irradiation-induced segregation. The presented results shed light on a very complex chain of radiation-induced defects in neutron-induced microstructures in both materials at high temperatures, and provide information that will enable better design of more radiation tolerant materials in the future.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301326-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): L. Zhao, K.C. Chan, S.H. Chen, S.D. Feng, D.X. Han, G. Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report that the tensile ductility of metallic glass (MGs) is tunable by introducing gradient rejuvenated amorphous structures (GRASs) using large-scale atomistic simulations. The results reveal that the ductile GRASs promote the formation and propagation of new shear bands in the interior unrejuvenated region by suppressing the catastrophic propagation of individual shear bands across the GRASs, thus resulting in a more dispersed plastic shearing throughout the sample. It is also demonstrated that increasing both the volume fraction and degree of structural disordering of GRASs can improve the tensile ductility of MGs and lead to a brittle-to-ductile transition of the deformation mode, although at the expense of some strength. Moreover, the critical volume fraction of GRASs required for switching the transition is found to depend on the specific degree of structural disordering. The observed structural state-dependent transition of the deformation mode is further understood from a mechanical perspective by considering the competition between the macroscopic yield strength and the critical stress of the material required for shear delocalization, based on which a criterion is developed to predict the critical transition boundary in MGs with GRASs across a wide range of structural states. The findings provide a detailed atomistic understanding of the relationship between the structural state and mechanical properties in MGs with partially rejuvenated amorphous structures, which may offer useful insights for designing and processing MGs with a sought-after combination of ductility and strength.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301405-fx1.jpg" width="428" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Minxia Fang, Shailendra Rajput, Zhonghua Dai, Yuanchao Ji, Yanshuang Hao, Xiaobing Ren〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Lead zirconate titanate (PZT) ceramics have been widely used because of their large piezoelectric response and good temperature-stability in the vicinity of morphotropic phase boundary (MPB). However, the understanding on the mechanism of temperature-stable piezoelectricity in PZT is still blur and thus needs to explore properly since the urgently-needed Pb-free systems designed by the same MPB method commonly show poor temperature stability. In this paper, we investigate the property-microstructure relationship for the thermal-stable piezoelectricity through comparative studies of the phase diagram, 〈em〉in-situ〈/em〉 piezoelectricity and microstructure evolution in both Pb-based and Pb-free systems. Our results show that the piezoelectric thermal-stability is closely related to the verticality of MPB and the doping elements. Tilted MPB shows worse thermal-stability as compared with that of the vertical MPB. While acceptor doped commercial PZTs show better thermal-stability when compared with that of the donor doped PZT. Moreover, our 〈em〉in-situ〈/em〉 TEM reveals that it is the thermal-stable fine microstructure (〈em〉i.e.〈/em〉, no degeneration) that corresponds to the thermal-stable high-performance piezoelectricity. Our work provides an in-depth understanding of the relationship between the phase diagram, microstructure and thermal-stable piezoelectricity, which shall benefit the designing of new lead-free temperature-stable piezoelectric materials.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301442-fx1.jpg" width="381" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): A.M. Cassell, J.D. Robson, C.P. Race, A. Eggeman, T. Hashimoto, M. Besel〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Zirconium (Zr) is used in modern aluminum alloys to form dispersoids that control grain structure. The interaction of these dispersoids with the alloying elements used to strengthen aluminum remains poorly understood. We have used high resolution imaging and composition analysis via electron microscopy to study the Zr-rich dispersoids in AA7010, a commercial Al-Zn-Mg-Cu alloy, addressing this knowledge gap.〈/p〉 〈p〉We show that the dispersoids are not of the ideal Al〈sub〉3〈/sub〉Zr stoichiometry, and contain Zn up to approximately 15 at%. Copper also concentrates in the dispersoids up to approximately 6 at%. Atomistic simulation was used to predict the partitioning, demonstrating favourable substitution of Zn and Cu onto the Al sublattice in the dispersoid phase, consistent with the measurements. We have also observed larger, facetted dispersoids, which are not of a phase observed in the binary Al–Zr system. Instead, we have found a previously unreported dispersoid structure (tI10 Ni〈sub〉4〈/sub〉Mo structure type), which we propose is stabilized by the presence of Zn.〈/p〉 〈p〉We have calculated the total loss in Zn and Cu due to partitioning into the dispersoids. We show this is too small to directly have a detrimental effect on age hardening, but may have a secondary effect in promoting heterogeneous nucleation for undesirable quench induced precipitation.〈/p〉 〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301338-fx1.jpg" width="258" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Asaya Fujita〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hydrogen redistribution between the two magnetic states that coexist at transition temperature, which is called the splitting phenomenon, was investigated in La(Fe,Si)〈sub〉13〈/sub〉-based magnetocaloric compounds. Asymmetric splitting into two portions with different volumes and H concentrations was observed on analysis of thermomagnetization curves, and this asymmetric growth was determined, using first-principle calculations, to be correlated with the electronic band structure. Excellent entropy change was observed in (Ce,La)(Fe,Mn,Si)〈sub〉13〈/sub〉H specimens, on which full hydrogenation was performed to inhibit the splitting phenomenon. The splitting phenomenon was found to be incompletely suppressed in these specimens after a short duration of homogenization annealing, and it gradually disappeared with increasing annealing duration. Magneto-optical observation with a magnetic transfer film shows that first heterogeneous nucleation appears at the spot-like sites; this is followed by the instant growth of a cloud-shaped droplet. The pinning of the droplet boundary by the spot-like nucleation site was also confirmed. Comparison of the metallographic structure revealed that the spot-like heterogeneous nucleation sites correspond to the grain-boundary triple points formed by imperfect grains, which would be swallowed by other stable grains during a ripening action. The droplet boundary, which is central to the hydrogen diffusion process, runs inside of crystalline grains, and in consequence, the main diffusion path of hydrogen was considered to be the body diffusion.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301399-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Jun Takahashi, Kazuto Kawakami, Kaoru Kawasaki〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Cr- and Cu-added steel sheets enable hardening of the surface region (the diffusion layer) by nitriding and simultaneously strengthening of the inner region due to thermally activated Cu precipitation during nitriding. Atom probe tomography (APT) analysis revealed that precipitate pairs, each composed of a Cu particle and CrN particle, were generated in the nitrided surface region. The influence of the complex precipitation of Cu and CrN on the precipitation kinetics and hardening properties was investigated by experimentally varying the initial state of Cu atoms in the steel by pre-aging before nitriding. It was found that Cu particles and CrN particles acted as heterogeneous nucleation sites for CrN and Cu, respectively, and the complex precipitation adversely affected the hardening of the nitrided surface region depending on the initial state of Cu. In addition, it was found that there were two types of precipitate pair forms (type-A/B), which were considered to be determined by the relation between the {001}-facet-size of the Cu particle and the size of the CrN nucleus in heterogeneous nucleation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301351-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Oleksandr Glushko, Gerhard Dehm〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301375-fx1.jpg" width="248" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 12 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Cheng Wen, Yan Zhang, Changxin Wang, Dezhen Xue, Yang Bai, Stoichko Antonov, Lanhong Dai, Turab Lookman, Yanjing Su〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We formulate a materials design strategy combining a machine learning (ML) surrogate model with experimental design algorithms to search for high entropy alloys (HEAs) with large hardness in a model Al-Co-Cr-Cu-Fe-Ni system. We fabricated several alloys with hardness 10% higher than the best value in the original training dataset via only seven experiments. We find that a strategy using both the compositions and descriptors based on a knowledge of the properties of HEAs, outperforms that merely based on the compositions alone. This strategy offers a recipe to rapidly optimize multi-component systems, such as bulk metallic glasses and superalloys, towards desired properties.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉In the present study, we proposed a data-driven approach combining machine learning, experimental design and feedback from experiment to accelerate the search for multi-component alloys with target properties. We demonstrated the efficiency of our approach by rapidly obtaining several alloys with hardness 10% higher than the best value in the original training dataset via only seven experiments. In Iteration Loop I, a machine learning surrogate model is trained to learn the property-composition, relationship, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mi〉y〈/mi〉〈mi〉i〈/mi〉〈/msub〉〈mo〉=〈/mo〉〈mi〉f〈/mi〉〈mrow〉〈mo〉(〈/mo〉〈msub〉〈mi〉c〈/mi〉〈mi〉i〈/mi〉〈/msub〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉, with associated uncertainties. The model is applied to the vast unexplored space and a utility function is employed to recommend the most informative candidate for the next experiment, which balances the exploitation and exploration. Feedback from experimental synthesis and characterization allows the subsequent iterative improvement of the surrogate model. Iteration Loop II is essentially same as Iteration Loop I, except that a features pool was introduced to the Iteration Loop I and a surrogate model is trained from composition (c〈sub〉i〈/sub〉) and the preselected physical features (p〈sub〉i〈/sub〉), 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mi〉y〈/mi〉〈mi〉i〈/mi〉〈/msub〉〈mo〉=〈/mo〉〈mi〉f〈/mi〉〈mrow〉〈mo〉(〈/mo〉〈msub〉〈mi〉c〈/mi〉〈mi〉i〈/mi〉〈/msub〉〈mo〉,〈/mo〉〈msub〉〈mi〉p〈/mi〉〈mi〉i〈/mi〉〈/msub〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉. We found that the approach using both the composition and the descriptors based on domain knowledge can more effectively accelerate material optimization compared to the approach using only the compositions.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301430-fx1.jpg" width="281" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 169〈/p〉 〈p〉Author(s): Jiaying Jin, Mi Yan, Yongsheng Liu, Baixing Peng, Guohua Bai〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Two-step post-sinter annealing (PSA) treatment, due to its universal applicability, has been one of the most effective strategies for enhancing the coercivity of Nd-Fe-B sintered magnets. Here we report a peculiar phenomenon that the as-sintered multi-main-phase (MMP) Nd-La-Ce-Fe-B magnet exhibits the highest reported to date coercivity of 13.0 kOe upon 25 wt% La-Ce substituting for Nd, combined with 〈em〉B〈/em〉〈sub〉r〈/sub〉 = 13.12 kG, and (〈em〉BH〈/em〉)〈sub〉max〈/sub〉 = 41.67 MGOe. Further annealing cannot enhance the coercivity whereas the remanence and energy product are drastically lowered, i.e. to 11.0 kOe, 12.57 kG, and 38.71 MGOe after two-step PSA, being remarkably different from the conventional Nd-Fe-B. Microstructural analysis reveals that the as-sintered Nd-La-Ce-Fe-B magnet with coexisting REFe〈sub〉2〈/sub〉 (〈em〉Fd〈/em〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉3〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈em〉m〈/em〉) and RE-rich (〈em〉Fm〈/em〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉3〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈em〉m〈/em〉) intergranular phases possesses continuous GB layer isolating adjacent ferromagnetic grains. Hence the beneficial role of PSA on modifying the microstructure and strengthening the coercivity is tiny. Additionally, high-temperature PSA destroys the initial chemical heterogeneity of MMP magnets, leading to the formation of REs homogeneously distributed grains and resultant reduction of the intrinsic magnetic properties. As evidenced by in-situ Lorentz TEM characterization, PSA sample with reduced chemical gradient exhibits quick domain wall motion, compared to the as-sintered one with basically unchanged magnetic domain structure upon the same applied field. The proof-of-principle micromagnetic simulation further confirms that retaining the inhomogeneous La/Ce/Nd distribution is essential to suppress the magnetic dilution effect. These findings demonstrate the realistic prospect of skipping PSA treatment in MMP magnets, which can not only reduce the production process and cost, but also delight the scenario for designing high-performance 2:14:1-type sintered magnets based on abundant La/Ce.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉The as-sintered MMP magnet exhibits a highest reported to date coercivity of 13.0 kOe at 25 wt% La-Ce substitution level, with strengthened remanence of 13.12 kG, and maximum energy product of 41.67 MGOe (a-b). The preferable magnetic performances are attributed to two main features, first is the coexisting REFe〈sub〉2〈/sub〉 (〈em〉Fd〈/em〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉3〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈em〉m〈/em〉) and RE-rich (〈em〉Fm〈/em〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉3〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〈em〉m〈/em〉) intergranular phases to form continuous GB layer, as evidenced by the smooth and clear interface between RE〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B/RE〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B, RE〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B/REFe〈sub〉2〈/sub〉, and RE〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B/fcc-REO〈sub〉x〈/sub〉 phases (c), second is the retained chemical gradient to form core-shell morphology within an individual grain and intergrain REs deviations (d). The former feature weakens the intergranular exchange coupling, and the latter strengthens the intrinsic magnetic properties, as revealed by the reverse magnetic domain structure across the GB layer and the stable domain walls upon the applied field (e).〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301387-fx1.jpg" width="437" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 170〈/p〉 〈p〉Author(s): Ilias Bikmukhametov, Hossein Beladi, Jiangting Wang, Peter D. Hodgson, Ilana Timokhina〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930165X-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 170〈/p〉 〈p〉Author(s): Yuechao Chen, Cuiping Wang, Jingjing Ruan, Toshihiro Omori, Ryosuke Kainuma, Kiyohito Ishida, Xingjun Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The γ/γ′ two-phase Co–Al–V-base superalloys are investigated in detail, including their phase equilibria, alloying effects, and alloy design. The experimental proof of the existence of a ternary compound with an L1〈sub〉2〈/sub〉-ordered γ′-Co〈sub〉3〈/sub〉(Al, V) phase is provided in this study, and the γ/γ′ two-phase Co–5Al–14V (at.%) alloy is designed. The γ′-Co〈sub〉3〈/sub〉(Al,V) phase is proved to be thermodynamically stable at 900 °C, and a wide γ/γ′ two-phase composition region in the Co–Al–V ternary phase diagram at 900 °C is identified. The performed investigation of Co–5Al–14V-2X (X: Ti, Cr, Nb, Mo, Ta, and W) and Co-xNi-8Al–12V (x = 10, 20, and 30) quaternary alloys revealed that Ti, Nb, Mo, Ta, W, and Ni tend to concentrate in the γ′ phase, whereas Cr tends to partition to the γ matrix. Besides, the γ/γ′ two-phase Co–30Ni–10Al–5V–4Ta–2Ti (30Ni4Ta2Ti) alloy is designed. The 30Ni4Ta2Ti alloy exhibits a very high γ′ solvus temperature (namely, 1242 ± 3 °C) combined with a low density (8.46 ± 0.08 g cm〈sup〉−3〈/sup〉), which characteristics are comparable with those of the Mar-M-247 Ni-base superalloy. Moreover, the nominal and the specific yield strength values of the obtained 30Ni4Ta2Ti alloy within the temperature range from 600 to 900 °C are also close to those of the Mar-M-247 superalloy.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301466-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 170〈/p〉 〈p〉Author(s): Beilin Ye, Tongqi Wen, Manh Cuong Nguyen, Luyao Hao, Cai-Zhuang Wang, Yanhui Chu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The formation possibility of a new (Zr〈sub〉0.25〈/sub〉Nb〈sub〉0.25〈/sub〉Ti〈sub〉0.25〈/sub〉V〈sub〉0.25〈/sub〉)C high-entropy ceramics (ZHC-1) was first analyzed by the first-principles calculations and thermodynamical analysis and then it was successfully fabricated by hot pressing sintering technique. The first-principles calculation results showed that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of ZHC-1 was in the range of 0.693R–1.040R. The thermodynamical analysis results showed that ZHC-1 was thermodynamically stable above 959 K owing to its negative mixing Gibbs free energy. The experimental results showed that the as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal structure, some interesting nanoplate-like structures, and high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it showed a relatively low thermal conductivity of 15.3 ± 0.3 W/(m⋅K) at room temperature, which was due to the presence of solid solution effects, nanoplates and porosity. Meanwhile, it exhibited the relatively high nanohardness of 30.3 ± 0.7 GPa and elastic modulus of 460.4 ± 19.2 GPa and the higher fracture toughness of 4.7 ± 0.5 MPa m〈sup〉1/2〈/sup〉, which were attributed to the solid solution strengthening mechanism and nanoplate pullout and microcrack deflection toughening mechanism.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉A new (Zr〈sub〉0.25〈/sub〉Nb〈sub〉0.25〈/sub〉Ti〈sub〉0.25〈/sub〉V〈sub〉0.25〈/sub〉)C high-entropy ceramic (ZHC-1) with a single rock-salt crystal structure of metal carbides, some interesting nanoplate-like structures and high compositional uniformity from nanoscale to microscale was fabricated and investigated in detailed and it exhibited the superior mechanical and thermal physical performances, which would endow ZHC-1 a promising candidate for extreme environmental applications.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301648-fx1.jpg" width="404" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 170〈/p〉 〈p〉Author(s): Zhiqi Fan, Yitian Zhao, Qiyang Tan, Ning Mo, Ming-Xing Zhang, Mingyuan Lu, Han Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The present work proposed a novel 3D printing approach for fabricating ternary eutectic ceramics (Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉-YAG-ZrO〈sub〉2〈/sub〉, or AYZ in short) using laser engineered net shaping (LENS) technique. Highly dense (≥98%) thin wall components with refined eutectic microstructures were successfully fabricated. The as-fabricated ceramic eutectics mainly consist of cellular eutectics with three phases interpenetrated. In each deposited layer, the solidified microstructure changes from planar to cellular along building direction. The evolution of characteristic dimension of microstructure can be well interpreted using Jackson-Hunt relationship. Irregular interpenetrating to regular fibrous eutectic structure transition occurs at boundary region of LENSed specimens, primarily due to much higher solidification rate. The texture of 〈0001〉〈sub〉Al2O3〈/sub〉//〈001〉〈sub〉YAG〈/sub〉//〈001〉〈sub〉ZrO2〈/sub〉 is recognized at the interior region of LENSed eutectic part along building direction, but gradually tilts from central to edge due to change of cellular growth direction. The LENSed AYZ samples exhibit almost isotropic property, and have mechanical performances comparable to those fabricated using conventional direction solidification methods.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301636-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): F. Fischer, G. Schmitz, S.M. Eich〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this atomistic study on the copper–nickel system, a new embedded-atom alloy potential between copper and nickel is fitted to experimental data on the mixing enthalpy, taking available potentials for the pure components from literature. The resulting phase boundaries of the new potential are in very good agreement with a recent CALPHAD prediction. Using this new potential, a high angle symmetrical tilt 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉5〈/mn〉〈/mrow〉〈/math〉 and a coherent 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉 twin grain boundary (GB) are chosen for a systematic investigation of equilibrium GB segregation in the semi-grandcanonical ensemble at temperatures from 400 K to 800 K. Applying thermodynamically accurate integration techniques, the GB formation energies are calculated exactly and as an absolute value for every temperature and composition, which also enables the evaluation of GB excess entropies. The thorough thermodynamic model of GBs developed by Frolov and Mishin is excellently confirmed by the simulations quantitatively, if the impact of both segregation and GB tension on the change in GB formation energy is accounted for. In the case of the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉 coherent GB, it turns out that the change in GB formation energy at low temperatures is for the most part attributed to the GB tension, while segregation only has a small influence. This demonstrated effect of GB tensions should also be taken into account in the interpretation of experiments.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303945-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉