ALBERT

Description: 〈p〉Publication date: 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 174〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Cong Yang, Qingyan Xu, Xianglin Su, Baicheng Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Understanding the solidification behavior in nickel-based single crystal superalloy solidification is essential for determining the following heat treatment process, while the experimental findings have led to contradictory results. On the one hand, the fine γ/γ′ structure in the interdendritic zone suggests a eutectic reaction. On the other hand, the coarse γ′ precipitates indicate a peritectic reaction. In this work, the CALPHAD-based multiphase-field model was used to investigate the superalloy solidification behavior and complex γ/γ′ morphology generation. The fine γ/γ′ and coarse γ′ patterns were reproduced in the γ/γ′ colony, which were compared with the interdendritic microstructure observed by experiments. And the eutectic and peritectic reactions were verified by analyzing the chemical driving force at the phase interfaces. The simulation results suggest that the coarsening of γ′ in the γ/γ′ colony was due to the fast decrease of chemical driving force at the γ/〈em〉l〈/em〉 and γ'/〈em〉l〈/em〉 interfaces. Besides, remelting of the γ dendrite was found near the bulk γ′, which can be mainly ascribe to the increasing of Mo concentration rejected by the growing γ'. The simulated microsegregation patterns of the alloy components were also obtained and the results were in good agreement with the experimental findings.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303921-fx1.jpg" width="391" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Susumu Fujii, Tatsuya Yokoi, Masato Yoshiya〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Grain boundaries often have a decisive effect on the macroscopic properties of polycrystalline materials, but the wide variety of their atomic structures, interfacial lengths, and compositions makes them difficult to characterize all-encompassingly. An indispensable example in which an understanding of the relationship between grain boundary structures and properties would greatly facilitate development of superior materials is thermal transport, especially with respect to microstructure evolution, thermoelectrics and thermal barrier coatings. To contribute to a more comprehensive understanding, we performed a systematic study of lattice thermal conduction across a wide range of symmetric tilt grain boundaries in MgO using perturbed molecular dynamics. It was found that thermal conductivities vary significantly with grain boundary structure but are strongly correlated with excess volume, which is a measure of the number density of atoms in the vicinity of the grain boundary planes. Real-space analysis revealed that dislocation densities determine the phonon transport paths and thermal conductivity in low-angle boundaries whereas it is the amount of open volume rather than the shape of structural units in high-angle boundaries that determine thermal conductivity. We also found that low-angle boundaries mainly reduce phonon transports at low frequencies whereas high-angle boundaries also reduce it at intermediate and high frequencies effectively, regardless of the shape of structural units. These insights are expected to be applicable to other close-packed oxide systems, and should aid the design of next-generation thermal materials through tailoring of grain boundaries.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302010-fx1.jpg" width="482" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Meng-Jun Zhou, Yi Wang, Yanzhou Ji, Zi-Kui Liu, Long-Qing Chen, Ce-Wen Nan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉It was recently found that nanowires of PbTiO〈sub〉3〈/sub〉 synthesized through an intermediate pre-perovskite phase exhibit enhanced spontaneous polarization. Here we investigated the pre-perovskite PbTiO〈sub〉3〈/sub〉 (PP-PTO) nanowire phase at finite temperatures employing first-principles quasiharmonic calculations. We calculated its band gap, phonon dispersions, phonon density of states, Debye temperature, and thermodynamic properties. The corresponding calculations for cubic and tetragonal PbTiO〈sub〉3〈/sub〉 were also carried out for comparison. In the current calculations, the amount of imaginary frequencies associated with the ideal cubic PTO structure, i.e., a cubic cell shape with ion positions at the ideal cubic perovskite lattice sites, was decreased to a negligible level by employing a constrained cubic structure, a structure with the same cubic cell shape as the ideal cubic PTO structure but allowing the ion positions to relax to thermodynamically more stable tetragonal positions at 0 K. In contrast to the general observation that a higher volume phase would have relatively higher entropy, it is found that the PP-PTO phase possesses the lowest entropy while having the largest volume compared to cubic and tetragonal PbTiO〈sub〉3〈/sub〉 phases. Furthermore, the temperature-pressure phase diagram for the three PbTiO〈sub〉3〈/sub〉 phases was obtained, which demonstrates that PP-PTO could be stabilized under a large volume or a negative pressure. This study provides insights to experimentally synthesizing the PP-PTO phase and to better understanding its phase transition into the converted tetragonal PbTiO〈sub〉3〈/sub〉 nanowires with enhanced piezoelectric and ferroelectric properties.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302009-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Wenjia Song, Shanjie Yang, Masahiro Fukumoto, Yan Lavallée, Siddharth Lokachari, Hongbo Guo, Yancheng You, Donald B. Dingwell〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The turbine technology incorporated in jet engines is inherently vulnerable to attack by environmental silicate debris. Amongst the various kinds of such debris, volcanic ash is a particular threat as its glass softens to a liquid at temperatures of 500–800 °C, far below jet engine operating temperatures of ∼1500 °C. As a result, ingested re-molten droplets impact and form splats on the protective thermal barrier coatings (TBCs). Investigation of the damage to jet engines ensuing from this process has, to date been restricted to forensic observations after critical encounters. Here, we employ a thermal spray technology to recreate the ‘〈em〉in-situ〈/em〉’ generation of molten volcanic ash droplets and observe their morphological evolution and interaction with TBCs. The mechanism of splat formation is found to depend both on substrate topography and on in-flight droplet characteristics, whereby splat circularity increases with surface roughness and with the product of the Weber and Reynolds numbers. The experiments reveal that the molten ash droplet adhesion rate is dictated by droplet temperature and viscosity, ash concentration and substrate roughness. A new dimensionless number, 〈em〉S〈/em〉, is developed to quantify the molten ash droplet adhesion rate to both substrate topography and in-flight droplet characteristics. These findings provide a greatly improved basis for the quantification of the hazard potential of volcanic ash to jet engines and should be incorporated into protocols for operational aviation response during volcanic crises.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302034-fx1.jpg" width="310" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): M.G. Tsoutsouva, G. Stokkan, G. Regula, B. Ryningen, T. Riberi – Béridot, G. Reinhart, N. Mangelinck-Noël〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically 〈em〉in situ,〈/em〉 by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111}/facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302022-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): 〈/p〉

Description: 〈p〉Publication date: 15 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 172〈/p〉 〈p〉Author(s): S. Irukuvarghula, H. Hassanin, C. Cayron, M. Aristizabal, M.M. Attallah, M. Preuss〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effect of powder size distribution and oxygen content on the extent of multiple twinning and spatial distribution of oxide inclusions in hot isostatic pressed (HIPed) 316L steels was investigated using powders with different characteristics. Modifications to, and differences in their microstructural topology, were tracked quantitatively by evaluating the metrics related to twin related domains (TRDs) on specimens produced by interrupting the HIPing process at various points in time. Results revealed that powder size distribution has a strong effect on the extent of multiple twinning in the fully HIPed microstructure, with specimens produced using narrow distribution showing better statistics (i.e., homogeneously recrystallized) than the ones produced using broad size distribution. The oxide inclusion density in fully HIPed microstructures increased with the amount of oxygen content in the powders while prior particle boundaries (PPBs) were only observed in the specimens that were HIPed using broad powder distribution. More importantly, results clearly revealed that the spatial distribution of the inclusions was strongly affected by the homogeneity of recrystallization. Implications of the results are further discussed in a broader context, emphasizing the importance of utilizing the occurrence of solid state phase transformations during HIPing for controlling the microstructure evolution.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301909-fx1.jpg" width="289" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): P.E. Seiler, H.C. Tankasala, N.A. Fleck〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Rapid prototyping is an emerging technology for the fast make of engineering components. A common technique is to laser cut a two-dimensional (2D) part from polymethyl methacrylate (PMMA) sheet. However, both manufacturing defects and design defects (such as stress raisers) exist in the part, and these degrade its strength. In the present study, a combination of experiment and finite element analysis is used to determine the sensitivity of the tensile strength of PMMA hexagonal lattices to both 〈em〉as-manufactured〈/em〉 and 〈em〉as-designed〈/em〉 defects. The 〈em〉as-manufactured〈/em〉 defects include variations in strut thickness and in Plateau border radius. The knockdown in lattice tensile strength is measured for lattice relative density in the range of 0.07 to 0.19. A systematic finite element (FE) study is performed to assess the explicit role of each type of as-manufactured defect on the lattice strength. 〈em〉As-designed〈/em〉 defects such as randomly perturbed joints, missing cells, and solid inclusions are introduced within a regular hexagonal lattice. The notion of a transition flaw size is used to quantify the sensitivity of lattice strength to defect size.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301880-fx1.jpg" width="290" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Mohamed Fares Slim, Akram Alhussein, Elia Zgheib, Manuel François〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The scope of this work is to propose a methodology allowing the determination of the single-crystal elasticity constants of a phase included in a multiphase thin film taking into account its microstructure (crystallographic and morphological texture, porosity and multiphase aspect). The methodology is based on the use of a macro-mechanical test, the impulse excitation technique, a micro-mechanical test, X-ray diffraction and the Kröner-Eshelby scale transition model. As a supporting example, it was applied to determine the single-crystal elasticity constants of the W〈sub〉β〈/sub〉 tungsten metastable phase embedded in a two phases (α+β) tungsten thin film deposited on a steel substrate by DC magnetron sputtering. The effects of the grain-shape, the crystallographic texture, the porosity and the W〈sub〉β〈/sub〉 volume fraction on the macroscopic elasticity constants were studied. Among all these effects, it was found that the effect of the W〈sub〉β〈/sub〉 volume fraction was the most pronounced. The effects of the crystallographic and morphological texture on the microscopic elastic behavior of the film were evaluated. No dominance of the crystallographic or morphological texture effect was observed and their contributions depend on the crystallographic plane and the measurement direction.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Schematic representation of the methodology used in the determination of the single-crystal elasticity constants.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304082-fx1.jpg" width="265" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): X. Xu, D. Lunt, R. Thomas, R. Prasath Babu, A. Harte, M. Atkinson, J.Q. da Fonseca, M. Preuss〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Metals with a hexagonal close packed structure can deform by several different slip modes with different Critical Resolved Shear Stresses, which provides a great deal of complexity when considering mechanical performance of Mg, Ti and Zr alloys. Hence, an accurate but also statistically meaningful analysis of active slip systems and their contribution to plasticity is of great importance for the understanding of deformation mechanism. In the present study, a correlative scanning electron microscopy-based method of slip trace analysis has been utilised to provide statistical, accurate information of slip behaviour in a weakly textured Ti–6Al–4V alloy with a plastic strain of ∼2%. This is achieved through grain orientation mapping by Electron Backscatter Diffraction and strain mapping by High Resolution Digital Image Correlation. The initial identification of slip mode was performed by comparing the slip trace captured in the high-resolution effective shear strain map with all theoretical slip planes with an angle acceptance criterion of ±5°. Ambiguity in slip mode identification was further resolved using the Relative Displacement Ratio method, which enables the determination of the Burgers vector directly from the displacement data. The correctness of the identified slip modes has been confirmed by detailed dislocation analysis using Bright Field Scanning Transmission Electron Microscopy on thin foils extracted from specific grains employing Focused Ion Beam. This detailed investigation demonstrates the robustness of the slip trace analysis based on grain orientation and high-resolution strain mapping.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930391X-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Mingqiang Li, Bo Wang, Heng-Jui Liu, Yen-Lin Huang, Jingmin Zhang, Xiumei Ma, Kaihui Liu, Dapeng Yu, Ying-Hao Chu, Long-Qing Chen, Peng Gao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Reversible switching of non-180° ferroelastic domains that largely alters the local strain distribution enables many electromechanical, electromagnetic and electroacoustic applications. However, in thin films, the ferroelastic domain walls are usually believed to be immobile because of the interface clamping and/or dislocation pinning. Here, using 〈em〉in situ〈/em〉 and aberration-corrected transmission electron microscopy, we directly observe reversible switching of individual 90° domains in dislocation-free PbTiO〈sub〉3〈/sub〉 thin films and uncover the weakened interface clamping effect. We find the tetragonality is suppressed to ∼1.017 while the polarization vectors rotate 45° in the 〈em〉a〈/em〉-domain near the interface. These huge structural distortions at the interface is mainly responsible for the weakened clamping effect and thus the ability to switch ferroelastic domains. The switching is fully reversible (i.e., either electric field or mechanical stress can re-establish the erased domain) regardless of polarization orientation of the 〈em〉c〈/em〉-domain matrix. Phase-field modeling also shows excellent agreement with experimental observations. Our study reveals the mechanism of controllable and reversible ferroelastic domain switching, enabling the design of new actuators, sensors, and electromagnetic devices.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301958-fx1.jpg" width="326" 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): U. Hecht, J. Eiken, S. Akamatsu, S. Bottin-Rousseau〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Solid-solid phase boundary anisotropy is a key factor controlling the selection and evolution of non-faceted eutectic patterns during directional solidification. This is most remarkably observed during the so-called maze-to-lamellar transition. By using serial sectioning, we followed the spatio-temporal evolution of a maze pattern over long times in a large Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Al〈sub〉2〈/sub〉Cu eutectic grain with known crystal orientation of the Al and Al〈sub〉2〈/sub〉Cu phases, hence known crystal orientation relationship (OR). The corresponding phase boundary energy anisotropy (γ-plot) was also known, as being previously estimated from molecular-dynamics computations. The experimental observations reveal the time-scale of the maze-to-lamellar transition and shed light on the processes involved in the gradual alignment of the phase boundaries to one distinct energy minimum which nearly corresponds to one distinct plane from the family 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mrow〉〈mn〉120〈/mn〉〈/mrow〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈/mrow〉〈mrow〉〈mi〉A〈/mi〉〈mi〉l〈/mi〉〈/mrow〉〈/msup〉〈mo〉|〈/mo〉〈mo〉|〈/mo〉〈msup〉〈mrow〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mrow〉〈mn〉110〈/mn〉〈/mrow〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈/mrow〉〈mrow〉〈mi〉A〈/mi〉〈mi〉l〈/mi〉〈mn〉2〈/mn〉〈mi〉C〈/mi〉〈mi〉u〈/mi〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉. This particular plane is selected due to a crystallographic bias induced by a small disorientation of the crystals relative to the perfect OR. The symmetry of the OR is thus slightly broken, which promotes lamellar alignment. Finally, the maze-to-lamellar transition leaves behind a network of fault lines inherited from the phase boundary alignment process. In the maze pattern, the fault lines align along the corners of the Wulff shape, thus allowing us to propose a link between the pattern defects and missing orientations in the Wulff shape.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301879-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 173〈/p〉 〈p〉Author(s): Zian Jia, Lifeng Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Recent researches show that material microstructural designs mimicking biomaterials offer an effective way to produce tougher composites. To reveal the underlying microstructure-toughness relationship, five bioinspired material microstructures are investigated experimentally, including the brick-and-mortar, cross-lamellar, concentric hexagonal, and rotating plywood microstructures. The feature sizes of these microstructures are controlled to be one order smaller than the specimen size, providing better pictures of how crack resistance interacts with heterogeneity. Fracture theories are further used to analyze the toughening mechanisms and find the design criteria. Results show that the rotating plywood structure presents a “J” shaped 〈em〉R〈/em〉-curve, while other structures show “Γ” shaped 〈em〉R〈/em〉-curves. The “J” shaped 〈em〉R〈/em〉-curve gives a larger critical energy release rate and tolerates a longer crack, thus preferable for crack arresting. By contrast, the “Γ” shaped 〈em〉R〈/em〉-curve provides a larger critical failure stress and is beneficial for preventing crack initiation. Moreover, combined experimental results and theoretical analysis suggest that heterogeneity improves toughness by 1) creating stiffness variations to slow down crack propagation and prevent crack penetration and 2) guiding cracks along weak interfaces to promote progressive damage. Our results shed new light on the structure-property relationship that will facilitate the design of tougher and better crack resistant composites.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930254X-fx1.jpg" width="497" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 173〈/p〉 〈p〉Author(s): P. Godard, D. Faurie, T. Sadat, M. Drouet, D. Thiaudière, P.O. Renault〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The dependence on thermal history of the plasticity mechanisms occurring in nanocrystalline gold thin films is evidenced thanks to relaxation tests combined with 〈em〉in-situ〈/em〉 synchrotron X-ray diffraction. The two techniques complement one another. The activation parameters show that the films deform mainly by dislocations and give indications about their mean free paths, whereas the Bragg peak positions, widths and areas bring invaluable information on residual stress as well as on some plasticity properties, like dislocation storage inside the grains or grain rotations. For the demonstration, two sputter-deposited nanocrystalline 50 nm-thin films deposited onto stretchable substrates are studied. It is shown that an as-grown sample (at a homologous temperature of 0.22) presents a stress-assisted annealing, thus decreasing its initial defect density, whereas if a thermal annealing (3 h at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msup〉〈mrow〉〈mn〉200〈/mn〉〈/mrow〉〈mrow〉〈mo〉∘〈/mo〉〈/mrow〉〈/msup〉〈mi〉C〈/mi〉〈/mrow〉〈/math〉, corresponding to a homologous temperature of 0.35) has been applied to the sample before the tensile test, it deforms by conventional plasticity, and the dislocations are not stored inside the grains. These mechanisms lead to different work-hardening properties. This work shows how a moderate annealing can have a profound influence on the mechanical behaviour of these thin films.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302265-fx1.jpg" width="294" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Jaclyn T. Avallone, Thomas J. Nizolek, Benjamin B. Bales, Tresa M. Pollock〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Metallic multilayer systems show promising performance in extreme environments, with high stability of bi-metal interfaces down to nanometer length scales. The creep behavior of bulk, accumulative roll bonded (ARB) Copper-Niobium (Cu-Nb) composites has been studied at 400 °C as a function of layer thickness, ranging from 2 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mi〉μ〈/mi〉〈mi〉m〈/mi〉〈/mrow〉〈/math〉 to 65 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mi〉n〈/mi〉〈mi〉m〈/mi〉〈/mrow〉〈/math〉. Similar to single phase metallic systems, three regimes are observed during creep: transient, steady-state and tertiary. The mechanism controlling minimum creep rate for all conditions tested has a strong dependence on stress, consistent with dislocation-dominated creep. Unlike the conventional effect of grain size on creep resistance, this study reveals that 〈em〉decreasing〈/em〉 length scale 〈em〉increases〈/em〉 creep resistance.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303969-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Reiner Kirchheim〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Transient and steady state electrotransport and diffusion of ions being produced and annihilated by internal reactions are discussed and exemplified for Yttria Stabilized Zirconia (YSZ). These phenomena are important for understanding flash sintering. They will also play a role in solid oxide fuel cell (SOFC) and solid oxide electrolysis cells (SOEC), where current densities may exceed the reaction rates with gases at the porous electrodes. The characteristic time for attaining steady state transport contains two parts, one depending on the length of the sample and one depending on the field strength. This characteristic time is derived in this study for a linear increase of temperature for the first time. By assuming that the characteristic time is a measure of the onset of flash sintering, yields – without fitting parameters – incubation times or onset temperatures of flash sintering in good agreement with experimental results for YSZ. Thus concentration changes building up during the incubation period are driving forces for internal reactions generating or consuming holes and electrons. A concomitant increase of conductivity leads to Joule heating which further accelerates reaction rates and ion mobility.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930401X-fx1.jpg" width="258" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 20 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Ying Liu, Junhai Xia, Peter Finkel, Scott D. Moss, Xiaozhou Liao, Julie M. Cairney〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This paper reports on the real-time observation of mechanical stress-induced micro and nano domain evolution in a single crystal relaxor ferroelectric [011] poled 24PIN-PMN-PT “3-2 mode” nano-sized lamella. Mechanical loading in the [100] direction was applied to lamella in situ within a transmission electron microscope, with a [0〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉] viewing direction selected for the recording of real-time videos of the domain evolution within the lamella. The observed dominant behavior under loading is a reversible response composed of the movement of microdomains and disappearance of the nanodomains. Changes in the selected area diffraction pattern down the [0〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉1〈/mn〉〈/mrow〉〈/math〉] zone axis are reported, and provide evidence of a significant increase in crystal symmetry under compression. A qualitative insight into the behaviour of the lamella’s mechanical response is obtained via predictions of the non-uniform stress distribution within the lamella found using simple finite element analysis. The correlation between the observed morphology changes, diffraction pattern changes, and the known mechanical stress induced polydomain-rhombohedral to monodomain-orthorhombic phase transition of bulk [011] poled 24PIN-PMN-PT “3-2 mode” single crystal is discussed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303908-fx1.jpg" width="292" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 18 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Y.F. Zhang, S. Xue, Q. Li, Jin Li, Jie Ding, T.J. Niu, R. Su, H.Wang, X. Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mechanical behavior of metallic multilayers has been intensively investigated. Here we report on the study of magnetron-sputtered highly textured Al/Ti multilayer films with various individual layer thicknesses (h = 1 - 90 nm). The hardness of Al/Ti multilayers increases monotonically with decreasing layer thickness without softening and exceeds 7 GPa, making it one of the strongest light-weight multilayer systems reported to date. High resolution transmission electron microscopy and X-ray diffraction pole figure analyses confirm the formation of high-density nanotwins and 9R phase in Al layers. The density of nanotwins and stacking faults scales inversely with individual layer thickness. In addition, there is an HCP-to-FCC phase transformation of Ti when h ≤ 4.5 nm. The high strength of Al/Ti multilayers primarily originates from incoherent interface, high-density twin boundaries, as well as stacking faults.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303957-fx1.jpg" width="200" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Luis M. Moreno-Ramírez, Carlos Romero-Muñiz, Jia Yan Law, Victorino Franco, Alejandro Conde, Iliya A. Radulov, Fernando Maccari, Konstantin P. Skokov, Oliver Gutfleisch〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Materials with a large magnetocaloric response require a large magnetic moment. However, we show in this paper that it is possible to retain both the isothermal entropy change and the adiabatic temperature change even using dopants that reduce the magnetic moment of the parent alloy, provided that the first order character of the transition is enhanced. In this work, a combination of first-principles calculations, experimental determination of the magnetocaloric response (direct and indirect) as well as a new criterion to determine the order of the phase transition are applied to Cr-doped La(Fe,Si)〈sub〉13〈/sub〉 compounds. Despite a reduction in magnetic moment, the magnetocaloric response is retained up to x ≈ 0.3 in LaFe〈sub〉11.6-x〈/sub〉Cr〈sub〉x〈/sub〉Si〈sub〉1.4〈/sub〉. Unlike other transition metal dopants, Cr occupy 8b sites and couple antiferromagnetically to Fe atoms. The cross-over of first to second order transition is achieved for a Cr content of x = 0.53, larger in comparison to other dopants (e.g. Ni or Mn). A direct relation between the first order character and the hysteresis is observed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303891-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): Siyang Wang, Finn Giuliani, T. Ben Britton〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Microstructure and crystallography of δ phase hydrides in as-received fine grain and ‘blocky alpha’ large grain Zircaloy-4 (average grain size ∼11 μm and 〉200 μm, respectively) were examined using electron backscatter diffraction (EBSD). Results suggest that the matrix-hydride orientation relationship is 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mn〉0001〈/mn〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈mrow〉〈mi〉α〈/mi〉〈/mrow〉〈/msub〉〈mo〉|〈/mo〉〈mo〉|〈/mo〉〈msub〉〈mrow〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈/mrow〉〈mrow〉〈mi〉δ〈/mi〉〈/mrow〉〈/msub〉〈mo〉;〈/mo〉〈mo〉〈〈/mo〉〈mn〉11〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉2〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉0〈/mn〉〈msub〉〈mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈mrow〉〈mi〉α〈/mi〉〈/mrow〉〈/msub〉〈mo〉|〈/mo〉〈mo〉|〈/mo〉〈mo〉〈〈/mo〉〈mn〉110〈/mn〉〈msub〉〈mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈mrow〉〈mi〉δ〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mo stretchy="true"〉{〈/mo〉〈mn〉10〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉7〈/mn〉〈mo stretchy="true"〉}〈/mo〉〈/mrow〉〈/math〉 of the surrounding matrix. The morphology of intergranular hydrides can vary depending upon the angle between the grain boundary and the hydride habit plane. The misfit strain between α-Zr and δ-hydride is accommodated mainly by high density of dislocations and twin structures in the hydrides, and a mechanism of twin formation in the hydrides has been proposed. The growth of hydrides across grain boundaries is achieved through an auto-catalytic manner similar to the growth pattern of intragranular hydrides. Easy collective shear along 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"〉〈mrow〉〈mo〉〈〈/mo〉〈mn〉1〈/mn〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈/mrow〉〈mn〉00〈/mn〉〈mo〉〉〈/mo〉〈/mrow〉〈/math〉 makes it possible for hydride nucleation at any grain boundaries, while the process seems to favour grain boundaries with low (〈40°) and high (〉80°) 〈em〉c〈/em〉-axis misorientation angles. Moreover, the angle between the grain boundary and the adjacent basal planes does not influence the propensity for hydride nucleation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301284-fx1.jpg" width="361" 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): Guodong Li, Jiangang He, Qi An, Sergey I. Morozov, Shiqiang Hao, Pengcheng Zhai, Qingjie Zhang, William A. Goddard, G. Jeffrey Snyder〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Tuning phonon transport to reduce the lattice thermal conductivity (〈em〉κ〈/em〉〈sub〉L〈/sub〉) is crucial for advancing thermoelectrics (TEs). Traditional strategies on 〈em〉κ〈/em〉〈sub〉L〈/sub〉 reduction focus on introducing scattering sources such as point defects, dislocations, and grain boundaries, that may degrade the electrical conductivity and Seebeck coefficient. We suggest here, a novel twin boundary (TB) strategy that can decrease the 〈em〉κ〈/em〉〈sub〉L〈/sub〉 of Mg〈sub〉2〈/sub〉Si by ∼90%, but which may not degrade the electrical properties significantly. We validate this suggestion using density functional theory (DFT). We attribute the mechanism of TB induced 〈em〉κ〈/em〉〈sub〉L〈/sub〉 reduction to (i) the lower phonon velocities and larger Grüneisen parameter, (ii) “rattling” of the Mg〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Mg pair induced soft acoustic and optical modes, (iii) shorter phonon lifetime and higher phonon scattering rate. We predict that the size of nanotwinned structure should be controlled between 3 nm and 100 nm in the Mg〈sub〉2〈/sub〉Si matrix for the most effective 〈em〉κ〈/em〉〈sub〉L〈/sub〉 reduction. These results should be applicable for other TE or non TE energy materials with desired low thermal conductivity, suggesting rational designs of high-performance Mg〈sub〉2〈/sub〉Si TE materials with low 〈em〉κ〈/em〉〈sub〉L〈/sub〉 for the energy conversion applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301272-fx1.jpg" width="271" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Xiaolong Ma, Suhas Eswarappa Prameela, Peng Yi, Matthew Fernandez, Nicholas M. Krywopusk, Laszlo J. Kecskes, Tomoko Sano, Michael L. Falk, Timothy P. Weihs〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Precipitation of fine intermetallic particles during conventional thermal aging can significantly enhance the mechanical properties of Al alloys. However, this method offers only limited strengthening in Mg alloys as thermal aging usually leads to intermetallic particles that are too coarse in size and too sparse in spacing. Dynamic precipitation during low-temperature deformation processing offers a chance to rectify this limitation. In addition, mechanical processing often drives precipitation and recrystallization concurrently, therefore, a careful analysis of their interaction and interdependent thermodynamic driving forces is needed. Herein, we investigate dynamic precipitation and recrystallization in a coarse-grained, fully solutionized Mg-9wt.%Al alloy following low-temperature Equal Channel Angular Extrusion (ECAE) using electron microscopy, theoretical calculations, and mechanical property evaluations. Through comparisons with conventionally aged samples, we find that dynamic precipitation during extrusion produces continuous, nanoscale Mg〈sub〉17〈/sub〉Al〈sub〉12〈/sub〉 particles within grain interiors with a high number density and a low aspect ratio due to strain-induced, defect-assisted nucleation. We quantitatively analyzed the dislocation-accelerated nucleation rate, and the excess vacancy concentration in comparison to the reports in Al alloys. We also find a combined set of reactions that includes discontinuous precipitation and recrystallization along grain boundaries due to the extrusion process. The volume fraction of the combined-reaction region that contains submicron Mg grains and submicron intergranular Mg〈sub〉17〈/sub〉Al〈sub〉12〈/sub〉 particles grows as the number of passes increases. Using a thermodynamic analysis, we estimate the individual and combined driving forces for precipitation and recrystallization processes at grain boundaries. We identify the chemical energy of the supersaturated Mg matrix as a major driving force for the combined reactions, which indirectly promotes recrystallization and the formation of submicron Mg grains. Our results offer key insights into the evolution of microstructure during dynamic precipitation and recrystallization, and thus provide guidance for the design of improved microstructures in Mg alloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302484-fx1.jpg" width="476" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Christian Huber, Hossein Sepehri-Amin, Michael Goertler, Martin Groenefeld, Iulian Teliban, Kazuhiro Hono, Dieter Suess〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Laser powder bed fusion is a well-established additive manufacturing method that can be used for the production of net-shaped Nd-Fe-B magnets. However, low coercivity has been one of the drawbacks in the laser powder bed fusion processed Nd-Fe-B magnets. In this work, we have demonstrated that the grain boundary diffusion process using low-melting Nd-Cu, Nd-Al-Ni-Cu, and Nd-Tb-Cu alloys to the selective laser sintered NdFeB magnets can results in a substantial enhancement of coercivity from 0.65 T to 1.5 T. Detailed microstructure investigations clarified that the formation of Nd-rich grain boundary phase, the introduction of Tb-rich shell at the surface of Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B grains, and maintaining the grain size in nano-scale are responsible for the large coercivity enhancement.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302393-fx1.jpg" width="293" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Hao-Wen Dong, Sheng-Dong Zhao, Peijun Wei, Li Cheng, Yue-Sheng Wang, Chuanzeng Zhang〈/p〉 〈div xml:lang="en"〉〈div〉〈p〉Double-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. However, the systematic design and realization of broadband double-negative AMMs are stilling missing, which hinder their practical implementations. In this paper, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework involving different microstructure symmetries, minimal structural feature sizes and dispersion extents of effective parameters. The optimization framework is applied to conceive the heuristic resonance-cavity-based and space-coiling metamaterials with broadband double negativity. Meanwhile, we demonstrate the essences of double negativity derived from the novel artificial multipolar LC (inductor-capacitor circuit) and Mie resonances which can be induced by controlling mechanisms in optimization. Furthermore, abundant numerical simulations validate the corresponding double negativity, negative refraction, enhancement of evanescent waves and subwavelengh imaging. Finally, we experimentally show the desired broadband subwavelengh imaging by using the 3D-printed optimized space-coiling metamaterial. The present design methodology provides an ideal approach for constructing the constituent “atoms” of metamaterials according to any artificial physical and structural requirements. In addition, the optimized broadband AMMs and superlens lay the structural foundations of subwavelengh imaging technology.〈/p〉〈/div〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302447-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): J.J. Bhattacharyya, T.T. Sasaki, T. Nakata, K. Hono, S. Kamado, S.R. Agnew〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Microalloyed, age-hardenable Mg alloys can exhibit an excellent balance between strength and ductility. One such alloy, AXM10304 (Mg-1.31Al-0.33Ca-0.46Mn in wt. %), possesses high extrudability with mechanical properties comparable to 6xxx-series Al alloys at considerably lower density. The mechanical properties are due to the presence of a high number density of disc-shaped GP zones parallel to the basal planes. The present work focuses on providing a mechanistic understanding of the effect of these GP zones, on both basal and prismatic 〈a〉 slip. Using crystal plasticity simulations in conjunction with analytical strength modeling, the anisotropy in shear resistance of disc-shaped GP zones is quantitatively evaluated for the first time. It is shown that the passage of dislocations parallel to the zone (basal slip) experience a lower resistance compared to the case when the shearing occurs perpendicular to the zone (prismatic slip). This is not simply a geometrical effect; in fact, the geometry of basal discs favors the strengthening of basal slip when dislocations are not required to bow around the obstacles. Rather, it is hypothesized that the GP zone coherency strain fields contribute less resistance to basal slip as compared to prismatic slip. Furthermore, the possibility of a chemical effect is only present for the prismatic. Despite the fact that the harder mode (prismatic slip) is strengthened more than the soft, it is shown that the net strengthening effects still cause a decrease in the overall anisotropy of the individual grains, which may help to explain why the ductility is good, despite the high strength.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930237X-fx1.jpg" width="276" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 20 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): T.-H. Kim, T.T. Sasaki, T. Ohkubo, Y. Takada, A. Kato, Y. Kaneko, K. Hono〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dy distributions in Dy-free and Dy-containing Nd-Fe-B sintered magnets in the course of the Dy-vapor grain boundary diffusion (GBD) process have been investigated in order to understand the origin of the high coercivity of 3.0 T that is reachable only when the initial magnet is alloyed with Dy. We have discovered the formation of a secondary Dy-rich shell within the well-known primary Dy-rich shell, which is a key contributor to the 3.0 T coercivity. The coercivity increment of the Dy-containing magnet after the GBD treatment was only 0.08 T, much lower than 0.87 T for the Dy-free magnet; however, it was substantially enhanced by a post-diffusion annealing. Compared to the Dy-free magnet, a larger amount of Dy atoms were diffused from the Nd-rich grain boundary (GB) phase to the primary Dy-rich shell in the Dy-containing magnet after the annealing, resulting in the formation of a secondary Dy-rich shell with a higher Dy-concentration at the GB phase/secondary shell interfaces.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302344-fx1.jpg" width="265" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Congbing Tan, Jun Ouyang, Xiangli Zhong, Jinbin Wang, Min Liao, Lunjun Gong, Chuanlai Ren, Gaokuo Zhong, Shuaizhi Zheng, Hongxia Guo, Yichun Zhou〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hierarchical polydomain nanostructures, usually formed in epitaxial films, serve to fully relax the misfits between the film and substrate, as well as the strains between the poly-twin variants of the film material. The minimization of the elastic energy endows this type of structure superior stability over other domain architectures. However, the existence of energetically degenerate polytwin variants and their intersecting domain boundaries often led to undesirable functional properties in non-engineered films. Here we show that such nanostructures in epitaxial perovskite ferroelectric films can be crystallographically engineered to eliminate intersecting domain boundaries, thereby achieving an improved ferroelectric performance. In addition to a reduced coercive field and increased dielectric and piezoelectric responses, this type of ferroelectric polydomain nanostructure shows an enhanced ferroelectric polarization with a substantially improved fatigue resistance, as opposed to non-engineered nanostructures with intersecting domain boundaries. These findings suggest an alternative route for the fabrication of highly-reliable nonvolatile ferroelectric memory devices, and may find broader applications in piezoelectrics and energy storage.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302319-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 172〈/p〉 〈p〉Author(s): R. Dai, A.K. Gangopadhyay, R.J. Chang, K.F. Kelton〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The glass transition temperature, 〈em〉T〈/em〉〈sub〉g〈/sub〉, is important for predicting glass formation and stability and for designing processing steps for tailoring the glass to specific applications. It is conventionally determined from measurements of the shear viscosity or specific heat. This requires that the glass first be made, significantly limiting the use of this parameter for the prediction of glass formation. Further, viscosity or specific heat measurements can only be made for glasses that have a high stability against crystallization at 〈em〉T〈/em〉〈sub〉g〈/sub〉. It is demonstrated here that it is possible to accurately predict 〈em〉T〈/em〉〈sub〉g〈/sub〉 from measurements of the shear viscosity and thermal expansion coefficient of the high temperature equilibrium liquid. In addition to the practical usefulness of this, the connection between the glass transition and the properties of the equilibrium liquid may shed light into the processes that determine glass formation in metallic alloys and of the nature of the glass transition.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302368-fx1.jpg" width="261" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 28 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): D.C. Johnson, B. Kuhr, D. Farkas, G.S. Was〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Localized deformation has emerged as a key factor in the crack initiation process for irradiated steels, as cracks are observed to nucleate preferentially at these sites. Using high resolution electron backscatter diffraction (HREBSD), the local stress tensor surrounding the dislocation channel-grain boundary interaction sites was quantified and coupled with fully determined grain boundary plane orientation information to determine, for the first time, the relationship between grain boundary normal stress and intergranular crack initiation in irradiated austenitic stainless steel. A Fe-13Cr-15Ni alloy was strained in simulated boiling water reactor, normal water chemistry after quantifying the residual stress tensor at discontinuous dislocation channel – grain boundary interaction sites where grain boundaries were determined to be well oriented with respect to the loading axis. Local stresses at the grain boundary were observed to reach magnitudes greater than 1.5 GPa at a distance of 200 nm from the intersection between the dislocation channel and the grain boundary. A pseudo-threshold stress of 0.9 GPa was measured, below which no cracking was observed. As the stress acting normal to the grain boundary increased above this value, the susceptibility to cracking increased with the cracking fraction reaching 100% at the high end of the stress range. This study shows for the first time that not only does intersection between discontinuous dislocation channels and grain boundaries result in peak local stresses, but the magnitude of the local tensile stress drives the crack initiation process.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301181-fx1.jpg" width="496" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Josh Kacher, Julian E. Sabisch, Andrew M. Minor〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Using electron backscatter diffraction, over 1,000 grain boundary/twin interactions were investigated in pure Re after uniaxial compression. Crystallographic factors such as grain boundary disorientation angle, displacement gradient tensor accommodation, and twin plane and shear vector alignment were taken into account as well as the macroscopic Schmid factor. It was found that the crystallographic relationship between coincident twin pairs at grain boundaries fall into two categories. In the first category, the twin pairs satisfied two criteria: the twin plane alignment was maximized across the boundary and the twin variant was kept constant across the boundary. In the second category, comprising approximately 5% of the characterized interactions, twin variant selection appeared to be driven by the macroscopically applied stress state, as resolved by the Schmid factor. In comparison to published results on twin/grain boundary interactions in Mg and Zr, it was found that twins transmit across grain boundaries in Re far more readily, regardless of the grain boundary disorientation angle. This is likely to be a function of the anomalously low twin boundary energy in Re as well as the difference in magnitude of the twinning-induced deformation that must be accommodated at the boundary, with the {11〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉2〈/mn〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉1}〈11〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉26〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〉 system being dominant in Re and the {10〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mn〉1〈/mn〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉2}〈10〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.gif" overflow="scroll"〉〈mrow〉〈mover accent="true"〉〈mrow〉〈mn〉11〈/mn〉〈/mrow〉〈mo stretchy="true"〉¯〈/mo〉〈/mover〉〈/mrow〉〈/math〉〉 being dominant in Mg and Zr.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302538-fx1.jpg" width="376" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): H.J. Kong, C. Xu, C.C. Bu, C. Da, J.H. Luan, Z.B. Jiao, G. Chen, C.T. Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Aging treatments at 400–550˚C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300–600˚C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittlement regime can improve the impact toughness but at a great expense of strength. In this work, the strengthening mechanisms as well as the toughening of a low cost weldable HSLA steel with a low content of carbon (C ∼0.08 wt.%), nickel (Ni =0.78 wt.%), and copper (Cu =1.3 wt.%) were carefully investigated. Our findings show that the low-C-Ni-Cu HSLA steel is insensitive to the aging temperatures and can achieve a yield strength (YS) and ultimate tensile strength (UTS) over 1000 and 1100 MPa, respectively, with tensile ductility 〉10% (reduction of area 〉60%) at a heat-treat temperature of 640˚C through multiple strengthening mechanisms. Besides, a good low-temperature (-40˚C) impact performance (∼200 J) with high YS (∼900 MPa) and UTS (∼1000 MPa) can be obtained by seeking a strength balance among the fine grain size (∼2.5 μm), medium-sized (∼14 nm) overaged Cu-rich precipitates, tempered martensite, and fresh martensite (or carbides). Moreover, a relatively lower YS (∼800 MPa) and UTS (∼900 MPa) useful for steel manufacturing can be attained by a prolonged aging at 640˚C. In addition, the dislocation-precipitate interactions were also explored based on the dislocation theories in this study.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302435-fx1.jpg" width="291" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Lei Ding, Dmitry D. Khalyavin, Pascal Manuel, Joseph Blake, Fabio Orlandi, Wei Yi, Alexei A. Belik〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in 〈em〉half-metallic ferrimagnets〈/em〉 such as the known case Sr〈sub〉2〈/sub〉FeMoO〈sub〉6〈/sub〉 where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material−Tl〈sub〉2〈/sub〉NiMnO〈sub〉6〈/sub〉, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis−showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the material is a 〈em〉ferromagnetic insulator〈/em〉. Hence the ordered Tl〈sub〉2〈/sub〉NiMnO〈sub〉6〈/sub〉 (∼70% of Ni〈sup〉2+〈/sup〉/Mn〈sup〉4+〈/sup〉 cation ordering) represents the first realization of a 〈em〉ferromagnetic〈/em〉 insulating double perovskite, showing CMR. The study of the relationship between structure and magnetic properties allows us to clarify the nature of spin glass behaviour in the disordered Tl〈sub〉2〈/sub〉NiMnO〈sub〉6〈/sub〉 (∼31% of cation ordering), which is related to the clustering of antisite defects and associated with the short-range spin correlations. Our results highlight the key role of the cation ordering in establishing the long range magnetic ground state and lay out new avenues to exploit advanced magnetic materials in double perovskites.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302460-fx1.jpg" width="268" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Hanliang Zhu, Mengjun Qin, Robert Aughterson, Tao Wei, Gregory Lumpkin, Yan Ma, Huijun Li〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The irradiation microstructure of a titanium aluminide (TiAl) alloy subjected to in situ transmission electron microscope (TEM) irradiation with 1 MeV Kr ions at the elevated temperature of 873K was investigated. Triangle and large hexagon shaped volume defects were observed within the γ-TiAl phase in the TEM images of the irradiated microstructure. High resolution TEM images and composition analyses revealed that the volume defects were vacancy-type stacking fault tetrahedra (SFTs). Molecular dynamic simulations showed that the increased diffusion coefficient at the elevated temperature promoted the movement and aggregation of vacancies, leading to the formation and growth of SFTs in the irradiated FCC γ phase. The lamellar interfaces in the irradiation microstructure were more effective for acting as strong sinks to absorb the primary point defects and defect clusters at the elevated temperature. The initial defects at the interfaces of the TiAl alloy enhanced the sink strength of the material and played an important role in refining SFTs near the lamellar interfaces.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302459-fx1.jpg" width="334" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Fernando L. Reyes Tirado, Spencer Taylor, David C. Dunand〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The recently-discovered metastable γ’-Co〈sub〉3〈/sub〉(Ta〈sub〉0.76〈/sub〉V〈sub〉0.24〈/sub〉) phase formed on aging in a Co-6Ta-6V (at.%) ternary alloy is stabilized partial replacement of Ta and V with Al and Ti. In two alloys with composition Co-6Al-3Ta-3V and Co-5Al-3Ta-3V-1Ti with γ+γ’ microstructure, the γ’-precipitates remain stable for up to 168 h at 850 and 900 °C, with no precipitation of additional phases. Adding Ni and B and doubling the Ti concentration produces a γ/γ’ superalloy, Co-10Ni-5Al-3Ta-3V-2Ti-0.04B (at.%), with γ’ precipitates which are stable up to 6 weeks of aging at 850 °C, while slowly coarsening and coalescing from cuboidal to elongated shapes. After 1 day of aging at 850 °C, the γ’ nanoprecipitates have (Co〈sub〉0.87〈/sub〉Ni〈sub〉0.17〈/sub〉)〈sub〉3〈/sub〉(Ta〈sub〉0.42〈/sub〉Al〈sub〉0.23〈/sub〉Ti〈sub〉0.19〈/sub〉V〈sub〉0.15〈/sub〉B〈sub〉0.01〈/sub〉) composition, with Al and Ti replacing at the same rate both Ta and V in the original metastable Co〈sub〉3〈/sub〉(Ta〈sub〉0.76〈/sub〉V〈sub〉0.24〈/sub〉) phase. To improve oxidation resistance, 4% Cr is added to the new superalloy, resulting in a somewhat higher volume fraction of finer cuboidal γ’ precipitates after one week of aging at 850 ºC, but no other deleterious phases. These W- and Mo-free γ/γ’ superalloys show good creep resistance at 850 °C, on par with two other recent Co-base γ/γ’ superalloys: (i) Co-9W-9Al-8Cr (at.%) which has higher density due to its high W content, and (ii) Co-30Ni-10Al-5Mo-2Nb (at.%) which has lower density (as it is W-free) but contains triple the Ni concentration.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302332-fx1.jpg" width="466" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Rasool Ahmad, Binglun Yin, Zhaoxuan Wu, W.A. Curtin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The thermally activated pyramidal-to-basal (PB) transition of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mo〉〈〈/mo〉〈mi mathvariant="bold-italic"〉c〈/mi〉〈mo〉+〈/mo〉〈mi mathvariant="bold-italic"〉a〈/mi〉〈mo〉〉〈/mo〉〈/mrow〉〈/math〉 dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mo〉〈〈/mo〉〈mi mathvariant="bold-italic"〉c〈/mi〉〈mo〉+〈/mo〉〈mi mathvariant="bold-italic"〉a〈/mi〉〈mo〉〉〈/mo〉〈/mrow〉〈/math〉 dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302198-fx1.jpg" width="291" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): D. Simon, H. Wuest, S. Hinderberger, T. Koehler, A. Marusczyk, S. Sawatzki, L.V.B. Diop, K. Skokov, F. Maccari, A. Senyshyn, H. Ehrenberg, O. Gutfleisch〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉In order to find a promising trade-off permanent magnet material regarding a performance/cost-ratio, the Ce〈sub〉1−x〈/sub〉Sm〈sub〉x〈/sub〉Fe〈sub〉11−y〈/sub〉Ti〈sub〉1〈/sub〉V〈sub〉y〈/sub〉-phase (x=0–1; y=0, 1) is analyzed in detail. In the first part, its existence range is studied (1000〈sup〉∘〈/sup〉C) and the intrinsic magnetic properties are comprehensively determined. Diffraction experiments localize both structure-stabilizing transition metals on 8i-sites, explaining the measured reduction in saturation polarization as V is added. Curie temperatures increase upon SM-substitution with a negligible dependence on V. Annealings of nanocrystalline material produced via intensive milling and melt-spinning show that V especially raises the obtainable maximum coercivities for SM-rich phases (924 kA/m).〈/p〉 〈p〉In the second part, the promising magnetic properties of the nanocrystalline material are successfully transferred to the bulk state via hot-pressing. The isotropic Ce〈sub〉0.5〈/sub〉Sm〈sub〉0.5〈/sub〉Fe〈sub〉10〈/sub〉Ti〈sub〉1〈/sub〉V〈sub〉1〈/sub〉-magnet (coercivity = 425 kA/m) is characterized by various means. Magnetic measurements, structural investigations and calculations of the elastic constants consider necessary factors for a successful texturing by die-upsetting (as accepted for Nd-Fe-B). The results are fundamental for further considerations in this active field of research.〈/p〉 〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301983-fx1.jpg" width="280" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 23 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Germán Martínez-Ayuso, Michael I. Friswell, Hamed Haddad Khodaparast, James I. Roscow, Christopher R. Bowen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉High piezoelectric coupling coefficients enable the harvesting of more energy or increase the sensitivity of sensors which work using the principle of piezoelectricity. These coefficients depend on the material properties, but the manufacturing process can have a significant impact on the resulting overall coefficients. During the manufacturing process, one of the main steps is the process of polarization where a poling electric field aligns the ferroelectric domains in a similar direction in order to create a transversely isotropic material able to generate electric fields or deformations. The degree of polarization depends on multiple factors and it can strongly influence the final piezoelectric coefficients. In this paper, a study on the electric field distribution on the sensitivity of the main piezoelectric and dielectric coefficients to the polarization process is performed, focusing on porous piezoelectric materials. Different inclusion geometries are considered, namely spherical, ellipsoidal and spheres with cracks. The electric field distribution at the micro scale within a representative volume element is modelled to determine the material polarization level using the finite element method. The results show that the electric field distribution is highly dependent on the inclusion geometries and cracks and it has a noticeable impact on the equivalent piezoelectric coefficients. These results are compared with experimental measurements from published literature. Good agreement is found between the ellipsoidal model and the experimental data.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Jia-Hong Ke, George A. Young, Julie D. Tucker〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The development of long range order in nickel-chromium alloys is of great technological interest but the kinetics and mechanisms of the transformation are poorly understood. The present research utilizes a combined computational and experimental approach to elucidate the mechanism by which phosphorus accelerates the ordering rate of stoichiometric Ni〈sub〉2〈/sub〉Cr in Ni-Cr alloys. A series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with phosphorus concentrations, x = 〈0.005-0.1 at.% and aged between 373 and 470°C for times up to 3000 h. The first-principles modeling considers fcc Ni with dilute P as a reasonable approximation for the complex Ni-Cr-P alloy system. Calculation results show a pronounced enhancement of vacancy transport by vacancy-solute pair diffusion via consecutive exchange and rotation jumps of vacancies associated with the phosphorus atom. The energy barriers of these two migration paths are at least 0.35 eV lower than that of vacancy-atom exchange in pure Ni solvent. The analytical diffusion model predicts enhanced solvent diffusion by 2 orders of magnitude for 0.1 at.% P at 400-500°C. The model prediction is in good agreement with the evolution of micro-hardness. We characterize the micro-hardness result by a kinetic ordering model, showing a significant decrease of the activation energy of ordering transformation. These results help gauge the risk of industrial alloys developing long range order which increases strength but degrades ductility and toughness. Specifically, minor alloying additions that bind with excess vacancies and lower the vacancy migration barrier can greatly accelerate hardening via Ni〈sub〉2〈/sub〉Cr precipitation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302381-fx1.jpg" width="282" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): Ye-Chuan Xu, Chengchao Hu, Liwang Liu, Jian Wang, Wei-Feng Rao, John W. Morris, Armen G. Khachaturyan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A vast amount of physically-different materials near their structural phase transitions have been widely used due to their rich and extraordinary properties, e.g., superelasticity, elastic softening and invar/elinvar effects. These pre-transitional materials are known to have complex microstructures consisting of stress-generating defects such as dislocations and coherent nano-precipitates. However, effects of such defects on properties have not been well understood, which hinders fully exploiting the potential applications of these materials. In this paper we investigated a nano-embryonic mechanism in a generic case of pre-transitional materials with stress-generating defects at temperatures close but above the starting temperature of phase transformation, 〈em〉M〈/em〉〈sub〉〈em〉s〈/em〉〈/sub〉. We demonstrated that the stress concentration generated by defects could induce localized displacive phase transformation near defects, producing equilibrium nano-size embryos of orientation variants of the product phase. The obtained mixed state consisting of nano embryos is in a thermoelastic equilibrium in which the total volume and sizes of embryos are equilibrium internal thermodynamic parameters. The subsequent imposition of an applied stress causes these embryos to grow, generating superelastic responses with an increasing applied field. If the defects are stationary the growth maintains thermoelastic equilibrium, and is, hence, fully reversible and anhysteretic. Moreover, cooling toward the 〈em〉M〈/em〉〈sub〉〈em〉s〈/em〉〈/sub〉 also causes embryo growth resulting in a diffuse phase transformation, which increases the volume and softens the modulus. These effects counteract the thermal contraction and modulus increase in the untransformed matrix, and may explain the invar and elinvar affects in alloys with low-temperature displacive transformations.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302290-fx1.jpg" width="274" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 168〈/p〉 〈p〉Author(s): Matias Acosta, Nikola Novak, Wook Jo, Jürgen Rödel〈/p〉

Description: 〈p〉Publication date: Available online 27 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Darko Makovec, Matej Komelj, Goran Dražić, Blaž Belec, Tanja Goršak, Sašo Gyergyek, Darja Lisjak〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this investigation we analyze an unprecedented difference in the behavior of nanoparticles when compared to the corresponding bulk. We have found that a chemical substitution can have the opposite effect on the magnetic properties of nanoparticles compared to the bulk, as revealed for the first time in the case of Sc-substituted barium-hexaferrite nanoplatelets. Even though the Sc substitution is known to greatly decrease the saturation magnetization, 〈em〉M〈/em〉〈sub〉S〈/sub〉, of the bulk barium hexaferrite, it showed the opposite effect for nanoplatelets. The 〈em〉M〈/em〉〈sub〉S〈/sub〉 values of the nanoplatelets (in average 50 nm wide and approximately 3 nm thick) increased to over 38 Am〈sup〉2〈/sup〉/kg, compared to ∼16 Am〈sup〉2〈/sup〉/kg for unsubstituted nanoplatelets of comparable average size. The Sc incorporation was investigated with a combination of atomic-resolution imaging and elemental mappings in a scanning-transmission electron microscope. As in the bulk, the Sc〈sup〉3+〈/sup〉 ions showed a clear preference for incorporation into an R block of the hexaferrite SRS〈sup〉∗〈/sup〉R〈sup〉∗〈/sup〉 structure for the nanoplatelets (R and S represent a hexagonal (BaFe〈sub〉6〈/sub〉O〈sub〉11〈/sub〉)〈sup〉2-〈/sup〉 and a cubic (Fe〈sub〉6〈/sub〉O〈sub〉8〈/sub〉)〈sup〉2+〈/sup〉 structural block, respectively). A clear difference between the nano and the bulk observed for the first time was in the partial substitution of the Sc〈sup〉3+〈/sup〉 for the Ba〈sup〉2+〈/sup〉 in the nanoplatelets; however, this cannot explain the large increase in 〈em〉M〈/em〉〈sub〉S〈/sub〉. 〈em〉Ab-initio〈/em〉 calculations suggest that the opposite effect of the Sc substitution in the nanoplatelets to that in the bulk can be ascribed to specific, two-dimensional magnetic ordering in the platelets.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302526-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): A. Brian Aebersold, Lorenzo Fanni, Aïcha Hessler-Wyser, Sylvain Nicolay, Christophe Ballif, Cécile Hébert, Duncan T.L. Alexander〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The grain size evolution of polycrystalline thin films, which form by competitive grain overgrowth as commonly interpreted using the van der Drift model, is understood to follow a power-law scaling of the average grain size 〈em〉d〈/em〉 with film thickness 〈em〉h〈/em〉, i.e. 〈em〉d〈/em〉 ∝ 〈em〉h〈/em〉〈sup〉〈em〉α〈/em〉〈/sup〉. While simulations have identified a growth exponent 〈em〉α〈/em〉 = 0.4 for three-dimensional growth, previous experimental studies did not confirm this value, instead finding 〈em〉α〈/em〉 values in the range of ∼0.5–0.7. Here we study competitive grain overgrowth using a system of ZnO thin films grown by low-pressure metal–organic chemical vapor deposition. We present quantitative data on the evolution of grain size and orientation across the thickness of thin films, obtained by automated crystal orientation mapping of “double-wedge” transmission electron microscopy samples. The data from 〈em〉a〈/em〉-textured ZnO films, grown under three different conditions, are compared against van der Drift model predictions of self-similarity of the grain size distribution and the power-law scaling. The results are further interpreted by comparing to simulations of facetted polycrystalline film growth, which we adapt to the ZnO system by including idiomorphic growth shapes with a six-fold symmetry and random or biased nuclei orientations. As well as showing the predicted self-similarity of grain size distributions during growth, for the first time our experimental data confirm a power-law growth exponent of 〈em〉α〈/em〉 = 0.4, as also predicted by the simulations using randomly oriented nuclei. Nevertheless, interpretation of this result is contingent on the absence of factors such as textured nucleation and renucleation during film growth. Indeed, only one film, grown at a higher ratio of H〈sub〉2〈/sub〉O/DEZ precursor gases, displaying random initial nucleation, and minimal grain renucleation during growth, shows a proper conformance to the model nature and predictions.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302514-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 24 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Sang-Hyun Yu, Sang-Min Lee, Sukjin Lee, Jae-Hoon Nam, Jae-Seung Lee, Chul-Min Bae, Young-Kook Lee〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The hydrogen embrittlement (HE) and H trapping sites of pearlitic steel specimens with various lamellar spacings (λ) were evaluated through slow strain rate tensile testing and thermal desorption analysis. When λ decreases, both tensile strength and resistance to HE were unusually improved. This is because tearing, which is the initiation of H cracking, was delayed in the specimen with fine λ and short cementite (θ) platelets. Undeformed H-charged specimens showed a peak (peak 1), which is separable into two sub-peaks (peak 1-1 and peak 1-2) in their H desorption rate curves, regardless of λ. Peak 1-1 and peak 1-2 were generated by H atoms detrapped from F〈sub〉P〈/sub〉/θ interfaces and from dislocations inside F〈sub〉P〈/sub〉, respectively. The 〈em〉E〈/em〉〈sub〉〈em〉a〈/em〉〈/sub〉 values of H desorption for peak 1-1 and peak 1-2 were 23.2 kJ/mol, and 26.1 kJ/mol, respectively. Meanwhile, deformed H-charged specimens exhibited the second peak (peak 2) with peak temperature (T〈sub〉P〈/sub〉) of ∼600 K, as well as peak 1 with T〈sub〉P〈/sub〉 of ∼375 K. When tensile strain increased, peak 2 increased at the expense of peak 1. Primary H trapping sites for peak 2 are strained F〈sub〉P〈/sub〉/θ interfaces with interfacial dislocations.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302423-fx1.jpg" width="483" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 171〈/p〉 〈p〉Author(s): S. Hémery, P. Villechaise〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Identification of operating deformation processes and assessment of the resulting strain partitioning are critical concerns for mechanical properties prediction and microstructure optimization in complex alloys such as α/β titanium alloys. Lattice rotation relative to the initial orientation was presently used as a marker of slip activity. A Ti-6Al-4V specimen with a bi-modal microstructure was tested in tension in a scanning electron microscope. Crystallographic orientations were characterized in situ using electron back-scattered diffraction (EBSD). A successful prediction of activated slip systems was achieved using the rotation axis associated with plastic activity. The combination of this procedure and slip traces analysis offers an insight into the determination of both slip plane and slip direction of active slip systems. Based on classical crystal plasticity formulations, the magnitude of the rotation relative to the initial orientation was interpreted in terms of plastic shear magnitude. A quantitative assessment of plastic strain at the microstructure scale was then carried out using lattice rotation data. This approach enabled to discuss strain partitioning in Ti-6Al-4V considering the influence of microstructural features and active slip modes.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302356-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 17 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Julian Buchinger, Nikola Koutná, Zhuo Chen, Zaoli Zhang, Paul Heinz Mayrhofer, David Holec, Matthias Bartosik〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419302307-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 168〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 170〈/p〉 〈p〉Author(s): Dikai Guan, Bradley Wynne, Junheng Gao, Yuhe Huang, W. Mark Rainforth〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Tension twinning nucleation and evolution in Mg WE43 alloy over a large sampling area was investigated using a 〈em〉quasi-in-situ〈/em〉 EBSD/SEM method during interrupted compression testing. The results showed tension twins with both high and low macroscopic Schmid factor (MSF) were activated under a compressive stress of 100 MPa with a strain rate of 10〈sup〉−1〈/sup〉 s〈sup〉−1〈/sup〉. Basal slip in most grains dominated at this stress, so nucleation of twin variants required little interaction with non-basal slip, which was different from other studies that reported prismatic slip and/or tension twinning were required to activate some low MSF tension twin variants. The geometric compatibility factor (m') was demonstrated to be an important factor to determine tension twin variant selection assisted by basal slip. The analysis indicated m' played a critical role over MSF in tension twin variant selection during twin nucleation stage, and final twin variant types were insensitive to increasing stress, but they inherited twin variant types determined at twin nucleation stage. Moreover, which specific grain boundary of a grain with hard orientation for basal slip would nucleate which twin variant could be also validated by m' and largely depended on two factors: (a) high value of m' with 1st or 2nd rank between the tension twinning of nucleated twin variant and basal slip in adjoining grains; and (b) intensive basal slip activity in the neighbouring grains before twin nucleation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301612-fx1.jpg" width="289" 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): F. Wang, A. Inoue, F.L. Kong, S.L. Zhu, E. Shalaan, F. Al-Marzouki, W.J. Botta, C.S. Kiminami, Yu.P. Ivanov, A.L. Greer〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Heating-induced crystallization of high-entropy (HE) (Fe〈sub〉0.25〈/sub〉Co〈sub〉0.25〈/sub〉Ni〈sub〉0.25〈/sub〉Cr〈sub〉0.125〈/sub〉Mo〈sub〉0.125〈/sub〉)〈sub〉86‒89〈/sub〉B〈sub〉11‒14〈/sub〉 amorphous (am) alloys is examined to develop new structural materials with low B contents. The crystallization of 11B alloy occurs in three stages: first nanoscale bcc precipitates form in the amorphous matrix, second nanoscale fcc precipitates form, and the residual amorphous phase disappears in the third stage which yields borides in addition to the bcc and fcc phases. Crystallization of 14B alloy is the same, except that the order of appearance of bcc and fcc is reversed. The bcc and fcc particle diameters are 5–15 nm and remain almost unchanged up to ∼960 K. On annealing, ultrahigh hardness of 1500–1550 (unprecedented for boride-free structures) is attained just before the third crystallization stage. This hardening and the thermal stability of the novel [am + bcc + fcc] structures are remarkable at such low boron content and encouraging for development as ultrahigh-strength alloys. The results are interpreted in terms of the nature and extent of partitioning of elemental components between the bcc/fcc phases and the amorphous matrix, and the size and defect structures of the bcc and fcc precipitates. The magnetic flux density at room temperature increases by precipitation of bcc and decreases by appearance of fcc. Slower quenching of the 11B alloy shows a pseudo-polymorphic crystallization that may be characteristic of multicomponent HE systems.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301624-fx1.jpg" width="448" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 17 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Fei Wang, Patrick Altschuh, Alexander M. Matz, Johann Heimann, Bettina S. Matz, Britta Nestler, Norbert Jost〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Magnesium silicide has been widely exploited in thermoelectric and photovoltaic devices. In contrast to the fabrication of the magnesium silicide phase on flat Si-substrates in literature, we here concentrate on the growth of the Mg〈sub〉2〈/sub〉Si phase on the surface of Si-foams by solid-state phase transformation. Based on the reactive diffusion mechanism, which is responsible for the growth of magnesium silicide, we adopt a grand-potential-based phase-field model to investigate the microstructural evolution during the solid-state phase transformation. The presently developed phase-field concept is capable to model the solid-state phase transformation between three stoichiometric phases, Mg〈sub〉2〈/sub〉Si, diamond and Mg-hcp phases. The simulated microstructures are scrutinized via a skeleton algorithm. The simulation results reveal that the thickness distribution of the Mg〈sub〉2〈/sub〉Si phase follows the one of the foam-ligaments and that the average thickness of the magnesium silicide phase strongly depends upon the surface-volume ratio of the Si-foam rather than the porosity. In addition, it has been found that for a constant porosity, the mean value for the thickness of the magnesium silicide is different when the thickness distribution of the foam-strut is different. The relationship between the local thickness of the magnesium silicide phase and the foam-strut is analyzed based on the skeleton of the microstructure.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301417-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): Andraž Bradeško, Lovro Fulanović, Marko Vrabelj, Mojca Otoničar, Hana Uršič, Alexandra Henriques, Ching-Chang Chung, Jacob L. Jones, Barbara Malič, Zdravko Kutnjak, Tadej Rojac〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Electrocaloric fatigue, i.e., the degradation of the electrocaloric temperature change of an active material under continuous electric-field cycling, has not been addressed in detail so far, despite the elevated electric fields expected for EC cooling devices. Here, we investigate the electrocaloric fatigue mechanism of a prototype relaxor material, i.e., Pb(Mg〈sub〉1/3〈/sub〉Nb〈sub〉2/3〈/sub〉)O〈sub〉3〈/sub〉, by directly measuring its temperature response under device-relevant electric-field conditions. We show that after a critical number of field cycles the temperature of the sample begins to increase dramatically, leading to a significant degradation of the cooling properties. The degradation of cooling properties is investigated using a combination of multiscale characterization techniques, revealing that the origin of the degradation is the increased grain boundary conductance caused by an unexpected electric-field-induced phase transformation to a ferroelectric phase. We further show that this transformation and thus the fatigue can be regulated by careful control of the temperature and electric-field conditions. By revealing a previously unexplored fatigue mechanism, this study provides the first guidelines for the integration of high-performance relaxors into cooling devices.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301600-fx1.jpg" width="240" 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): Wenjiang Huang, Pedro Martin, Houlong L. Zhuang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉High-entropy alloys (HEAs) have been receiving intensive attention due to their unusual properties that largely depend on the selection among three phases: solid solution (SS), intermetallic compound (IM), and mixed SS and IM (SS + IM). Accurate phase prediction is therefore crucial for guiding the selection of a combination of elements to form a HEA with desirable properties. It is widely accepted that the phase selection is correlated with elemental features such as valence electron concentration and the formation enthalpy, leading to a set of parametric phase-selection rules [1]. Previous studies on predicting the phase selection employed density functional theory (DFT) calculations to obtain some correlated parameters. But DFT calculations are time consuming and exhibit uncertainties in terms of treating the 〈em〉d〈/em〉 orbitals of transition-metal atoms that are often components of HEAs. Here we employ machine learning (ML) algorithms to efficiently explore phase selection rules using a comprehensive experimental dataset consisting of 401 different HEAs including 174 SS, 54 IM, and 173 SS + IM phases. We adopt three different ML algorithms: K-nearest neighbours (KNN), support vector machine (SVM), and artificial neural network (ANN). To avoid overfitting, we divide the whole dataset into four nearly equal portions to perform a cross validation. For the classification of the three phases at the same time, the testing accuracy values from the KNN, SVM and ANN calculations achieve 68.6%, 64.3% and 74.3%, respectively. We then focus on the classification of two of the three phases using SVM and ANN. We find that the testing accuracy values using ANN in classifying the SS and IM phases, the SS + IM and IM phases, and the SS and SS + IM phases, are 86.7%, 94.3%, and 78.9%, respectively, which are higher than the corresponding testing accuracy values using SVM. As such, the trained ANN model performs the best among the three ML algorithms and is useful for predicting the phases of new HEAs. Our work provides an alternative route of computational design of HEAs, which is also applicable to accelerate the discovery of other metal alloys for modern engineering applications.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301454-fx1.jpg" width="269" 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): Rémy Besson, Jérôme Dequeker, Ludovic Thuinet, Alexandre Legris〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the context of Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉 and Mn-doped ferritic steels, we progressively elaborate an atomic-scale energy model to reproduce the thermodynamic behaviour of quaternary Fe〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Mn〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉C on a bcc lattice. This model is built on physical concepts: DFT calculations, pair Hamiltonians, non-configurational thermal effects, these elements being combined in a reasoned way to lead to a mastered and predictive formulation. In particular, this approach allows to explore the correlation between ordering in substitutional ternary Fe〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Al〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Mn and interstitial carbon, which brings new elements to the metallurgy of carbon in ferritic steels.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301569-fx1.jpg" width="265" 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): XiaoYu Chong, MingYu Hu, Peng Wu, Quan Shan, Ye Hua Jiang, Zu Lai Li, Jing Feng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉As the main strengthening phases in high-chromium cast irons (HCCIs), the elastic and ductile-brittle properties of M〈sub〉7〈/sub〉C〈sub〉3〈/sub〉 carbides are critical for the wear-resistance and application of HCCIs. The M〈sub〉7〈/sub〉C〈sub〉3〈/sub〉 carbides are characterized to be Cr〈sub〉3.87〈/sub〉Fe〈sub〉3.04〈/sub〉C〈sub〉3.09〈/sub〉 and hexagonal system (P6〈sub〉3〈/sub〉mc) in Fe-25.81 wt% Cr-4.45 wt% C alloy. Based on the elemental ratio and distribution, the crystals are built by a non-dilute ordered model. Mulialloying of Fe, Cr, W, Mo and B is adopted to design the mechanical properties of M〈sub〉7〈/sub〉C〈sub〉3〈/sub〉 carbides. Results from first-principles calculations and nanoindentation show that the W + B and W + Mo doping can increase the ductility but not significantly decrease the mechanical modulus of Cr〈sub〉4〈/sub〉Fe〈sub〉3〈/sub〉C〈sub〉3〈/sub〉, and Mo + B and Mo + W + B doping can improve the hardness of Cr〈sub〉4〈/sub〉Fe〈sub〉3〈/sub〉C〈sub〉3〈/sub〉 in HCCIs with finite decrease of ductility, which are all effective strategy to balance the ductility and strength of Cr〈sub〉4〈/sub〉Fe〈sub〉3〈/sub〉C〈sub〉3〈/sub〉 and enhance the wear-resistance of HCCIs. The relationship between intrinsic hardness (〈em〉H〈/em〉〈sub〉V〈/sub〉) and Pugh ratio (〈em〉B〈/em〉/〈em〉G〈/em〉) are fitted as 〈em〉H〈/em〉〈sub〉V〈/sub〉 = 29.4 GPa-7.6 GPa × 〈em〉B〈/em〉/〈em〉G〈/em〉, from which the maximum 〈em〉H〈/em〉〈sub〉V〈/sub〉 and 〈em〉B〈/em〉/〈em〉G〈/em〉 are 29.4 GPa and 3.87 by multialloying strategy, respectively. The bulk, shear, Young's modulus and hardness are largest during 0.61–0.63 electrons/Å〈sup〉3〈/sup〉 range of the effective density of valence electrons, while 〈em〉B〈/em〉/〈em〉G〈/em〉 and Poisson's ratio (〈em〉σ〈/em〉) are smallest. Considering that M〈sub〉7〈/sub〉C〈sub〉3〈/sub〉 carbide is rod-like monocrystal with strong orientation in HCCIs, the calculated and experimental Young's modulus from nanoindentation along non-[0001] direction is smaller than other directions, which provides guidance to achieve high wear-resistance of HCCIs by directional solidification. The elastic anisotropy is determined by the different atomic arrangement and chemical bonding along different crystallographic orientation. The decrease of mechanical modulus is attributed to the C-Mo and C-W bonds in M〈sub〉7〈/sub〉C〈sub〉3〈/sub〉 multicomponent carbides weaker than C-Fe and C-Cr bonds in Cr〈sub〉4〈/sub〉Fe〈sub〉3〈/sub〉C〈sub〉3〈/sub〉.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301582-fx1.jpg" width="286" 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): Bulat N. Galimzyanov, Dinar T. Yarullin, Anatolii V. Mokshin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Control of the crystallization process at the microscopic level makes it possible to generate the nanocrystalline samples with the desired structural and morphological properties, that is of great importance for modern industry. In the present work, we study the influence of supercooling on the structure and morphology of the crystalline nuclei arising and growing within a liquid metallic film. The cluster analysis allows us to compute the diffraction patterns and to evaluate the morphological characteristics (the linear sizes of the homogeneous part and the thickness of the surface layer) of the crystalline nuclei emergent in the system at different levels of supercooling. We find that the liquid metallic film at the temperatures corresponded to low supercooling levels crystallizes into a monocrystal, whereas a polycrystalline structure forms at deep supercooling levels. We find that the temperature dependence of critical size of the crystalline nuclei contains two distinguishable regimes with the crossover temperature 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈mi〉T〈/mi〉〈mo〉/〈/mo〉〈msub〉〈mrow〉〈mi〉T〈/mi〉〈/mrow〉〈mrow〉〈mi〉g〈/mi〉〈/mrow〉〈/msub〉〈mo〉≈〈/mo〉〈mn〉1.15〈/mn〉〈/mrow〉〈/math〉 (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi〉T〈/mi〉〈/mrow〉〈mrow〉〈mi〉g〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 is the glass transition temperature), which appears due to the specific geometry of the system.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419301429-fx1.jpg" width="389" 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): Ritwik Bandyopadhyay, Michael D. Sangid〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Two competing failure modes, namely inclusion- and matrix-driven failures, are studied in a Ni-base superalloy, RR1000, subjected to fatigue loading using crystal plasticity finite element (CPFE) simulations. Each individual factor related to the inclusion, which may contribute to crack initiation, is isolated and systematically investigated. Specifically, the role of the inclusion stiffness, loading regime, loading direction, a debonded region in the inclusion-matrix interface, microstructural variability around the inclusion, inclusion size, dissimilar coefficient of thermal expansion (CTE), temperature, residual stress, and distance of the inclusion from the free surface are studied in the emergence of two failure modes. The CPFE analysis indicates that the emergence of a failure mode is an outcome of the complex interaction between aforementioned factors. We observe the possibility of a higher probability of failure due to inclusions with increasing temperature, if the CTE of the inclusion is higher than the matrix, and vice versa. We do not find any overall correlation between the inclusion size and its propensity for damage, based on an inclusion that is of the order of the mean grain size. Finally, the CPFE simulations indicate that the surface inclusions are more damaging than the interior inclusions for similar surrounding microstructures.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304641-fx1.jpg" width="399" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 16 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Celal Soyarslan, Vincent Blümer, Swantje Bargmann〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Thin-walled metamaterials based on triply periodic surfaces are relatively simple, light-weight structures that, as shown in the following, possess extraordinary material properties. As opposed to their filled counterparts, these structures can be tuned to be elastically isotropic and isotropically auxetic - the latter is the material property of extending in all directions under tensile loading in one direction. Considering level surfaces topologically equivalent to the triply periodic minimal surfaces of types Primitive, Diamond, Gyroid and I-WP, we focus on stiffness, symmetry, auxeticity, Cauchy pressure and proximity to Born mechanical instability. Our findings show that core-shell structures respond drastically differently not only in their stiffness but also for each of these observed properties compared to their counterparts with complete filling. Only core-shell topology makes elastic isotropy possible. Diamond core-shell structures are the only ones which show negative Cauchy pressure 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msub〉〈mi〉p〈/mi〉〈mtext〉C〈/mtext〉〈/msub〉〈mo〉〈〈/mo〉〈mn〉0〈/mn〉〈/mrow〉〈/math〉. Most notably, for the Diamond core-shell structures we observe an auxetic behavior spanning over the whole range from non-auxetic to isotropically auxetic. For the structures possessing auxeticity, negativity in the Poisson’s ratio is retained for a wide deformation range from infinitesimal tension to finite compression.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304537-fx1.jpg" width="366" 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): Prafull Pandey, Sanjay Kashyap, Dhanalakshmi Palanisamy, Amit Sharma, Kamanio Chattopadhyay〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The paper reports evolution of two-phase microstructure quantitatively in a high entropy alloy of composition Co〈sub〉37.6〈/sub〉Ni〈sub〉35.4〈/sub〉Al〈sub〉9.9〈/sub〉Mo〈sub〉4.9〈/sub〉Cr〈sub〉5.9〈/sub〉Ta〈sub〉2.8〈/sub〉Ti〈sub〉3.5〈/sub〉. A very high-volume fraction of ordered L1〈sub〉2〈/sub〉 precipitates forms below its solvus temperature of 1156 °C that cannot be suppressed during quenching. The quench microstructure contains nanometric bimodal size distribution of spheroidal and irregular precipitates, both having L1〈sub〉2〈/sub〉 ordering (γ’) and a Co + Ni concentration close to 75 at. %, thus representing a stoichiometry of (Ni,Co)〈sub〉3〈/sub〉 (Al,Cr,Ta,Ti,Mo). The alloy shows hardening on aging with a peak in hardness value at 900 °C for 50 h of annealing. The morphology of the precipitates at this stage evolves into cuboidal shape having rounded corners with a lattice mismatch of +0.22% with the matrix. A detailed study of the coarsening behaviour of these precipitates in the matrix of complex solid solution indicates that the coarsening follows a modified LSW mechanism with estimated activation energy of 360 ± 50 kJ/mol between 900 and 1000 °C. This is slightly higher than the activation energy of most of the solutes in γ Co matrix that does not contain heavy elements like W or Re. The high-volume fraction of the precipitates together with coarsening resistance lead to an attractive high temperature strength that is higher than many of the known superalloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304410-fx1.jpg" width="289" 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): B.S. Li, Shenghui Xie, Jamie J. Kruzic〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this study, as-cast and cold-rolled Zr〈sub〉63.78〈/sub〉Cu〈sub〉14.72〈/sub〉Ni〈sub〉10〈/sub〉Al〈sub〉10〈/sub〉Nb〈sub〉1.5〈/sub〉 bulk metallic glass (BMG) samples were subjected to 20, 70, 120, and 170 cryogenic thermal cycles prior to fracture toughness testing. Thermal cycling raised the fracture toughness by promoting plastic deformation and stable crack growth, with the most significant increase occurring after the first 20 thermal cycles. Thermally cycled samples showed more tortuous crack paths and stable crack propagation left periodic blunting marks spaced at ∼57.5 μm on the fracture surface corresponding to each increment of crack advance. Microhardness mapping revealed a microstructure of hard and soft domains (∼63 μm × 105 μm), and thermal cycling heterogeneously softened the hard domains while the soft domains remained apparently unchanged. In addition to the observed softening, large increases in the relaxation enthalpy, and thus the average free volume, were found over the first ∼70 thermal cycles. While cold rolling the samples prior to the thermal cycling did not raise the mean fracture toughness values, the scatter was reduced compared to as-cast thermal cycled samples. This was attributed to the introduction of shear bands giving a more repeatable initial microstructure than casting.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304422-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): Philipp Frint, Martin F.-X. Wagner〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report on recurrent shear localization by formation of strictly alternating shear and matrix bands during equal-channel angular pressing (ECAP) of a 6000 series aluminum alloy. The strain partitioning process is documented by analyzing the deformation of a grid of indents as well as reconstructing the corresponding flow lines. Interestingly, shear strains of ∼3.6 in the shear bands considerably exceed the conventional maximum shear strain achievable in a single ECAP pass, whereas much lower strains occur in the matrix bands, maintaining on average the macroscopic deformation that is expected for ECA-pressing with a 90° die. Microstructural analysis by electron back-scatter diffraction (EBSD) and scanning transmission electron microscopy documents the different stages of microstructural evolution in shear and matrix bands and confirms the pronounced differences associated with the novel strain partitioning process. Furthermore, an EBSD-based analysis of texture evolution for billets with different orientations with respect to the initial extrusion direction demonstrates the important role that texture softening plays in triggering shear localization in two characteristic orientations as opposed to homogeneous deformation in the third orientation. Shear banding during ECAP is often interpreted in the light of failure mechanisms and cracking; the present study demonstrates that stable strain partitioning facilitates the fabrication of bulk laminated materials by ECAP.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304392-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): Siwei Chen, Yuichi Miyahara, Akiyoshi Nomoto, Kenji Nishida〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The effects of low-fluence neutron irradiation on hardening and microstructure evolution in ferrite of solution annealed or thermally aged CF3, CF3M, CF8 and CF8M cast austenitic stainless steels (CASSs) have been investigated by means of nanoindentation tests and atom probe tomography (APT). Thermal aging was performed at 400 °C for 500 h. Neutron irradiation was carried out to a fluence of 4.84 × 10〈sup〉18〈/sup〉 n/cm〈sup〉2〈/sup〉 (E 〉 1 MeV) at the temperature ranging from 289 to 292 °C in the LVR-15 research reactor. Irradiation hardening in thermally-aged specimens was found to be similar with or smaller than that in the corresponding solution annealed specimens. Phase decomposition and formation of solute clusters acted two major factors for the hardening in ferrite with thermal aging and/or neutron irradiation. The phase decomposition of ferrite increased with either the thermal aging or the neutron irradiation for the solution annealed materials; however, the change in the phase decomposition of ferrite was neither significant nor apparent with the low-fluence neutron irradiation for the thermally-aged materials. Ni–Si–Mn enriched solute clusters were observed in the matrix of ferrite in the aged specimens, and the irradiated specimens with/without thermal aging. Mo in the CASSs appeared to inhibit the formation of solute clusters under the neutron irradiations. In the thermally-aged specimen with low-C and without Mo, neutron irradiation enhanced the formation of solute clusters significantly. For the first time we discussed the relationship between hardening and microstructure evolution in ferrite of CASSs with consideration of both thermal aging and neutron irradiation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305415-fx1.jpg" width="290" 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): Ping-Jiong Yang, Qing-Jie Li, Wei-Zhong Han, Ju Li, Evan Ma〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Single-phase metals can be strengthened via cold work, grain refinement, or solid solution hardening. But the yield strength elevation normally comes at the expense of ductility, i.e., a conspicuous decrease of the uniform elongation in uniaxial tension. This strength-ductility trade-off is often a result of inadequate strain hardening rate that can no longer keep up with the elevated flow stress to prevent plastic instability. Here we alleviate this dilemma by designing oxygen interstitial solution hardening in body-centered-cubic niobium: the strain hardening rate is exceptionally high, such that most of the uniform tensile ductility of Nb can be retained despite of quadrupled yield strength. The oxygen solutes impose random force field on moving dislocation line, promoting the formation of cross-kinks that dynamically accumulate vacancy-oxygen complexes. These obstacles enhance the trapping/multiplication of screw dislocations as well as cross-slip, all promoting strain hardening and strain de-localization. This approach utilizes only a low concentration of interstitial solutes to achieve effective strengthening and strain hardening simultaneously, and is an inexpensive and scalable route amenable to the processing of bulk samples.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419305361-fx1.jpg" width="500" 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): 〈/p〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 178〈/p〉 〈p〉Author(s): Ruitao Qu, Dominik Tönnies, Lin Tian, Zengqian Liu, Zhefeng Zhang, Cynthia A. Volkert〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Upon reducing the sample size into micrometer scale, an obvious brittle-to-ductile transition accompanied by a drastic change of failure mode from shattering to shear-banding was observed when compressing the brittle but strong Co〈sub〉55〈/sub〉Ta〈sub〉10〈/sub〉B〈sub〉35〈/sub〉 bulk metallic glass (BMG). The shattering failure under macroscopic compression is dominated by splitting cracking, which completely differs from shear-banding and originates from extrinsic defects like inclusions. To reveal the critical conditions for shear-banding and splitting cracking, various micropillar specimens with intentionally introduced holes as extrinsic defects were tested, and the stress distributions at the failure moment were analyzed with finite element simulation. The shear plane criterion was found to be quite effective to estimate the nominal stress required for the failure dominated by shear-banding. However, brittle splitting cracking does not occur although the maximum tensile stress reaches the critical value, which is different from traditional brittle solids. To initiate splitting cracking, a high-tensile-stress region over a critical distance, which depends on defect size and fracture toughness of the BMG, is required. The critical conditions for shear failure and splitting cracking demonstrated in this approach can be used to estimate the failure conditions of various BMG components with complex geometries in a wide range of length scales, and to design tough composites based on brittle BMGs. As an example, a design criterion to avoid brittle splitting fracture of porous BMG 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-S1359645419305312-fx1.jpg" width="499" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 177〈/p〉 〈p〉Author(s): H.L. Che, S. Tong, K.S. Wang, M.K. Lei, Marcel A.J. Somers〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The formation of a metastable and nitrogen-supersaturated f.c.c. interstitial solid solution layer on Fe–Cr–Ni austenitic stainless steel at a moderate temperature around 650–720 K is not entirely understood. In the present work, three groups of austenitic Fe–Cr–Ni alloys, containing systematic variations of chromium, nickel and iron contents were nitrided by plasma-based low-energy ion implantation at 653 K for 4 h and investigated with light-optical microscopy (LOM), electron probe microanalysis (EPMA), (grazing incidence) X-ray diffraction (XRD) and transmission electron microscopy (TEM). Commercial AISI 304L was included for comparison. For the austenitic alloys with a Cr content below 12 wt.%, a duplex layer is observed in the nitrided case where the interface between the top layer, consisting of γ〈sup〉'〈/sup〉-Fe〈sub〉4〈/sub〉N like ordered γ〈sup〉'〈/sup〉〈sub〉N〈/sub〉, and the disordered γ〈sub〉N〈/sub〉 (nitrogen enriched austenite) zone underneath is associated with a decrease in N content. For the alloys with a Cr-content over 12 wt.%, a featureless continuous zone is observed with LOM and a gradual decrease in nitrogen content is measured with EPMA. Nevertheless, a similar duplex structure of outer γ〈sup〉'〈/sup〉-Fe〈sub〉4〈/sub〉N like ordered γ〈sup〉'〈/sup〉〈sub〉N〈/sub〉 and inner γ〈sub〉N〈/sub〉 is confirmed by XRD and TEM for all nitrided alloys, irrespective of the Cr content. The results are discussed in terms of the short-range order (SRO) promoted by the Cr–N interaction and long-range order (LRO) caused by the Fe–N interaction.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304598-fx1.jpg" width="478" 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): J. Wang, H. Sepehri-Amin, Y.K. Takahashi, T. Ohkubo, K. Hono〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A systematic study is conducted on the effect of MgO substrate/underlayer in terms of the surface roughness, grain boundary, and crystallographic texture on the magnetic in-plane components (medium noise) of the FePt-based heat-assisted magnetic recording (HAMR) media. Cross-sectional transmission electron microscope observations revealed that good (001) texture can be developed in FePt nanogranular films even when they are grow on a rough surface of a single crystal-MgO (001) substrate or across the grain boundaries of a (001)-textured polycrystalline-MgO underlayer. The misoriented FePt grains with magnetic easy axes (c-axes) tilted away from the film normal direction mainly originate from their epitaxial growth on the initially misoriented MgO underlayer grains. Micromagnetic simulation results revealed that the FePt grains with misorientation angle larger than ∼20° are attributed to the detected magnetic in-plane components. Based on these experimental results, we discuss how to improve the (001) texture of the polycrystalline underlayer as well as to develop FePt-based HAMR media with enhanced signal-to-noise ratio.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304574-fx1.jpg" width="280" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 18 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Alexander H. Bork, Erwin Povoden-Karadeniz, Alfonso J. Carrillo, Jennifer L.M. Rupp〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In the search of new materials for the solar-to-fuel technology, we turn to the material class of perovskites that offer wide possibilities in manipulation of its chemistry and redox activity. Here, we access the role of Cr in the La〈sub〉0.6〈/sub〉Sr〈sub〉0.4〈/sub〉Mn〈sub〉1-y〈/sub〉Cr〈sub〉y〈/sub〉O〈sub〉3-〈em〉δ〈/em〉〈/sub〉 perovskite solid solution hitherto unexplored for two-step solar thermochemical fuel production. A multi-component Calphad defect model for the system La-Sr-Cr-Mn-O is therefore optimized and used for computations of oxygen nonstoichiometries and redox thermodynamics of the La〈sub〉0.6〈/sub〉Sr〈sub〉0.4〈/sub〉Mn〈sub〉1-y〈/sub〉Cr〈sub〉y〈/sub〉O〈sub〉3-〈em〉δ〈/em〉〈/sub〉 solution series in the temperature range of 1073 to 1873 K as a potential operation window for solar-to-fuel conversion. Modeling results reveal two advantages of substituting manganese by chromium. Firstly, it is possible to reduce the heat capacity with up to 10 %, to a value of 132 J mol〈sup〉-1〈/sup〉 K〈sup〉-1〈/sup〉. Secondly, the thermodynamic driving force for solar-to-fuel conversion increases and the Cr-doped materials provide higher yield and efficiency at isothermal operation. The proposed model allows for continuous simulative scanning of redox thermodynamics from zero Cr-doping to a fully substituted chromite perovskite. For isothermal water splitting, the composition La〈sub〉0.6〈/sub〉Sr〈sub〉0.4〈/sub〉Mn〈sub〉0.2〈/sub〉Cr〈sub〉0.8〈/sub〉O〈sub〉3-〈em〉δ〈/em〉〈/sub〉 displays the highest fuel yield and efficiency of 2.7 % due to a high thermodynamic driving force at elevated temperature for this composition. These predictive insights give prospects for engineering the thermodynamics of the oxygen release reaction in perovskites towards higher fuel production and efficiency in solar-to-fuel reactors with isothermal operation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304628-fx1.jpg" width="469" 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): Bin Hu, Haiwen Luo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, we present a novel two-step intercritical annealing process, 〈em〉i.e.〈/em〉 a-few-minute interruption during heating followed by a short intercritical annealing, to improve the mechanical properties of medium Mn steel without prolonging the intercritical annealing (IA) period. Both experimental investigations and numerical simulations were performed to investigate the microstructural inheritance between the two steps and its influence on the tensile properties. The increase of interruption temperature led to a gradual change of tensile flow curve from the discontinuous yielding to the continuous yielding at lower yield strength due to more martensite and less austenite formed after the IA at 670 °C. It is concluded that the two-step IA process can improve the mechanical properties of medium Mn steel when it is optimized, compared to the non-interrupted IA process. The strategy for optimizing this novel process is to carefully tailor cementite precipitates before annealing as the nuclei of austenite for promoting relatively coarse austenite grains formed with Mn gradients inside. This kind of austenitization on the deliberately introduced nuclei finally led to a considerable fraction of austenite grains retained adjacent to martensite, causing strength and ductility improved simultaneously. Moreover, we demonstrate that such a deliberate design of two-step IA process can be guided by the quantitative modeling of austenitization kinetics on a cementite nucleus.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304525-fx1.jpg" width="329" 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): C. Shashank Kaira, Tyler J. Stannard, Vincent De Andrade, Francesco De Carlo, Nikhilesh Chawla〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Even after nearly a century of extensive use of aluminum alloys in structural applications, our understanding of such precipitation-strengthened materials is far from complete. With the advent of next generation advanced characterization techniques, our ability to probe materials in unique ways and at different length scales has established a new paradigm for devising new pathways to alloy design by engineering materials and tailoring specific properties at the nanoscale. Here, we perform 〈em〉in situ〈/em〉 nanomechanical testing in conjunction with synchrotron-based hard X-ray nanotomography to capture initiation and evolution of damage in 3D in Al–Cu alloys. Precipitates in these alloys are seen to exhibit unprecedented localized deformation in compression, which is attributed to novel observations of kinking in these brittle second-phase particles, accompanied with the generation of a fine polycrystalline texture in the adjacent matrix. We observe a size-dependent transition in precipitate deformation behavior that has been thoroughly investigated using a comprehensive correlative approach.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304549-fx1.jpg" width="290" 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): Gionata Schneider, Ludger Weber, Andreas Mortensen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We explore how reaction at the interface between a solid porous ceramic and an infiltrating molten metal influences wetting in pressure infiltration, wetting being characterized by a drainage curve that plots the metal saturation versus the applied metal pressure. Specifically, we infiltrate Cu-46at.pct. Si into graphite preforms at 1050 °C, 1100 °C, 1150 °C or 1200 °C. The Si in the copper alloy reacts with the graphite to form SiC, which is better wetted by the alloy compared to the initial graphite. We show that, unlike what is observed in non-reactive systems, at fixed applied pressure reaction prevents stabilization of the metal saturation and causes the metal to continuously flow into the preform. Interpreting the data under the assumption that the applied pressure influences the local rate of thermally activated triple line motion as does the applied stress the rate of thermally activated motion of dislocations, measured infiltration velocities can be exploited to deduce both an activation volume and an activation energy for the interfacial process that governs reaction-driven motion of the triple line in this system. The resulting activation volume is on the order of one to a few 100 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mi〉n〈/mi〉〈msup〉〈mrow〉〈mi〉m〈/mi〉〈/mrow〉〈mrow〉〈mn〉3〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉, leading to estimated activation energy values of a few times 100 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mfrac〉〈mrow〉〈mi〉k〈/mi〉〈mi〉J〈/mi〉〈/mrow〉〈mrow〉〈mi〉m〈/mi〉〈mi〉o〈/mi〉〈mi〉l〈/mi〉〈/mrow〉〈/mfrac〉〈/mrow〉〈/math〉. Both are realistic for a process that is limited by the rate of SiC growth along the metal/graphite interface.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304409-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): Kazuki Shibanuma, Yuta Suzuki, Kazuya Kiriyama, Katsuyuki Suzuki, Hiroyuki Shirahata〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The first 3D model to simulate cleavage crack propagation in a body center cubic (BCC) polycrystalline solid is presented. The model was developed based on the extended finite element method (XFEM). Crack shape as well as a polycrystal were modeled independently from the finite element mesh. Cleavage crack planes formed on the {100} planes based on the local fracture stress criterion. Crack propagation was simulated by an iterative calculation with updating cleavage planes. Model validations were conducted with experimental fractography using a ferrite-pearlite steel. The results showed that the proposed model successfully simulated complicated cleavage crack propagation behavior, including the wraparound behavior noted in local crack propagation direction as well as the formation of numerous micro-cracks under the main fracture surface. In particular, the comparison of the distribution of the cleavage plane directions quantitatively validated the proposed model. The proposed model in the present study showed potential to help clarify the relationship between microstructure and cleavage crack propagation resistance.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304513-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): DeAn Wei, Michael Zaiser, Zhiqiang Feng, Guozheng Kang, Haidong Fan, Xu Zhang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work, we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar cross-section. The simulations show significant strengthening effects for TB orientation angles less than 45°, where the interaction between mixed dislocations and TBs plays an important role. For angles larger than 45°, the interaction between screw dislocations and TB dominates, leading to weak strengthening. At 45°, dislocations on the dominant slip systems glide parallel to the TB and no strengthening is observed.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304379-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): N.I. Vázquez-Fernández, T. Nyyssönen, M. Isakov, M. Hokka, V.-T. Kuokkala〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this work, the effects of strain rate and adiabatic heating on the strain induced martensitic phase transformation were uncoupled and individually evaluated. Tension tests were performed at different strain rates ranging from 2 × 10〈sup〉−4〈/sup〉 s〈sup〉−1〈/sup〉 to 1400 s〈sup〉−1〈/sup〉, covering both isothermal and adiabatic conditions. The adiabatic temperature rise of a sample tested at a high strain rate was replicated with heating resistors in a normally isothermal low strain rate test. This test allows studying the mechanical behavior and microstructural evolution of the material at a very low strain rate at thermal conditions similar to that of a high strain rate test. The phase transformation rates from austenite to α′-martensite were measured with the magnetic balance method. The phase transformation rate drops significantly with increasing strain rate. At a higher strain rate, the α′-martensite nucleates primarily on a single habit plane parallel to the primary slip plane of the parent austenite, while at a lower strain rate the α′-martensite nucleation occurs on several habit planes. At the studied plastic strains, the strain rate seems to have a stronger effect on the α′-martensite formation than the adiabatic heating. This is supported by thermodynamic stacking fault calculations, which indicate that the increase in the stacking fault energy due to adiabatic heating at low strains is small and therefore unlikely the only reason for the reduced phase transformation rate. Therefore, the strain rate itself seems to have an important role in the strain induced martensitic phase transformation rate.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304276-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): H. Chen, A. Kauffmann, S. Seils, T. Boll, C.H. Liebscher, I. Harding, K.S. Kumar, D.V. Szabó, S. Schlabach, S. Kauffmann-Weiss, F. Müller, B. Gorr, H.-J. Christ, M. Heilmaier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉High entropy alloys based on the Ta–Nb–Mo–Cr–Ti–Al system are expected to possess high creep and oxidation resistance as well as outstanding specific mechanical properties due to presumed high melting points and low densities. However, we recently reported that arc-melted and subsequently homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall. Mater. Trans. A 49 (2018) 772–781 and J. Alloys Cmpd. 661 (2016) 206–215]. Thermodynamic calculations suggest the formation of a B2-type ordered phase below the homogenization temperature. In the present article, we provide results of a detailed microstructural characterization of a series of Ta–Nb–Mo–Cr–Ti–Al derivatives and evaluate if B2-type ordering could be the origin for the observed lack of ductility. Backscatter electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT) were used to verify uniform elemental distribution after homogenization. X-ray diffraction (XRD) patterns indicate both, A2 or B2-type crystal structure, whereas transmission electron microscopy (TEM) diffraction experiments unambiguously confirm B2-type order in the as-homogenized state of all investigated alloys. In MoCrTiAl, planar defects that show antiphase boundary contrast with a {100}-type habit plane were detected by TEM dark field (DF) imaging. They are wetted by a Cr-enriched and Ti-depleted layer as confirmed by scanning transmission electron microscopy (STEM)-EDX line scans as well as APT analyses. The planar defects arise from a disorder-order solid-state phase transformation during cooling, as indicated by differential scanning calorimetry (DSC).〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304318-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): Y.S. Zhao, J. Zhang, Y.S. Luo, B. Zhang, G. Sha, L.F. Li, D.Z. Tang, Q. Feng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Nowadays, it is challenging to completely eliminate low angle or high angle grain boundaries (LAGBs or HAGBs) from Nickel-based single crystal (SX) superalloys manufactured using the conventional directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB) defects, and thus increasing the creep resistance. However, the strengthening mechanism through their additions is still unclear. In this study, a double-seed solidification technique with two misorientation levels, i.e., 5° and 20°, was used to produce a series of bicrystal superalloys with different contents of Hf and B. It is the first report of an alloy with joint Hf and B addition that demonstrates tolerance to GBs with a misorientation as high as ∼20° under all of the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. Interestingly, the effect of individual additions of Hf or B was not as pronounced as that of the joint Hf and B addition. To understand the influence of these additions on the creep mechanism in nickel-based superalloys with GB defects, a detailed characterization of the microstructures in the vicinity of the LAGBs or HAGBs was carried out, and the elemental distribution at the HAGBs was analyzed with various techniques. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304288-fx1.jpg" width="258" 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): Hang Su, Hiroyuki Toda, Kazuyuki Shimizu, Kentaro Uesugi, Akihisa Takeuchi, Yoshio Watanabe〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304306-fx1.jpg" width="247" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Hamidreza Zobeiri, Ridong Wang, Qianying Zhang, Guangjun Zhu, Xinwei Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This work reports the first results on the conjugated hot carrier diffusivity (〈em〉D〈/em〉) and thermal conductivity (〈em〉κ〈/em〉) of suspended nm-thick WS〈sub〉2〈/sub〉 structures. A novel nET-Raman technique is developed to distinguish and characterize these two properties by constructing steady and transient states of different laser heating and Raman probing sizes. The nET-Raman uses a nanosecond pulsed laser and a continuous wave laser for exciting Raman signals and heating samples. 〈em〉κ〈/em〉 is found to increase from 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mn〉15〈/mn〉〈mo〉.〈/mo〉〈msubsup〉〈mn〉1〈/mn〉〈mrow〉〈mo〉−〈/mo〉〈mn〉0.4〈/mn〉〈/mrow〉〈mrow〉〈mo〉+〈/mo〉〈mn〉0.3〈/mn〉〈/mrow〉〈/msubsup〉〈/math〉 to 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mn〉38〈/mn〉〈mo〉.〈/mo〉〈msubsup〉〈mn〉8〈/mn〉〈mrow〉〈mo〉−〈/mo〉〈mn〉2.4〈/mn〉〈/mrow〉〈mrow〉〈mo〉+〈/mo〉〈mn〉2.6〈/mn〉〈/mrow〉〈/msubsup〉〈/math〉 W·m〈sup〉−1〈/sup〉 K〈sup〉−1〈/sup〉 when the sample's thickness increases from 13 to 107 nm. This increase is attributed to the decreased effect of surface phonon scattering in thicker samples. Also, hot carrier diffusion length (Δ〈em〉r〈/em〉〈sub〉HC〈/sub〉) for these samples are measured without knowledge of hot carrier's lifetime (〈em〉τ〈/em〉). Measured 〈em〉D〈/em〉 of these four samples are in close range (except the thickest sample). This is due to the fact that lattice scattering for all these samples is similar and there is no substrate effect on our suspended films. nET-Raman is very robust and has negligible effect from laser absorption depth, sample thickness, and laser spot drift during measurement.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303702-fx1.jpg" width="449" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Ke Tong, Fei Ye, Ya Kun Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The structures of short-range ordered Fe–Cr–N clusters in face-centered cubic (f.c.c.) iron have been systematically studied by first-principles calculation to understand the atomic structure and phase stability of supersaturated nitrided layer on austenitic stainless steel. The clusters are formed by aggregation of Cr and N atoms due to their strong interaction. The unit of the clusters can be considered as a Fe〈sub〉6-〈em〉n〈/em〉〈/sub〉Cr 〈sub〉〈em〉n〈/em〉〈/sub〉 N octahedral cluster, in which the N atom occupies an octahedral interstitial site of the f.c.c. lattice and the metal atoms are at the first nearest neighbor sites to the N atom. Moreover, the Cr atoms prefer to distribute in pairs around the N atom. When the N concentration increases, a larger cluster may be formed by combination of the octahedral clusters through edge-shared mode, and Cr atoms prefer the shared sites. The cluster structure is affected by both the local lattice distortion around the cluster and Coulombic interaction in the cluster. Then, the atomic structure of supersaturated nitrided layer can be described as the f.c.c. iron dispersively embedded with the short-range ordered clusters. Furthermore, the stabilization mechanism of the metastable phase was examined based on the structure model. It was suggested that the stabilization of the metastable phase is mainly a chemically-driven mechanism by Cr and N atoms forming short-range ordered clusters.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303787-fx1.jpg" width="467" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Mingfeng Chen, Ji Ma, Ren-Ci Peng, Qinghua Zhang, Jing Wang, Yuhan Liang, Jialu Wu, Long-Qing Chen, Jing Ma, Ce-Wen Nan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Miniaturizing ferroic oxides is essential for studying the fundamental physics of low-dimensional topological defects such as flux-closure vortex as well as the applications of future nanoelectronic devices. Here, we successfully synthesized self-assembled BiFeO〈sub〉3〈/sub〉 nanoislands on LaAlO〈sub〉3〈/sub〉 (001) substrate by pulsed laser deposition. Center-type quad-domains are spontaneously stabilized in these nanostructures, and the polarization vectors of each quarter can be independently switched with robust retention properties over time and elevated temperature. As a result, the long-sought exotic domain structures such as anti-vertex can be created in these nanoislands by selectively switching polarization state of certain quarters. Resistive switching effect is also confirmed in quarters of the nanoisland, indicating the great potential as memory cells in high density ferroelectric random access memory.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303751-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): H. Sepehri-Amin, I. Dirba, Xin Tang, T. Ohkubo, T. Schrefl, O. Gutfleisch, K. Hono〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Based on a modified hydrogenation disproportionation desorption recombination (HDDR) process we propose and realize a novel top-down processing route to synthesize anisotropic nano-composite magnet powders. Selection of alloy compositions with Nd content lower than the stoichiometry of Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B phase led to the formation of spherical shaped nano-sized α-Fe phase within the Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B matrix after HDDR process. Next anisotropic nanocomposite Nd-Fe-B/α-Fe powders with substantial coercivity were developed with optimized HDDR conditions and subsequent grain boundary engineering which remedied the initial lack of Nd-rich intergranular phase. Specifically, the infiltration of Nd〈sub〉70〈/sub〉Cu〈sub〉30〈/sub〉 alloy increased the coercivity from 0.0 to 0.85 T. Note that low coercivity and the absence of texture in nanocomposite magnets have been the main challenges to realize the high (〈em〉BH〈/em〉)〈sub〉max〈/sub〉 postulated for many years for anisotropic nanocomposite magnets. We employed micromagnetic simulations for optimum microstructure design of a nanocomposite Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B/α-Fe magnet that gives a large maximum energy product, (〈em〉BH〈/em〉)〈sub〉max〈/sub〉. The simulations are then correlated with macroscopic hysteresis properties and high-resolution electron microscopy as well as atom probe tomography. Another very remarkable result is the observation that the formation of α-Fe phase with a size up to 200 nm within the matrix of Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B grains can still result in a significant coercivity of 0.85 T. This is in contrast to common understanding of exchange-coupled systems and we explain this observation with sharp and defect free 〈em〉α〈/em〉-Fe/Nd〈sub〉2〈/sub〉Fe〈sub〉14〈/sub〉B interface, the latter a result of the disproportionation and recombination reactions.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303763-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Jaber Rezaei Mianroodi, Pratheek Shanthraj, Paraskevas Kontis, Jonathan Cormier, Baptiste Gault, Bob Svendsen, Dierk Raabe〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dislocation-precipitate interaction and solute segregation play important roles in controlling the mechanical behavior of Ni-based superalloys at high temperature. In particular, the increased mobility of solutes at high temperature leads to increased dislocation-solute interaction. For example, atom probe tomography (APT) results [1] for single crystal MC2 superalloy indicate significant segregation of solute elements such as Co and Cr to dislocations and stacking faults in 〈em〉γ′〈/em〉 precipitates. To gain further insight into solute segregation, dislocation-solute interaction, and its effect on the mechanical behavior in such Ni-superalloys, finite-deformation phase field chemomechanics [2] is applied in this work to develop a model for dislocation-solute-precipitate interaction in the two-phase 〈em〉γ〈/em〉-〈em〉γ′〈/em〉 Ni-based superalloy model system Ni–Al–Co. Identification and quantification of this model is based in particular on the corresponding Ni–Al–Co embedded atom method (EAM) potential [3]. Simulation results imply both Cottrell- and Suzuki-type segregation of Co in 〈em〉γ〈/em〉 and 〈em〉γ'〈/em〉. Significant segregation of Co to dislocation cores and faults in 〈em〉γ′〈/em〉 is also predicted, in agreement with APT results. Predicted as well is the drag of Co by 〈em〉γ〈/em〉 dislocations entering and shearing 〈em〉γ'〈/em〉. Since solute elements such as Co generally prefer the 〈em〉γ〈/em〉 phase, Co depletion in 〈em〉γ′〈/em〉 could be reversed by such dislocation drag. The resulting change in precipitate chemistry may in turn affect its stability and play a role in precipitate coarsening and rafting.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303672-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): S. Alkan, H. Sehitoglu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉It is well known that interfaces play an important role in determining the mechanical response of materials. This paper focuses on the transforming shape memory alloy NiTi and is aimed towards a better understanding of austenite-martensite interface structure (steps and dislocation arrays) and the determination of transformation stress corresponding to the translation of this interface. In the present work, we characterize the defect content at the cubic-monoclinic interfaces via the Topological Model. The defect-induced displacement fields are generated within the framework of the Eshelby-Stroh formalism and further improved with Molecular Statics simulations accounting for interactions at the atomic level. The resulting defect core disregistry fields are employed as input to a modified Peierls-Nabarro framework for evaluating the transformation stress. We applied the proposed methodology to the particular case of NiTi alloy single crystals of specific orientations and predicted the transformation stress levels in close agreement with experiments. Moreover, the short-range interactions of dislocation core disregistry fields are shown to be responsible for the experimentally observed non-Schmid behavior of transformation stress levels. Overall, the paper represents an effort to improve our understanding of shape memory materials considering theory, computer simulation and experiment.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303684-fx1.jpg" width="320" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Aldo Marano, Lionel Gélébart, Samuel Forest〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We investigate the ability of local continuum crystal plasticity theory to simulate intense slip localization at incipient plasticity observed experimentally in metals exhibiting softening mechanisms. A generic strain softening model is implemented within a massively parallel FFT solver framework to study intragranular strain localization throughout high resolution polycrystalline simulations. It is coupled to a systematic analysis strain localization modes: Equivalent plastic strain and lattice rotation fields are processed to create binary maps of slip and kink bands populations, estimate their volume fraction and mean strain level. High resolution simulations show the formation of an intragranular localization band network. The associated localization maps are used to identify accurately slip and kink bands populations and highlight the distinct evolution of kink bands, influenced by lattice rotation. Results highlight that the analysis of the nature of localization bands in numerical studies is fundamental to asses the validity of polycrystalline simulations. Indeed, it is evidenced that selection between slip or kink localization modes is only due to grain to grain incompatibilities as these two localization modes are equivalent in classical crystal plasticity models. As a result they predict the formation of a large amount of kink bands in contradiction with experimental observations of softening metals. We show that this holds for complex physics based models too. Hence, the use of classical crystal plasticity for strain localization simulation should be reconsidered in order to predict realistic localization modes.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303696-fx1.jpg" width="332" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Xingwei Liu, Xuemei Liu, Hao Lu, Haibin Wang, Chao Hou, Xiaoyan Song, Zuoren Nie〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The influencing factors, distribution characteristics and evolution mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were demonstrated. Based on characterizations from various angles, the correlations of grain shape aspect ratio, WC contiguity and distribution of low-energy grain boundaries were analyzed for cemented carbides with addition of different grain growth inhibitors. It was found that the ∑2 grain boundaries have higher fraction in the cemented carbides with stable complexions, finer grain size and larger WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbides with anisotropic surface energy by coalescence of WC platelets with large shape aspect ratio. The results suggest that the fraction and distribution of low-energy grain boundaries in the cemented carbides can be modulated by matching grain growth inhibitor and sintering temperature of the initial WC-Co composite powder.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉The features of low-energy grain boundaries in the WC-Co cemented carbides were studied considering the influencing factors, distribution laws and formation and evolution mechanisms. The correlations of the distribution and fraction of low-energy grain boundaries with the shape aspect ratio, size, and contiguity of WC grains were disclosed. The principles to modulate the low-energy grain boundaries in the cemented carbides through matching the composition and sintering temperature of the initial powder were proposed, which may be applicable to a large variety of powder metallurgical materials.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930374X-fx1.jpg" width="385" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Fuminori Yanagimoto, Takuhiro Hemmi, Yuta Suzuki, Yasuhito Takashima, Tomoya Kawabata, Kazuki Shibanuma〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study investigates the contribution of grain size to cleavage crack propagation resistance in ferritic steels. A series of crack arrest tests were conducted using three kinds of steel, which had the same chemical composition but different grain sizes. Dynamic finite element analyses were used to simulate the respective crack arrest tests in order to evaluate local fracture stress. The results clearly showed that coarse-grained steel has higher local fracture stress. These results showed the opposite tendency to the well-known dependence of cleavage fracture initiation resistance on grain size. To understand these results from a micromechanics viewpoint, the energy dissipation during cleavage crack propagation was evaluated based on the consumed energy in the tear-ridge formation. Small-scale tensile experiments simulating tear-ridge formations were conducted using specimens with a pair of focused-ion-beam-machined slit-notches to simulate cleavage planes. The energy consumed during tear-ridge formation was quantified as a function of the cleavage plane distance based on the results of small-scale tests. Combining the quantified experimental results with the cleavage crack propagation model based on the extended finite element method, the energy dissipation in steels with various grain sizes was evaluated. The energy dissipation results showed the same tendency as the local fracture stress results. In other words, higher resistance against cleavage crack propagation can be obtained in coarse-grained steel.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304112-fx1.jpg" width="497" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 27 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Y. Wu, J. Yaacoub, F. Brenne, W. Abuzaid, D. Canadinc, H. Sehitoglu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The factors that affect the fatigue performance of shape memory alloys (SMAs), including fatigue crack growth (FCG) response, is far from being well-understood. In this study, we point to a mechanism that degrades the FCG performance considerably. We introduce the notion of FCG being affected by shielding and deshielding mechanisms, the former enhancing the resistance while the latter reducing the materials’ resistance. We show that the deshielding mechanism creates additional driving forces (positive K contribution) of both Mode II and Mode I types (as much as 5-10 MPa·m〈sup〉1/2〈/sup〉) which accelerates the crack advance. The origin of the positive K component is associated with the localized martensite variant formation that is highly asymmetric with respect to the crack tip. We derive a resultant ΔK in excellent agreement with that measured based on experimental displacement measurements. Overall, this study represents an advancement of our understanding in FCG of SMAs by quantifying the deshielding mechanism.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304161-fx1.jpg" width="401" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 26 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Jonathan Ehrler, Maciej Oskar Liedke, Jakub Čížek, Richard Boucher, Maik Butterling, Shengqiang Zhou, Roman Böttger, Eric Hirschmann, Thu Trang Trinh, Andreas Wagner, Jürgen Lindner, Jürgen Fassbender, Christoph Leyens, Kay Potzger, Rantej Bali〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The atomic arrangement in certain B2-ordered alloys, such as Fe〈sub〉60〈/sub〉Al〈sub〉40〈/sub〉, determines intrinsic material properties like magnetism. Here we have investigated the influence of open-volume defects on the atomic ordering process at elevated temperatures in Fe〈sub〉60〈/sub〉Al〈sub〉40〈/sub〉 thin films. A dependence of the ordering process on the type and concentration of defects is observed by positron annihilation spectroscopy combined with 〈em〉ab-initio〈/em〉 calculations. Comparing the lifetimes of positrons in the alloy for different annealing and irradiation treatments reveals the role of mono-vacancies, triple defects as well as large vacancy clusters: The rate of atomic ordering to the ordered B2 state is increased in the presence of mono-vacancies whereas triple defects and vacancy complexes decrease the ordering rate. Furthermore, an agglomeration of vacancies during annealing to di-vacancies and larger vacancy clusters is observed. The distribution of open-volume defects can be modified in such a way as to control the thermal stability 〈em〉via〈/em〉 ion-irradiation and thermal pre-treatments.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304100-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Haoren Wang, Tyler Harrington, Chaoyi Zhu, Kenneth S. Vecchio〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉FeAl-based MIL composites of various iron alloys were fabricated with an innovative “multiple-thin-foil” configuration and “two-stage reaction” strategy. Alternating stacked metal foils were reactive sintered via SPS at 600〈sup〉o〈/sup〉C and 1000〈sup〉o〈/sup〉C to grow intermetallics. The “multiple-thin-foil” configuration reduces reaction time, enables local chemical composition control and allows metal/intermetallic combinations, which cannot be produced via the conventional methods. Fe-FeAl, 430SS-FeAl, and 304SS-FeAl MIL composites can be synthesized with desired metallic/intermetallic ratios, where FeAl is the single intermetallic phase present in the composites. Microstructure analysis via SEM, EDS, and EBSD confirms phase identification and reveals the formation of transition layers. The transition layer, which incorporates the composition gradient between the metal (Fe, 430SS or 304SS) and the FeAl intermetallic phase, provides a gradual change in mechanical properties from the metal to intermetallic layers, and further functions as a chemical barrier into which other undesired intermetallics dissolve. Driven by diffusion-controlled growth, grains in the transition layers and FeAl regions exhibit ordered arrangement and sintering textures. Hardness profiles from the metal layer to FeAl region reveal the correlation between local mechanical properties and local chemical compositions. In compression testing, the compressive strength can reach 2.3 GPa with considerable plasticity, establishing the best mechanical properties of any MIL composites synthesized to date.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304124-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 25 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Oleg Shchyglo, Guanxing Du, Jenni K. Engels, Ingo Steinbach〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We present three-dimensional phase-field simulations of martensite microstructure formation in low-carbon steel. In this study, a full set of 24 Kurdjumov-Sachs symmetry variants of martensite is considered. Three different carbon compositions are investigated in order to reveal the effect of carbon content on the martensite microstructure formation. The simulations are performed using the finite strain framework which allows considering real martensite transformation strains. Using Neuber elasto-plastic approximation to the mechanical equilibrium solution, realistic stresses and strains can be obtained during martensite formation resulting in realistic mechanical driving forces for the transformation. The simulated microstructures are compared to experimental results for three carbon compositions. Good agreement between simulated and experimental results is achieved.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304094-fx1.jpg" width="386" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 22 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Mauricio R. Bonilla, Fabián A. García Daza, Javier Carrasco, Elena Akhmatskaya〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Garnet LiLaZrO (LLZO) is a promising solid electrolyte candidate for solid-state Li-ion batteries, but at room temperature it crystallizes in a poorly Li-ion conductive tetragonal phase. To this end, partial substitution of Li by Al ions is an effective way to stabilize the highly conductive cubic phase at room temperature. Yet, fundamental aspects regarding this aliovalent substitution remain poorly understood. In this work, we use molecular dynamics and advanced hybrid Monte Carlo methods for systematic study of the room temperature Li-ion diffusion in tetragonal and cubic LLZO to shed light on important open questions. We find that Al substitution in tetrahedral sites of the tetragonal LLZO allows previously inaccessible sites to become available, which enhances Li-ion conductivity. In contrast, in the cubic phase Li-ion diffusion paths become blocked in the vicinity of Al ions, resulting in a decrease of Li-ion conductivity. Moreover, combining the conductivities of individual phases through an effective medium approximation allowed us to estimate the conductivities of cubic/tetragonal phase mixtures that are in good agreement with those reported in several experimental works. This suggests that phase coexistence (due to phase equilibrium or gradients in Al content within a sample) could have a significant impact on the conductivity of Al-substituted LLZO, particularly at low contents of Al. Overall, by making a thorough comparison with reported experimental data, the theoretical study and simulations of this work advance our current understanding of Li-ion mobility in Al-substituted LLZO garnets and might guide future in-depth characterization experiments of this relevant energy storage material.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304045-fx1.jpg" width="485" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 21 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Inga Jonane, Andris Anspoks, Giuliana Aquilanti, Alexei Kuzmin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉X-ray absorption spectroscopy at the Cu and Mo K-edges was used to study the effect of heating on the local atomic structure and dynamics in copper molybdate (〈em〉α〈/em〉-CuMoO〈sub〉4〈/sub〉) in the temperature range from 296 to 973 K. The reverse Monte-Carlo (RMC) method was successfully employed to perform accurate simulations of EXAFS spectra at both absorption edges simultaneously. The method allowed us to determine structural models of 〈em〉α〈/em〉-CuMoO〈sub〉4〈/sub〉 being consistent with the experimental EXAFS data. These models were further used to follow temperature dependencies of the local environment of copper and molybdenum atoms and to obtain the mean-square relative displacements for Cu–O and Mo–O atom pairs. Moreover, the same models were able to interpret strong temperature-dependence of the Cu K-edge XANES spectra. We found that the local environment of copper atoms is more affected by thermal disorder than that of molybdenum atoms. While the MoO〈sub〉4〈/sub〉 tetrahedra behave mostly as the rigid units, a reduction of correlation in atomic motion between copper and axial oxygen atoms occurs upon heating. This dynamic effect seems to be the main responsible for the temperature-induced changes in the O〈sup〉2−〈/sup〉→Cu〈sup〉2+〈/sup〉 charge transfer processes and, thus, is the origin of the thermochromic properties of 〈em〉α〈/em〉-CuMoO〈sub〉4〈/sub〉 upon heating above room temperature.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304070-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 21 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Yuanchao Yang, Dezhen Xue, Ruihao Yuan, Yumei Zhou, Turab Lookman, Xiangdong Ding, Xiaobing Ren, Jun Sun〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Doping point defects into shape memory alloys (SMAs) influences their transformation behavior and mechanical properties. We propose a general Landau free energy model to study doping effects, which only assume that point defects produce local dilatational stresses coupled to the non-order parameter volumetric strain. Different dopants can be represented by their range of interaction and potency of dilatational stress. Time-dependent simulations based on our model successfully reproduce experimentally observed doping effects in SMAs, including the elevation or suppression of the transformation temperature, the modification of mechanical properties, the appearance of a cross-hatched tweed structure and the emergence of a frozen glassy state with local strain order. We predict that the temperature range for superelasticity will be enhanced in the crossover regime between martensite and strain glass. In addition, an Elinvar effect appears most likely in alloys with dopants tending to increase the transformation temperature, which needs to be verified experimentally. Moreover, the two dopant parameters in the Landau model, the interaction range and potency of the dilatational stress, inspire us to identify three material descriptors with which we can construct an empirical machine learning model. The model predicts the transformation temperature, and the slope of the change in transformation temperature as a function of doping composition, enabling an effective search for doped SMAs with targeted properties 〈em〉via〈/em〉 machine learning.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304021-fx1.jpg" width="316" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): O.N. Senkov, S. Gorsse, D.B. Miracle〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, 〈em〉T〈/em〉〈sub〉m〈/sub〉) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000 °C and 1200 °C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 〈em〉T〈/em〉〈sub〉m〈/sub〉, so that the alloys with higher 〈em〉T〈/em〉〈sub〉m〈/sub〉 retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000 °C is probably due to dissolution of secondary phases.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304033-fx1.jpg" width="262" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 18 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Qingsong Pan, Haofei Zhou, Qiuhong Lu, Huajian Gao, Lei Lu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉History-independent, stable and symmetric cyclic response has been detected in as-deposited bulk polycrystalline Cu with highly oriented nanotwins [Nature 551 (2017) 214-217]. In this study, to deepen the understanding of cyclic deformation in nanotwinned (NT) structures, small levels of tensile pre-strains were applied on NT-Cu, followed by strain-controlled symmetric tension-compression cyclic tests. Distinct from the symmetric cyclic response of as-deposited NT-Cu, the magnitude of the maximum stress in tension is much larger than that of the minimum stress in compression, indicating that the cyclic response of tensile pre-deformed NT-Cu is highly asymmetric. The degree of its cyclic asymmetry gradually decays as the number of cycles or the plastic strain amplitude is increased. The tensile pre-deformed NT-Cu recovers to its symmetric cyclic response after cyclic deformation at sufficiently large plastic strain amplitude, analogous to that detected in as-deposited NT counterparts. Molecular dynamics simulations and microstructural observations revealed that the observed asymmetric cyclic response is mainly related to the activation and movement of threading dislocations with extended misfit dislocation tails lying on the twin boundaries (TBs) during tensile pre-deformation. During cyclic deformation, threading dislocations in adjacent twin interiors tend to link their long tails with one another to form correlated necklace dislocations (CNDs) with a symmetric structure. The CNDs move back-and-forth along the twin boundaries without directional slip resistance, contributing to the transition from asymmetric to symmetric cyclic response of NT-Cu.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303933-fx1.jpg" width="226" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 174〈/p〉 〈p〉Author(s): Jun Liu, Yuanyuan Gong, Yurong You, Xinmin You, Bowei Huang, Xuefei Miao, Guizhou Xu, Feng Xu, Ekkes Brück〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉MnMX (M = Co or Ni, X = Si or Ge) alloys with strong magnetostructural coupling exhibit giant magnetic entropy change and are currently extensively studied. However, large thermal hysteresis results in serious irreversibility of the magnetocaloric effect in this well-known system. In this work, we report a low thermal hysteresis and large reversible magnetocaloric effect in a MnNiGe-based system. The introduction of Fe into both Ni and Mn sites can establish stable magnetostructural transitions from paramagnetic hexagonal to ferromagnetic orthorhombic phases. Fascinatingly, a low thermal hysteresis of 5.2 K is achieved in Mn〈sub〉0.9〈/sub〉Fe〈sub〉0.2〈/sub〉Ni〈sub〉0.9〈/sub〉Ge alloy with a large magnetization difference of 62.1 A m〈sup〉2〈/sup〉/kg between the two phases. These optimized parameters lead to a partially reversible phase transformation under a magnetic stimulus and bring about a large reversible magnetic entropy change of −18.6 Jkg〈sup〉−1〈/sup〉K〈sup〉−1〈/sup〉 under the field variation of 0–5 T, which is the largest value reported in MnMX system up to now. Moreover, this low-hysteresis magnetostructural transformation and large reversible magnetocaloric effect can be tuned by doping with Si in a wide temperature range covering room temperature. We also introduce geometrically nonlinear theory to discuss the origin of low hysteresis in MnMX alloys. A strong relation is found between thermal hysteresis and the change of 〈em〉c〈/em〉 axis in the orthorhombic structure during the transition. Our work greatly develops the potential of MnMX alloys as magnetocaloric materials and is meaningful to seek or design a MnMX system with low thermal hysteresis.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303581-fx1.jpg" width="500" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 175〈/p〉 〈p〉Author(s): Ana Šantić, Juraj Nikolić, Luka Pavić, Radha D. Banhatti, Petr Mošner, Ladislav Koudelka, Andrea Moguš-Milanković〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The scaling behaviour of six series of mixed ion-polaron glasses with the composition 〈em〉x〈/em〉WO〈sub〉3〈/sub〉/MoO〈sub〉3〈/sub〉-(30–0.5〈em〉x〈/em〉)Li〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O/Ag〈sub〉2〈/sub〉O-(30–0.5〈em〉x〈/em〉)ZnO-40P〈sub〉2〈/sub〉O〈sub〉5〈/sub〉, 0 ≤ 〈em〉x〈/em〉 ≤ 60 mol%, has been studied using Summerfield and Sidebottom scaling procedures that are model free. The validity of the Sidebottom scaling procedure for each individual glass confirms that all glasses obey time-temperature superposition principle implying that the conduction mechanism does not change with temperature. On the other hand, Summerfield scaling is not found valid for all glasses. First, this deviation is observed in all glasses containing Li〈sub〉2〈/sub〉O up to 20 mol% of MoO〈sub〉3〈/sub〉/WO〈sub〉3〈/sub〉. We relate this result to a non-typical interaction of Li〈sup〉+〈/sup〉 ion with the compact zinc phosphate network resulting either in change of its hopping distance or in available conduction pathways for it as a function of temperature. Second, non-Summerfield scaling was also observed for glasses containing Li〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O/Ag〈sub〉2〈/sub〉O with about 30 to 40 mol% of WO〈sub〉3〈/sub〉. Thus, the origin of this deviation lies in the significant amounts of both types of charge carriers - ions and polarons, and their differently thermally activated mobilities.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉Unusual Li〈sup〉+〈/sup〉 dynamics and mixed ionic-polaronic conduction in zinc phosphate glasses detected by scaling conductivity spectra.〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303593-fx1.jpg" width="350" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia, Volume 174〈/p〉 〈p〉Author(s): A. Nicolaÿ, G. Fiorucci, J.M. Franchet, J. Cormier, N. Bozzolo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Influence of strain rate on dynamic and post-dynamic recrystallization kinetics of Inconel 718 is investigated by performing hot compression tests at constant strain rate in the range 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈mrow〉〈mo stretchy="true"〉[〈/mo〉〈mrow〉〈mn〉0.001〈/mn〉〈mo〉;〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈mo stretchy="true"〉]〈/mo〉〈/mrow〉〈msup〉〈mrow〉〈mi〉s〈/mi〉〈/mrow〉〈mrow〉〈mo〉−〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 in the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈mi〉δ〈/mi〉〈/mrow〉〈/math〉 -subsolvus domain, with or without post-deformation holding at the deformation temperature. Dynamically and post-dynamically recrystallized grains are distinguished based on their internal misorientations, using EBSD data with enhanced angular resolution. For the applied deformation conditions (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mi〉T〈/mi〉〈mo linebreak="goodbreak" linebreakstyle="after"〉=〈/mo〉〈mn〉980〈/mn〉〈mo〉°〈/mo〉〈mi〉C〈/mi〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mi〉ε〈/mi〉〈mo linebreak="goodbreak" linebreakstyle="after"〉=〈/mo〉〈mn〉0.7〈/mn〉〈/mrow〉〈/math〉), dynamic recrystallization is inhibited at 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mrow〉〈mrow〉〈mover accent="true"〉〈mi〉ε〈/mi〉〈mo〉˙〈/mo〉〈/mover〉〈/mrow〉〈mo〉〉〈/mo〉〈mn〉0.1〈/mn〉〈msup〉〈mrow〉〈mi〉s〈/mi〉〈/mrow〉〈mrow〉〈mo〉−〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉. On the other hand, very fast post-dynamic recrystallization is promoted by high strain rates, with characteristic times which can be as short as few seconds to achieve full recrystallization. Most of previous works on the effect of strain rate on dynamic recrystallization kinetics were done by quenching samples right after deformation, without discriminating dynamically and post-dynamically recrystallized grains. Those works led to the conclusion that increasing strain rate beyond a critical value leads to an increase in dynamic recrystallization kinetics. Experimental quenching delays cannot be shorter that few seconds, which is shown here to be sufficient to get a significant increase in recrystallized fraction by post-dynamic mechanisms. Based on the present work, post-dynamic evolutions are actually very likely to be responsible for the apparent increase in dynamic recrystallization kinetics at high strain rates which has often been reported previously.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419303532-fx1.jpg" width="299" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 28 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): J. Nutter, H. Farahani, W.M. Rainforth, S. van der Zwaag〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The kinetic behaviour of austenite/ferrite interfaces in a low carbon – 0.5 mass% Mn containing steel during Cyclic Partial Phase Transformation (CPPT) experiments has been investigated using hot stage Transmission Electron Microscopy (TEM). Individual interfaces were observed to display behaviour typical of CPPT experiments as recorded in macroscopic dilatometry experiments and demonstrated i) the “normal”, ii) inverse transformations and iii) a stagnant stage in which the interface migrates at a very low velocity as a result of the interface passing through a Mn enriched zone due to the preceding transformation. The length of the stagnant stage determined from the TEM observations shows excellent agreement with that measured from dilatometry and kinetic modelling, whilst the distance migrated from the interface shows some disparities which are primarily attributed to differences in assumptions about grain geometry and nucleation. No special interface features were observed when the interface changed direction and passed through the previously Mn-enriched zones. General observations on the interaction of the transformation interface with microstructural features are also reported.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304951-fx1.jpg" width="398" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 27 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Paraskevas Kontis, Edouard Chauvet, Zirong Peng, Junyang He, Alisson Kwiatkowski da Silva, Dierk Raabe, Catherine Tassin, Jean-Jacques Blandin, Stéphane Abed, Rémy Dendievel, Baptiste Gault, Guilhem Martin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable Ni-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable Ni-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 μm enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419304896-fx1.jpg" width="499" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉