ALBERT

Beschreibung: Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ -ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths 〉1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades.

Beschreibung: The interface layer and discrete crystal growth in solidification are traced by analyzing the microsegregation of aluminum alloy. Based on the interface evolution and discrete crystal growth, the non-equilibrium microsegregation mechanism is qualitatively proposed. The solute content of solid is decided by the solidification temperature of interface. And the solidification temperature of interface is further decided by the evolution of interface compositions and interface temperature. The fluctuation of interface compositions and actual interface temperature persists in the whole solidification process.

Beschreibung: The weld metal of the ENiCrFe-7 nickel-based alloy-covered electrodes was investigated in terms of the microstructure, the grain boundary precipitation, and the ductility-dip cracking (DDC) susceptibility. Besides the dendritic gamma-Ni(Cr,Fe) phase, several types of precipitates dispersed on the austenitic matrix were observed, which were determined to be the Nb-rich MC-type carbides with “Chinese script” morphology and size of approximately 3 to 10 µ m, the Mn-rich MO-type oxides with size of approximately 1 to 2 µ m, and the spherical Al/Ti-rich oxides with size of less than 1 µ m. The discontinuous Cr-rich M 23 C 6 -type carbides predominantly precipitate on the grain boundaries, which tend to coarsen during reheating but begin to dissolve above approximately 1273 K (1000 °C). The threshold strain for DDC at each temperature tested shows a certain degree of correlation with the grain boundary carbides. The DDC susceptibility increases sharply as the carbides coarsen in the temperature range of 973 K to 1223 K (700 °C to 950 °C). The growth and dissolution of the carbides during the welding heat cycles deteriorate the grain boundaries and increase the DDC susceptibility. The weld metal exhibits the minimum threshold strain of approximately 2.0 pct at 1323 K (1050 °C) and the DTR less than 873 K (600 °C), suggesting that the ENiCrFe-7—covered electrode has less DDC susceptibility than the ERNiCrFe-7 bare electrode but is comparable with the ERNiCrFe-7A.

Beschreibung: Magnesium (Mg) alloys have been recently introduced as a biodegradable implant for orthopedic applications. However, their fast corrosion, low bioactivity, and mechanical integrity have limited their clinical applications. The main aim of this research was to improve such properties of the AZ91 Mg alloy through surface modifications. For this purpose, nanostructured fluoridated hydroxyapatite (FHA) was coated on AZ91 Mg alloy by micro-arc oxidation and electrophoretic deposition method. The coated alloy was characterized through scanning electron microscopy, transmission electron microscopy, X-ray diffraction, in vitro corrosion tests, mechanical tests, and cytocompatibility evaluation. The results confirmed the improvement of the corrosion resistance, in vitro bioactivity, mechanical integrity, and the cytocompatibility of the coated Mg alloy. Therefore, the nanostructured FHA coating can offer a promising way to improve the properties of the Mg alloy for orthopedic applications.

Beschreibung: Microstructural evolution of an Fe-22 pct Mn-0.4 pct C twinning-induced plasticity (TWIP) steel during high strain rate deformation has been investigated. When subjected to low strain rate deformation, the steel shows the typical TWIP phenomenon. On the other hand, when subjected to high strain rate deformation, there is a formation of nanostructured austenite, due to the occurrence of deformation twinning forming nanoscale twin/matrix lamellae followed by dynamic recovery induced by adiabatic heating during high strain rate deformation.

Beschreibung: The presence of Fe aids in establishing the mechanical and physical properties of AlSi alloys and is also one of the main impurities leading to formation of β -Al 5 FeSi intermetallics. This study aims to understand the effect of fluid flow on the dendritic microstructure with intermetallics in Al-5/7/9 wt pct Si-0.2/0.5/1.0 wt pct Fe alloys that are directionally solidified under defined thermal and fluid flow conditions. We made extensive use of 3D X-ray tomography to obtain a better insight into the morphology and formation of the intermetallics. Three-dimensional (3-D) distribution of intermetallics presented here shows that the growth of large β -Al 5 FeSi due to forced flow occurs in the eutectic specimen center and together with an increase in the number density of β precipitates. The 3D reconstructions have verified the β shaped to be curved, bent with twining, branched, and to have imprints, holes, and propeller-shaped platelets. The 3D views showed that hole-shaped β arose from the lateral growth around α -Al dendrites. These views also confirmed the phenomenon of shortening of β as an effect of flow in the dendritic region, where β could be fragmented or completely remelted, and ultimately resulting in microstructures with shorter β -Al 5 FeSi and increases in number density. The analysis revealed an interaction between melt flow, 3D distribution, and the morphology of β -Al 5 FeSi. The growth of a large and complex group of β intermetallics can reduce the melt flow between dendrites and strengthen pore nucleation and eutectic colonies nucleation, leading to lower permeability of the mushy zone and increased porosity in the castings.

Beschreibung: Due to the miniaturization of the solder joints in micro/nanoelectronic devices, the volume ratio of intermetallic (IMCs) materials has substantially increased. This increased ratio could affect the reliability of solder joints depending on the regime and the rate of the loading. Cu 6 Sn 5 is the primary IMC layer in the solder joint, and the primary crack initiation is observed in Cu 6 Sn 5 site in the literature. As the size of the joints becomes closer to the grain size, joints may only contain a few numbers of grains of Cu 6 Sn 5 . This manifests itself in statistical grain size effects, as well as anisotropy. Modeling these joints using bulk properties of Cu 6 Sn 5 does not capture the actual behavior of these joints especially when plastic deformation is involved. Plastic deformation, starting at yield point, happens to be associated with the activation of slip systems. Deformation of a slip system of single crystal largely rests on the slip parameters such as critical resolved shear stress (CRSS), initial hardening modulus, and saturation stress (Stage I stress when large plastic flow occurs). However, no efforts have been made to capture the slip parameters of Cu 6 Sn 5 experimentally or analytically because of the difficulties of using conventional mechanical tests to measure the slip parameters of HCP single crystals. Due to wide range of CRSS values, it becomes difficult to isolate a specific slip system in testing without activating the other slip systems. The crystal plasticity finite-element (CPFE) method takes into account the effect of anisotropy and slip system behavior in modeling materials. This work uses a combined strategy based upon experiments, modeling, and a comparative analysis to obtain slip system parameters that could predict the slip process of Cu 6 Sn 5 . Nanoindentation tests were performed on Cu 6 Sn 5 single crystal to extract the load–displacement curves, and a CPFE nanoindentation model analysis along with custom user material was utilized to obtain set of crystal plasticity material parameters which can represent the plastic behavior of Cu 6 Sn 5 IMC. These parameters were then used to predict shear yield strength and shear modulus of Cu 6 Sn 5 , and the findings were compared with the previously published values in the literature.

Beschreibung: Most research related to dendrite coherency point (DCP) has been done on cast aluminum alloys and at a low cooling rate condition. In this research, the DCP of a wrought aluminum alloy is calculated in the range of high cooling rates used in the direct-chill casting process. The two-thermocouple thermal analysis technique was used to determine the DCP of Al2024 alloy. The aim of this work is to investigate the effect of different cooling rates on the dendrite coherency characteristics of Al2024. The cooling rates used in the present study range from 0.4 to 17.5 °C s −1 . Also, the effect of 1.2 wt pct Al-5Ti-1B grain refiner on the DCP was studied. To calculate the solid fraction at dendrite coherency, solid fraction versus time is plotted based on Newtonian technique. The results show that by increasing the cooling rate, both time and temperature of dendrite coherency are decreased. Also, by adding the Al-5Ti-1B master alloy, dendrite coherency temperature is reduced and dendrite impingement is postponed. To reduce casting defects occurring during equiaxed solidification, e.g ., macrosegregation, porosities, and hot tearing, these two operations which lead to postpone the transition from mass to inter-dendritic feeding, or dendrite coherency, can be useful. By increasing the cooling rate, solid fraction at dendrite coherency increases initially and then decreases at higher cooling rates. Presence of grain refiner leads to increasing of solid fraction at DCP. Thus, by delaying the dendrite coherency and increasing the solid fraction at DCP, semi-solid forming can be performed on parts with higher solid fraction and less shrinkage. Microstructural evaluation was carried out to present the correlation between the cooling rate and solid fraction in 2024 aluminum alloy.

Beschreibung: Electron microscopy techniques have been used to investigate the cause of premature creep failure in the fusion zone of INCONEL ® Alloy 740H ® (INCONEL and 740H are registered trademarks of Special Metals Corporation) welds. The reduced creep rupture lives of all-weld-metal and cross-weld creep specimens (relative to base metal specimens) have been attributed to the presence of large grain boundary regions that were denuded in fine γ ′ but contained coarse, elongated particles. Investigation of creep rupture specimens has revealed four factors that influence the formation of these coarsened zones, and the large particles found within them have been identified as γ ′. Comparisons of the microstructural characteristics of these zones to the characteristics that are typical of denuded zones formed by a variety of mechanisms identified in the literature have been made. It is concluded that the mechanism of γ ′-denuded zone formation in alloy 740H is discontinuous coarsening of the γ ′ phase. The discontinuous reaction is catalyzed by the grain boundary migration and sliding which occur during creep and likely promoted by the inhomogeneous weld metal microstructure that results from solute segregation during solidification. The increased susceptibility to the formation of the observed γ ′-denuded zones in the weld metal as compared to the base metal is discussed in the context of differences in the contributions to the driving force for the discontinuous coarsening reaction.

Beschreibung: The activities of components of Sn-Zn system were predicted based on the molecular interaction volume model (MIVM). The separation coefficients and the vapor–liquid phase equilibrium of Sn-Zn system were also predicted using the MIVM. The predicted results indicated that the content of tin in the vapor phase was 0.000052 wt pct, while in the liquid phase, it was 99.98 wt pct at 1173 K (900 °C). Experimental investigations into the separation of Sn-Zn alloy by vacuum distillation were carried out for the proper interpretation of the predicted results. The effects of vacuum level (15 to 200 Pa), distillation temperatures [873  K to 1273  K (600 °C to 1000 °C)], and soaking time (20 to 60 minutes) were studied. The experimental results indicated that the content of tin in the vapor phase was 0.001 wt pct, while in the liquid phase, it was 99.98 wt pct at 1173 K (900 °C). The experimental results match well with the predicted data, suggesting that the MIVM is a suitable model for Sn-Zn system.

Beschreibung: In the current study, we detail a novel in situ X-ray diffraction-based bulk measurement technique, which allows for the continuous tracking of primary recrystallization kinetics. The approach is based on measuring the diffracted intensity that is correlated with the evolution of the volume fraction of particular texture components during annealing of a sample within a texture goniometer. The method is applied in an experimental study on a cold-rolled industrial Al-Fe-Si alloy. For comparison purposes, the macrotexture and the hardness evolution were monitored ex situ along isothermal and nonisothermal annealing. These measurements were then contrasted to the in situ obtained growth kinetics of recrystallizing grains in beta-fiber deformation and cube orientation. The results showed clearly that this method can be reliably utilized for the characterization of recrystallization kinetics in an industrial context.

Beschreibung: Fe-14Cr-1W oxide dispersion strengthened ferritic alloys have excellent mechanical properties at high temperatures due to a high density of thermally stable Y-Ti-O nanoclusters, which are rather stable at high temperature. The aim of this work was to study the evolutions of Y-Ti-O precipitation at high temperature as a function of nominal content of Y, Ti, and O. Different Fe-14Cr alloys were characterized by small-angle neutron scattering technique under magnetic field as well as transmission electron microscopy investigations before and after annealing at 1573 K to 1723 K (1300 °C to 1450 °C). Significant differences are observed depending on the Y/Ti/O ratios. For Ti/Y 〉1, the coarsening kinetics is low even at 1723 K (1450 °C). For Ti/Y 〈1, very large size particles are observed after annealing at 1573 K (1300 °C). We have shown that the oxygen content should be strictly controlled to avoid significant coarsening at 1573 K (1300 °C) and above.

Beschreibung: A thermodynamically consistent phase-field model is formulated for binary-alloy, particularly applicable to the systems which have a significant difference in mobilities of the solute and solvent atoms, e.g. , interstitial solutes. The model is based on the mixed-mode of growth which is considered to be the general mode of transformation. As an illustration, the model has been used to simulate austenite to ferrite transformation in one-dimension. The model is made ‘quantitative’ using a different diffusivity-interpolation function without using anti-trapping flux term. The simulation-result is compared with the experimental results, and good agreement with the mixed-mode of growth in the initial stage of the transformation (when the individual nuclei are far apart) has been observed.

Beschreibung: The unloading behavior was compared for three different steel grades: a dual-phase steel, a transformation-induced plasticity steel, and a twinning-induced plasticity steel. Steels that harden by phase transformation or deformation twinning exhibited a smaller component of microplastic strain during unloading and a smaller reduction in the chord modulus compared to the conventional hardening steel. As a result, unloading is closer to pure elastic unloading when the TRIP effect or TWIP effect is active.

Beschreibung: Interdiffusion in Ti-Al-Mo β solid solution was investigated at 1523 K (1250 °C) by analyzing diffusion couples. From the concentration profiles analytically represented by error function expansion (ERFEX), the ternary interdiffusion coefficients and impurity diffusivity were extracted by the Whittle–Green and generalized Hall methods. A comparison of the diffusion in five Ti-Al-X (Co, Cr, Fe, Mo, and V) ternaries reveals Ti-Al-Mo is comparably like Ti-Al-(Cr, V) while Ti-Al-(Co, Fe) are predominantly of interstitial nature.

Beschreibung: Sintering of Cu and thermoelectric Ca 3 Co 4 O 9 was tried using a modified pulsed electric current sintering (PECS) process, where an electrically nonconductive die was used instead of a conventional graphite die. The pulsed electric current flowed through graphite punches and sample powder, which caused the Joule heating of the powder compact itself, resulting in sintering under smaller power consumption. Especially for the Ca 3 Co 4 O 9 powder, densification during sintering was also accelerated by this modified PECS process.

Beschreibung: This work assesses the consequences of macrosegregation on microstructural evolution during solid-state transformations in a continuously cooled pressure-vessel steel (SA508 Grade 3). Stark spatial variations in microstructure are observed following a simulated quench from the austenitization temperature, which are found to deliver significant variations in hardness. Partial-transformation experiments are used to show the development of microstructure in segregated material. Evidence is presented which indicates the bulk microstructure is not one of upper bainite, as it has been described in the past, but one comprised of Widmanstätten ferrite and pockets of lower bainite. Segregation is observed on three different length scales, and the origins of each type are proposed. Suggestions are put forward for how the segregation might be minimized, and its detrimental effects suppressed by heat treatments.

Beschreibung: Experiments are carried out to measure the volume shrinkage during solid-state reaction in micro-joints for three-dimensional integrated circuit applications. Surface profilometer is employed to measure the volume shrinkage for the reaction between Ni and Sn. The shrinkage is correlated with the microstructural evolution during the reaction. It is found that the volume shrinkage is released through both joint height reduction and void formation. The resulting internal stress and the void formation might post potential reliability issues.

Beschreibung: Cyclic creep behavior of a type 316LN austenitic stainless steel was investigated in the temperature range from 823 K to 923 K (550 °C to 650 °C). A transition from fatigue-dominated to creep-dominated failure mode was observed with an increase in the mean stress. The threshold value of mean stress for the transition was seen to be a strong function of the test temperature. Occurrence of dynamic strain aging proved beneficial owing to a substantial reduction in the strain accumulation during cyclic loading.

Beschreibung: The present investigation discusses about the occurrence and activity of slip systems in low-cycle fatigue deformation of metastable austenite (fcc) at various total strain amplitudes (Δ ε t ) under ambient temperature, where the rate of deformation is kept constant. It has been found that with the increase in strain amplitude, the activity of the single slip system decreases and that of the multiple slip system increases.

Beschreibung: A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model.

Beschreibung: In this study, two Zr-based amorphous alloy matrix composites reinforced with STS304 stainless steel continuous fibers whose diameters were 110 and 250  μ m were fabricated by the liquid pressing process. Using a Hopkinson pressure bar, the compressive deformation behavior was investigated at a strain rate of about 10 3  s −1 , and the results were then compared with those obtained under quasi-static loading. 65 to 68 vol pct of STS fibers were homogeneously distributed in the amorphous matrix, in which considerable amounts of dendritic crystalline phases were present. According to the dynamic compressive test results, shear cracks were formed at the maximum shear stress direction in the 110- μ m-diameter-fiber-reinforced composite to reach the final failure. In the 250- μ m-diameter-fiber-reinforced composite, fibers were not cut by shear cracks because the fiber diameter was large enough to restrict the propagation of shear cracks, while taking over a considerable amount of compressive loads over 1500 MPa. This composite showed the higher yield and maximum compressive strengths and plastic strain than the 110- μ m-diameter-fiber-reinforced composite because of the sufficient ductility of STS fibers, the effective interruption of propagation of shear cracks, and the strain hardening of fibers themselves.

Beschreibung: Impact toughness testing was conducted on 10 and 30 mm plates of 2205 together with a 30 mm plate of LDX 2101 ® duplex stainless steel (DSS). The testing temperatures were between 153 K (−120 °C) and room temperature. Interrupted fracture toughness tests of the 10 mm plate and a 50 mm plate of 2205 were also performed. The conclusion from the fractographic investigation was that the delaminations that occur in hot-rolled DSSs were cleavage fractures. The toughness anisotropy can be explained by the cleavage fracture and the appearance of the microstructure. The result from the interrupted fracture toughness test revealed that the delaminations initiated prior to the maximum force plateau and propagated ahead of the stable crack growth during testing. Estimated upper limit for the fracture delamination initiation toughness at sub-zero temperatures for the 2205 base metal according to the crack-tip opening displacement method was 28 to 61  μ m for the 10 mm plate, 70 to 106  μ m for the 30 mm plate and below 100  μ m for the 50 mm plate.

Beschreibung: Thermal sprayed MCrAlY coatings are widely used as a bond coat in thermal barrier systems to protect the substrate from corrosion and high temperature oxidation and to improve the compatibility between the ceramic top coat and metallic substrate. In this paper, the high temperature oxidation resistance of MCrAlY coatings with modified compositions was evaluated; in particular, the effect of the addition of reactive and refractory elements (Ta, Re, Si, and Hf) was investigated. MCrAlY coatings were obtained by high velocity oxygen fuel spray and vacuum plasma spray techniques; samples were exposed to air at 1423 K (1150 °C) and the oxidation kinetics were evaluated by measuring the thickness of the thermally grown oxide (TGO) scale at several exposure times. Experimental data confirmed that the oxidation resistance of MCrAlY coatings is strictly related to the amount of the reactive and refractory elements in the starting powders and that a thorough understanding of the microstructural modifications taking place during oxidation is essential for controlling TGO growth and thermal barriers’ durability.

Beschreibung: The onset of α nucleation in a carbon containing β -titanium alloy has been investigated by coupling atom probe tomography (APT) with transmission electron microscopy. The analysis of the APT results indicates that in addition to ω precipitates that can act as potential α nucleation sites, carbon atoms tend to form clusters within the β -matrix, which in turn give rise to additional nucleation sites for α , resulting in finer scale α precipitates due to increased nucleation density.

Beschreibung: This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.

Beschreibung: This article provides a new model for predicting the surface tension of molten alkali halides, because the subject is worthy of investigation. A relationship exists between the surface tension ( γ ) at the melting point ( T m ), molar volume ( V ), internuclear distance ( D ), and radius ratio $ \left( {\frac{r^{+}}{r^{-} }} \right). $ The basic idea results from the assumption that all of the parameters are constants. The relation depends on the reliability and accuracy of all the constants on which it is based. The formula was examined and showed remarkable agreement between the calculated surface tension and experimental data within a difference of less than 10 pct for most of the salts studied.

Beschreibung: δ -TRIP is a low-alloy steel in which ferrite persists at all temperatures in the solid-state, with the remaining microstructure consisting of carbide-free bainite and carbon-enriched retained austenite. The present work explores for the first time, how changes in the intercritical annealing temperature and the transformation conditions associated with bainite influence the morphology and stability of the austenite, and hence the behavior of the microstructure during tensile deformation. It is found that the structure can be optimized to consist of a combination of blocky and film austenite that undergo transformation over a range of plastic strains, thus minimizing the possibility of plastic instabilities, and hence imparting considerable strength and uniform elongation.

Beschreibung: The variant grouping behavior of a simulated heat-affected zone in low-carbon steel was tracked in situ and analyzed by laser scanning confocal microscopy and electron backscatter diffraction. By visualizing the microstructural changes and crystallographic properties under rapid cooling conditions, the transition from Bain zone grouping to close-packed plane grouping was directly studied. The results were compared with those obtained under slow cooling and the link between the variant grouping behavior and the nested microstructure of lower bainite and martensite is clarified.

Beschreibung: Metallic–intermetallic laminate composites are promising materials for many applications, namely, in the aerospace industry. Ti/TiAl 3 laminates are one of the interesting laminate composites, which are mostly used in aerospace applications. In this work, commercially pure aluminum and titanium sheets were explosively joined. The multilayer samples were annealed between 1 and 260 hours at 903 K (630 °C) in ambient atmosphere, and the formation and growth of the intermetallic compound at the Ti/Al interface were monitored. Microstructural investigations were carried out using optical and scanning electron microscopes equipped with energy-dispersive spectroscopy and the X-ray diffraction technique. The microhardness profile of the layers was also determined. The thickness and type of Al-Ti intermetallics were determined. It was found that the only intermetallic phase observed in the interfaces was TiAl 3 . It was also shown that two mechanisms for TiAl 3 growth exist: reaction and diffusion controlled. The growth exponent was equal to 0.94 for the reaction-controlled mechanism (first step) and 0.31 for the diffusion-controlled mechanism (second step). These values were in good agreement with theoretical values (1 and 0.5 for the first and second steps, respectively). Based on the results of this research, a kinetic model for the formation and growth of TiAl 3 intermetallic phase was proposed.

Beschreibung: The final fractions of austenite after the isothermal austenite-to-ferrite transformation and its reverse transformations (including the martensite-to-austenite and the pearlite + ferrite-to-austenite transformations) in the intercritical annealing region have been studied using full equilibrium (FE), paraequilibrium (PE), and local equilibrium (LE) calculations and experiments. The LE model predictions are in a very good agreement with the experimental results, while the FE and PE model predictions deviate significantly. It is also found that the LEP/LENP transition for the austenite-to-ferrite transformation deviates from those of its reverse transformations. The magnitude of deviation increases with the increasing Mn concentration.

Beschreibung: This paper investigates the effects of post-weld heat treatment (PWHT) and temper bead welding (TBW) on hardness, microstructure and residual stresses in multi-layer welding on low carbon steel specimens made with two different weld geometries, viz. (1) smooth-contoured and (2) U-shaped. It was found that the PWHT technique gave overall lower hardness than the TBW technique, but the hardness values in both techniques were acceptable. Microscopy analysis showed that the TBW technique was more effective in tempering the heat affected zone as the grain size decreased slightly at the fusion line in spite of the higher temperature at the fusion line. Residual stresses measured using the hole-drilling method showed that the residual stress is not reduced below yield stress near the last bead solidified in TBW. Only PWHT gives low residual stress results in this area. High tensile residual stresses may result in sensitivity to fatigue loading.

Beschreibung: Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y 2 O 3 alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y 2 O 3 alloy and Fe-9Cr-1W-0.5Y 2 O 3 alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y 2 O 3 to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (〈5 nm).

Beschreibung: An investigation was carried out on the solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200- μ m thickness as an intermediate material prepared in vacuum in the temperature range from 973 K to 1073 K (700 °C to 800 °C) in steps of 298 K (25 °C) using uniaxial compressive pressure of 3 MPa and 60 minutes as bonding time. Scanning electron microscopy images, in backscattered electron mode, had revealed existence of layerwise Ti-Ni-based intermetallics such as either Ni 3 Ti or both Ni 3 Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) diffusion zone was free from intermetallic phases for all joints processed. Chemical composition of the reaction layers was determined in atomic percentage by energy dispersive spectroscopy and confirmed by X-ray diffraction study. Room-temperature properties of the bonded joints were characterized using microhardness evaluation and tensile testing. The maximum hardness value of ~800 HV was observed at TiA/Ni interface for the bond processed at 1073 K (800 °C). The hardness value at Ni/SS interface for all the bonds was found to be ~330 HV. Maximum tensile strength of ~206 MPa along with ~2.9 pct ductility was obtained for the joint processed at 1023 K (750 °C). It was observed from the activation study that the diffusion rate at TiA/Ni interface is lesser than that at the Ni/SS interface. From microhardness profile, fractured surfaces and fracture path, it was demonstrated that failure of the joints was initiated and propagated apparently at the TiA/Ni interface near Ni 3 Ti intermetallic phase.

Beschreibung: The aim of this work is to evaluate the influence of the prior austenite grain size (AGS) on the austenite-to-pearlite isothermal decomposition in a Fe-C-Mn hypoeutectoid steel. Due to the strong influence, grain boundaries have on pearlite transformation kinetics, morphological aspects of pearlite from two conditions with very different AGS were studied and characterized. Results allow us to conclude that the formation of pearlite and ferrite are favored for small AGS values, whereas a larger AGS led to an increase in the total amount of pearlite volume fraction. Furthermore, the average size of pearlitic colonies increased with increasing AGS, and it appears that the interlamellar spacing of the pearlite does not depend on AGS, but instead, is controlled by the isothermal decomposition temperature. Finally, it was observed that the ratio between lamellar thickness of ferrite and cementite depended on AGS.

Beschreibung: Ferrite-carbide and ferrite-martensite dual-phase microstructures have been produced in a low-carbon steel with different ferrite grain structures such as, uniform distribution of coarse- and very fine-ferrite grains, and bimodal distribution of ferrite grain sizes comprising of coarse grains (~12  μ m) and very fine grains (〈2  μ m). Very fine-grained dual-phase structure offered the best combination of tensile-strength and ductility among all the samples. The above microstructures have been compared in terms of their strain-hardening rate and the mechanism of plastic deformation.

Beschreibung: Copper zinc tin sulfide (Cu 2 ZnSnS 4 ) has been receiving a lot of attention in recent years as a new, alternative absorber for the production of cheap thin film solar cells owing to the high natural abundance of all the constituents, its tunable direct-band-gap energy, and its large optical absorption coefficient. In addition, to overcome the problem of expensive vacuum-based methods, solution-based approaches are being developed for Cu 2 ZnSnS 4 deposition. In this study, Cu 2 ZnSnS 4 thin films were grown on soda lime glass substrates via the sulfurization of solution grown Cu 2 SnS 3 /ZnS stacked sulfide layers. A new facile route to overcome the difficulty of depositing Cu 2 ZnSnS 4 thin film with a desired stoichiometric composition in a single cation solution has been presented. The influences of deposition cycles of layers on the morphological, compositional, structural, and optical properties of the samples were investigated. It was observed from scanning electron microscopy (SEM) images that the films were continuous and composed of homogenously distributed large grains. Possible chemical formulations of the best samples were predicted to be Cu 1.99 Zn 1.25 Sn 1.00 S 3.76 and Cu 1.97 Zn 1.03 Sn 1.29 S 3.71 via energy-dispersive X-ray spectroscopy (EDXS) results. The X-ray diffraction (XRD) patterns of the samples matched very well with the reference values. The Raman-scattering analysis of the films proved the phase purity of the CZTS samples. The optical absorption coefficient of the films was found to be about 10 4  cm −1 based on absorbance spectroscopy. The optical band gaps of the films were estimated to be between 1.36 and 1.50 eV. From these we are able to conclude that CZTS thin films can be effectively obtained via the vacuum-atmosphere sulfurization of Cu 2 SnS 3 /ZnS stacked sulfide layers.

Beschreibung: The rapid solidification of a peritectic alloy is studied. Various 2D and 3D characterization techniques were effectively utilized to investigate the effect of cooling rate on both the phase fractions and the shrinkage porosity. Particles of Al-36 wt pct Ni were produced using a drop tube impulse system. Neutron diffraction and Rietveld analysis were used to quantify the phases formed during solidification. The microstructure of the produced particles was analyzed using SEM and X-ray microtomography. It was found that increasing cooling rate resulted in decreasing the Al 3 Ni 2 to Al 3 Ni ratio. Also, quantitative analysis of the microtomography images revealed that the volume percent of porosity increased with increasing particle size. The distribution of porosity was found to be significantly different in small and large particles. It was concluded that the extensive growth of Al 3 Ni 2 at lower cooling rates followed by the peritectic reaction made the feeding of the shrinkages more difficult, and as a result, the volume percent of porosity increased. Other findings showed that high cooling rate during solidification would result in the formation of a quasicrystalline phase, known as D-phase, and suppression of the primary Al 3 Ni 2 . Also, investigation of the 3D structure of the solidified particles revealed that large particles of Al-36 wt pct Ni contain multiple nucleation sites, while smaller particles contain only one single nucleation site.

Beschreibung: Thin films of nanoporous palladium (np-Pd) were produced from binary palladium-nickel (Pd-Ni) precursor alloys. A suitable precursor alloy and a method of dealloying to yield optimum nanoporosity (average pore/ligament size of 7 nm) were developed by studying the effects of various processing parameters on final microstructure. To obtain crack-free np-Pd, a 100 nm thin film of 20 at. pct Pd (80 at. pct Ni) can be dealloyed for ~5 hours in a 1 M solution of sulfuric acid, with oleic acid and oleylamine added as surfactants. Both shorter and longer dealloying times, as well as heating, inhibit the formation of crack-free np-Pd. Stress measurements at different stages of dealloying revealed that the necessary dealloying time is determined by the diffusion-controlled corrosion reaction occurring within the thin film during dealloying. Strong interaction between hydrogen and np-Pd was reflected in the stress evolution during dealloying. A mechanism is proposed for the formation of a Ni-rich dense top layer that results from H-induced swelling during initial dealloying and permits the development of defect-free np-Pd beneath, by limiting the speed of dealloying.

Beschreibung: A detailed electrochemical study of cubic α -Fe 2 O 3 microparticles has been carried out in strong aqueous LiOH electrolyte. The α -Fe 2 O 3 was synthesized hydrothermally and investigated in the form of an electrochemical cell using an alkaline solution, ‘ α -Fe 2 O 3 |LiOH (saturated), ZnSO 4 (1 M)|Zn’. In this cell, the α -Fe 2 O 3 cathode showed a reversible capacity of ca 220 mAh/g within cut-off voltages of 0 and 1.5 V under the constant current of 0.3 mA. The electrochemical performance was attributed to the reversible formation of both proton and lithium intercalation products (FeOOH and LiFeO 2 ) detected in the cathode material. Interestingly, at a lower discharge current of 0.1 mA, some of the hematite phase was reduced to metallic iron after yielding 336 mAh/g. The various possible electro-reduction reactions, which have direct electro-hydrometallurgical implications, are analyzed and discussed.

Beschreibung: Possible phase transformation kinetic pathways from the high temperature β phase to the low temperature ( α  +  β ) two-phase Ti alloys were analyzed using the graphical thermodynamic method and the assumption that diffusionless and displacive transformations take place much faster than phase separation which requires long-range diffusion. It is shown that depending on the composition of a β -stabilizing element, many transformation mechanisms are possible, involving competing continuous and discontinuous displacive/diffusional transformations. We discuss the proposed phase transformation sequences employing existing experimental microstructures.

Beschreibung: We performed molecular dynamics simulations and investigated interactions between a Mg 17 Al 12 precipitate and a basal dislocation in magnesium. Modified embedded-atom method potentials for multiple-component systems were used in our simulations. The simulation results show that the basal dislocation is able to shear through the matrix and the precipitate/matrix interface, without creating a loop around the precipitate. The precipitate is only elastically deformed by the external shear strain. This interaction can be considered an extreme case of the Orowan mechanism when the strength of the precipitate/matrix interface is weak. Cross slip of the basal dislocation was observed when the precipitate size was 3.0 nm. The dislocation changed its slip plane to another basal plane via the $ (01\overline{1} 0) $ prismatic and the $ (0\overline{1} 11) $ pyramidal planes, creating jogs on these non-basal planes. The jogs had low mobility and debris was created when the jogs were dragged forward by the Shockley partial dislocations.

Beschreibung: The effect of Hf addition on microstructures, phase relationships, microhardness, and magnetic properties of Fe 50 Al 50− n Hf n alloys for n  = 1, 3, 5, 7, and 9 at. pct has been investigated. At all investigated compositions, the ternary intermetallic HfFe 6 Al 6 τ 1 phase forms due to the limited solid solubility of Hf in FeAl phase and tends to develop a eutectic phase mixture with the Fe-Al-based phase. The Hf concentration of the eutectic composition is found to be 7 at. pct from the microstructural examinations and the eutectic phase transition temperature is determined as 1521 K (1248 °C) independent of Hf amount by differential scanning calorimetry measurements. Furthermore, the enthalpies and activation energies (based on Kissinger and Ozawa methods) of eutectic phase transitions are reported. The minimum activation energy is calculated for the fully eutectic composition. Moreover, variation of the microhardness of Fe-Al-based alloys as a function of the Hf content is investigated, and its dependence on the thermal history of the alloys is explained.

Beschreibung: The anisotropic coefficients of Hill’s yield criterion are determined through a novel genetic algorithms-based multi-objective optimization approach. The classical method of determining anisotropic coefficients is sensitive to the effective plastic strain. In the present procedure, that limitation is overcome using a genetically evolved meta-model of the entire stress strain curve, obtained from uniaxial tension tests conducted in the rolling direction and transverse directions, and biaxial tension. Then, an effective strain that causes the least error in terms of two theoretically derived objective functions is chosen. The anisotropic constants evolved through genetic algorithms correlate very well with the classical results. This approach is expected to be successful for more complex constitutive equations as well.

Beschreibung: High-temperature oxidation of molten aluminum was investigated by high-resolution electron microscopes in order to determine the possibility of heterogeneous nucleation of aluminum grains on oxide for the grain refinement and structural uniformity of intensively melt-sheared aluminum alloys. High-resolution observations detect initial amorphous phase and gamma-alumina phase and show fine clusters with size of about 150 to 200 nm composed of extremely fine aluminum grains and gamma-alumina or amorphous aluminum oxide. Furthermore, high-resolution lattice images and diffraction patterns show no orientation relationship, although there is a specific orientation between gamma-alumina and aluminum along (111)[110] with high potency of heterogeneous nucleation. The volumetric shrinkage by the transformation of gamma- into alpha-alumina causes the surface oxide films to repeatedly rupture and leads to the creation of channels to the base melt surface for further oxidation of fresh metal. Based on the observations, the mechanism of high-temperature oxidation of molten aluminum and formation of the fine clusters as well as the possibility of the heterogeneous nucleation of aluminum grains are discussed.

Beschreibung: We report a W-rich alloy (W-7Cr-9Fe, at. pct) produced by high-energy ball milling, with alloying additions that both lower the densification temperature and retard grain growth. The alloy’s consolidation behavior and the resultant compacts’ microstructure and mechanical properties are explored. Under one condition, a 98 pct dense compact with a mean grain size of 130 nm was achieved, and exhibited a hardness of 13.5 GPa, a dynamic uniaxial yield strength of 4.14 GPa in Kolsky bar experiments, and signs of structural shear localization during deformation.

Beschreibung: The effect of simple strain path changes as well as post-deformation continuous cooling rate during thermomechanical-controlled processing of microalloyed steel was studied using laboratory physical simulation. The phase transformation characteristics were directly analyzed by dilatometry under various cooling rates. The microstructures of the transformation products were characterized quantitatively using EBSD. The results have shown that while strain path changes impose a considerable influence on the hot flow behavior of the austenite, the cooling rate following hot deformation is the determining factor of the phase transformation mechanism and behavior which establishes the final transformation products and subsequent mechanical properties.

Beschreibung: Synchrotron X-ray radiography at 125 frames per second was used to study deformation mechanisms in semi-solid Fe-C alloys at high solid fraction and shear strain rates of 10 −1 /s. An image correlation approach was also used to quantify the shear strain fields and study shear-induced dilation and the origin of shear cracking. It was shown that, at high solid fraction (90 to 93 pct solid), rearrangement including rotation and translation of solid particles became restricted and shear strain localized into narrow liquid-filled channels/fissures. Shear cracking was shown to originate from inadequate liquid flow into the expanding spaces between solid particles caused by shear-induced dilation. At lower solid fraction (~85 pct solid), solid particles rearranged with a significantly higher component of rotation leading to more shear-induced dilation and a wider shear band.

Beschreibung: Oxidation of Ti6Al4V is studied by X-ray photoelectron spectroscopy (XPS). Oxide layer growth was monitored on the Ti6Al4V surface for 24 hours. The surface was previously etched with Ar + ions under ultra-high vacuum conditions. XPS spectra show that TiO and Ti 2 O 3 , together with Al 2 O 3 , were the earliest oxides formed. Vanadium, despite being detected in its elementary form in the bulk, was not found in any of its oxidized states. TiO 2 , directly related to the good performance of Ti6Al4V for biomedical applications, did not contribute significantly to the passive layer at the beginning; nevertheless, it was identified after the oxidation process progressed to a more advanced stage. This behavior indicates that reoxidation of Ti6Al4V permits auto-healing of its passive layer, with the presence of TiO 2 , even in conditions of low oxygen availability.

Beschreibung: The boundary structures of twins in the hexagonal close-packed metal zirconium were studied. High-resolution transmission electron microscopy was used to characterize the boundary structure of \(\{10\bar{1}2\}\) (T1), \(\{\bar{1}\bar{1}21\}\) (T2), and \(\{\bar{1}\bar{1}22\}\) (C1) twins on the atomic level. Basal–prismatic (B–P) plane faceting is observed along the T1 twin boundaries, matching previous observations of T1 twins in magnesium. C1 twins are observed to form basal–pyramidal (B–Py) facets along otherwise perfect twin planes. T2 twins exhibit faceting that aligns prism planes with second-order pyramidal planes across the boundary (P–Py facets). As a function of the crystallography, T2 twins appear less likely to accommodate large deviations from perfect twin planes by P–Py faceting alone, and may rely on small dislocation-accommodated facets to achieve arbitrary boundary planes. The structure of these boundaries, specifically the modes by which faceting is permitted, has a direct impact on boundary mobility. In addition, the boundary structure of two C1 twins during a twin–twin interaction event is observed, and is compared to previous observations of tensile twin–twin interactions in magnesium.

Beschreibung: The effect of forward and reverse torsion on flow behavior and microstructure evolution, particularly dynamic and static spheroidization, on Ti-6Al-4V with an alpha lamella colony microstructure was studied. Testing was undertaken sub beta transus [1088 K (815 °C)] at strain rates of either 0.05 or 0.5 s −1 . Quantitative metallography and electron back scatter diffraction has identified that a critical monotonic strain ( ε c ) in the range of 0.3 to 0.6 is required to initiate rapid dynamic spheroidization of the alpha lamella. For material deformed to strains below ε c and then reversed to a zero net strain the orientation relationships between alpha colonies are close to ideal Burgers, enabling prior beta grains to be fully reconstructed. Material deformed to strains greater than ε c and reversed lose Burgers and no beta reconstruction is possible, suggesting ε c is the strain required to generate break-up of lamella. Static spheroidization is, however, sensitive to strain path around ε c . Annealing at 1088 K (815 °C) for 4 hours for material subjected to 0.25 forward + 0.25 forward strain produces 48 pct spheroidized grains while material with 0.25 forward + 0.25 reverse strain has 10 pct spheroidization. This is believed to be a direct consequence of different levels of the stored energy between these two strain paths.

Beschreibung: Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe 3 C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo 2 C (aspect ratio ranging from 10 to 25) and plate-shaped M 4 C 3 carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe 3 C, and interlath-retained austenite decomposed into ferrite and elongated carbide.

Beschreibung: Multilayered Ti/Al/Nb composites were produced by the accumulative roll bonding (ARB) process utilizing pure Ti, Al, and Nb element sheets. Up to four cycles of ARB were applied to the composites. The microstructure and texture evolution on the Nb phase were studied by X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy, and electron backscattered diffraction. Nb and Ti layers necked and fractured as the number of ARB passes increased. After four ARB cycles, a nearly homogeneous distribution of Nb and Ti layers in Al matrix was achieved. As-received Nb sheet exhibited a fully lamellar structure and had a strong cold-rolling texture. After subjecting to ARB, slight grain refining was observed and the high-angle boundary fraction was increased. The intensity of the α -fiber was weakened, while that of the γ -fiber was strengthened during ARB. The texture evolution was attributed to partial recrystallization during the ARB process as a result of adiabatic heating.

Beschreibung: The paper reports the findings in the development of “A” segregation streaks in large ingots which were cast under various solidification conditions: different bottom shapes of ingots and metal pouring using inoculated stream. It is shown that changed conditions bring about a decrease in the “A” segregation zones as well as reduced diameters and shorter lengths of “A” segregation streaks. A different design of the ingot bottom and smaller streaks reduce the chemical heterogeneity of ingots and improve the anisotropy of the forgings fabricated from the ingots.

Beschreibung: It has been suggested that microtextures related to deformation twinning can affect the stretch formability of hexagonal close-packed (hcp) metals. We study the evolution of strain localization and the formation of twin clusters in hcp polycrystals due to softening induced by twinning reorientation of the crystal lattice. We performed three-dimensional crystal plasticity finite element simulations of uniaxial and biaxial deformation using basal starting textures of various strengths. Weaker textures resulted in relatively homogeneous deformation, strong textured systems, however, showed significant shear band formation in biaxial tension. In this case, highly strained regions coincided very well with dense clusters of twins. Due to the scarcity of available slip systems, such shear localization due to twin clustering may lead to eventual crack initiation and failure. The spatial correlation of twins was analyzed and was found to be significantly affected by texture. The decay of the correlation function was well approximated by a power law, a characteristic of a wide range of phenomena associated with self-organized criticality.

Beschreibung: Amorphous Al 86 Ni 6 Y 6 Ce 2 (at. pct) flakes produced by melt spinning were consolidated using hot pressing at different conditions. The influence of pressing conditions on the crystallization behavior, thermal stability, and mechanical properties of the alloy has been studied through differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, microhardness, and compression test. The results show rapid solidification combined with the hot consolidation produce a highly dense sample (3.41 ± 0.2 g cm −3 ) with the ideal interflake bonding, good thermal stability, good microhardness (381 ± 12 HV), and remarkably high strength (910 ± 7 MPa) combined with 20 pct fracture strain were obtained at T  = 748 K (475 °C) and P  = 1.2 GPa in a lightweight Al-based material. The high mechanical properties mainly result from structural refinement during the controlled consolidation method.

Beschreibung: ZrB 2 -SiC composites were hot pressed at 2473 K (2200 °C) with graded amounts (5 to 20 wt pct) of SiC and the effect of the SiC addition on mechanical properties like hardness, fracture toughness, scratch and wear resistances, and thermal conductivity were studied. Addition of submicron-sized SiC particles in ZrB 2  matrices enhanced mechanical properties like hardness (15.6 to 19.1 GPa at 1 kgf), fracture toughness (2 to 3.6 MPa(m) 1/2 ) by second phase dispersion toughening mechanism, and also improved scratch and wear resistances. Thermal conductivity of ZrB 2 -SiC (5 wt pct) composite was higher [121 to 93 W/m K from 373 K to 1273 K (100 °C to 1000 °C)] and decreased slowly upto 1273 K (1000 °C) in comparison to monolithic ZrB 2  providing better resistance to thermal fluctuation of the composite and improved service life in UHTC applications. At higher loading of SiC (15 wt pct and above), increased thermal barrier at the grain boundaries probably reduced the thermal conductivity of the composite.

Beschreibung: A micro-tensile test system equipped with in situ monitoring of the in-plane displacements of a surface and an electron backscattered diffraction-based serial-sectioning technique were used to study the deformation (up to 2.4 pct axial plastic strain in tension) of a polycrystalline nickel micro-specimen. The experimental data include the global engineering stress-engineering strain curve, the local mesoscopic in-plane displacement and strain fields, the three-dimensional microstructure of the micro-specimen reconstructed after the tensile test, and the kernel-average misorientation distribution. The crystal plasticity finite element method using elasto-viscoplastic constitutive formulations was used to simulate the global and local deformation responses of the micro-specimen. Three different boundary conditions (BCs) were applied in simulation in order to study the effects of the lateral displacement (perpendicular to the loading direction) of the top and bottom faces of the specimen gage section. The simulation results were compared to the experimental results. The comparison between experiment and simulation results is discussed, based upon their implications for understanding the deformation of micro-specimens and the causes associated with uncertainties embedded in both experimental and numerical approaches. Also, the sensitivity of BCs to near-field and far-field responses of the micro-specimen was systematically studied. Results show that the experimental methodology used in the present study allows for limited but meaningful comparisons to crystal plasticity finite element simulations of the micro-specimen under the small plastic deformation.

Beschreibung: In this work, in-situ neutron diffraction measurements are performed for a martensitic steel in conjunction with crystal plasticity analysis. The results indicate that heterogeneous plastic contraction on transverse {200} grains is responsible for the observed nonlinear lattice strain evolution. The effect of slip properties on plastic contraction is computationally identified.

Beschreibung: The transformations of a high-strength 9Ni-Cr-Mo-V steel were characterized as a function of cooling rate by dilatometry, microhardness measurements, and microstructural characterization. The results demonstrate that this steel is extremely insensitive to changes in cooling rate, generating a duplex microstructure of coarse autotempered martensite within a matrix of fine lath martensite at nearly all cooling rates. The coarse autotempered martensite is observed even at very slow cooling rates, although the lath martensite becomes replaced by lath (or bainitic) ferrite.

Beschreibung: Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method ( i.e. , neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.

Beschreibung: Development of the methodology for creating reliable digital material representation (DMR) models of dual-phase steels and investigation of influence of the martensite volume fraction on fracture behavior under tensile load are the main goals of the paper. First, an approach based on image processing algorithms for creating a DMR is described. Then, obtained digital microstructures are used as input for the numerical model of deformation, which takes into account mechanisms of ductile fracture. Ferrite and martensite material model parameters are evaluated on the basis of micropillar compression tests. Finally, the model is used to investigate the impact of the martensite volume fraction on the DP steel behavior under plastic deformation. Results of calculations are presented and discussed in the paper.

Beschreibung: The influence of temperature on the dynamic tensile behavior of Zr has been investigated. Bullet-shaped Zr samples with two different textures were dynamically extruded at room temperature and 523 K (250 °C). A higher ductility was measured for samples deformed at elevated temperature as compared to those extruded at room temperature. This difference in ductility is discussed in terms of zirconium’s ability to accommodate plastic deformation via thermally enhanced slip activity, as evidenced by examination of the deformed microstructures.

Beschreibung: The tensile behavior of ductile ultra-high strength Fe-12 pctMn-0.3 pctC-2 pctAl austeno-ferritic steel was studied by in situ neutron diffraction measurement of the elastic lattice strains, dislocation density, stacking fault probability, and strain-induced transformation kinetics. Micro-yielding was observed in austenite, and the plastic deformation of ferrite remained very limited throughout the deformation. The analysis identified three contributions to the strain hardening: twinning-induced plasticity, transformation-induced plasticity, and the accumulation of a high density of geometrically necessary dislocations accommodating the strain mismatch at the phase boundaries.

Beschreibung: Low melting point Zr-based filler metals with melting point depressants (MPDs) such as Cu and Ni elements are used for titanium brazing. However, the phase transition of the filler metals in the titanium joint needs to be explained, since the main element of Zr in the filler metals differs from that of the parent titanium alloys. In addition, since the MPDs easily form brittle intermetallics, that deteriorate joint properties, the phase evolution they cause needs to be studied. Zr-based filler metals having Cu content from 0 to 12 at. pct and Ni content from 12 to 24 at. pct with a melting temperature range of 1062 K to 1082 K (789 °C to 809 °C) were wetting-tested on a titanium plate to investigate the phase transformation and evolution at the interface between the titanium plate and the filler metals. In the interface, the alloys system with Zr, Zr 2 Ni, and (Ti,Zr) 2 Ni phases was easily changed to a Ti-based alloy system with Ti, Ti 2 Ni, and (Ti,Zr) 2 Ni phases, by the local melting of parent titanium. The dissolution depths of the parent metal were increased with increasing Ni content in the filler metals because Ni has a faster diffusion rate than Cu. Instead, slow diffusion of Cu into titanium substrate leads to the accumulation of Cu at the molten zone of the interface, which could form undesirable Ti x Cu y intermetallics. This study confirmed that Zr-based filler metals are compatible with the parent titanium metal with the minimum content of MPDs.

Beschreibung: In the present study, bainite transformation kinetics was examined in low C-Mn steels with the addition of small amounts of B and Mo. This addition delays the onset of the bainite transformation. Mo addition causes transformation stasis at temperatures between 873 K and 823 K (600 °C and 550 °C) just below the bainite-start ( B s ) temperature, resulting from an incomplete bainite transformation. Post-stasis transformation after a prolonged hold proceeds by the formation of ferrite with a low dislocation density, and in Mo-containing alloys, often the formation of carbides. The volume fraction at which the transformation stops is higher for lower carbon contents and lower transformation temperatures. By contrast, at 773 K (500 °C), the bainite transformation accompanying cementite precipitation occurs regardless of microalloying and is completed after shorter hold times. EDX measurement performed on the Mo-added 0.15 pct C alloy with aberration-corrected STEM revealed that segregation at the bainite/austenite interphase boundary is small for Mn and negligible for Mo in the early stages of stasis, which does not support the incomplete transformation mechanism based on the solute drag theory for the alloys used.

Beschreibung: Hot compression tests were conducted on Ti-13V-11Cr-3Al beta-Ti alloy in the temperature range of 1203 K to 1353 K (930 °C to 1080 °C) and at strain rates between 0.001 and 1 s −1 The stress–strain curves showed pronounced yield point phenomena at high strain rates and low temperatures. The yield point elongation and flow stresses at the upper and lower yield points were related to the Zener–Hollomon parameter. It was found that dynamic recovery at low strain rates and dynamic recrystallization at high strain rates were the controlling mechanisms of microstructural evolution. The results also showed that strain rate had a stronger influence on the hot deformation behavior than temperature. The microstructural observations and constitutive analysis of flow stress data supported the change in the hot deformation behavior of the studied alloy varies with strain rate. For various applied strain rates, the activation energy for hot deformation was calculated in range of 199.5 to 361.7 kJ/mol. At low strain rates (0.001 and 0.01 s −1 ), the value of activation energy was very close to the activation energy for the diffusion of V, Cr, and Al in beta titanium. The higher value of activation energy for deformation at high strain rates (0.1 and 1 s −1 ) was attributed to the accumulation of dislocations and the tendency to initiate dynamic recrystallization.

Beschreibung: Although most of the research performed in bearing steel metallurgy aims to prevent crack nucleation and propagation, some applications require the exact opposite in order to study the role that disconnected surfaces inside the bulk material play when load is applied, or when fluids entrapped in surface cracks propagate tensile stresses or exacerbate corrosion. Four heat treatments have been designed to create controlled arrays of crack types and distributions in quenched and untempered steel normally used in the manufacture of bearings. The varieties of cracks studied include sparsely distributed martensite-plate cracks, fine-grain-boundary cracks, abundant martensite-plate cracks, and surface cracks. The intention was to create samples which can then be subjected to appropriate mechanical testing so that phenomena such as the appearance of “white-etching areas” or “white-etching cracks,” crack-lubricant interactions, or hydrogen trapping can be studied further.

Beschreibung: The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).

Beschreibung: MCrAlY coatings are usually adopted to improve the environmental resistance of Ni-based superalloy components of turbine engines against high-temperature oxidation and hot corrosion. In this work, CoNiCrAlYRe coatings were produced by atmospheric plasma spraying. The coatings exhibited relatively low oxygen content and porosity. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses revealed that the high-temperature exposure at 1383 K (1110 °C) promoted the growth of an oxide scale on the top surface being composed of a continuous and dense Al 2 O 3 inner layer followed by an outer mixed layer (Cr 2 O 3 and spinels). The oxide scale was mainly composed of Al 2 O 3 , while the formation of mixed oxides occurred at lesser extent. After high-temperature exposure, the formation of internal oxides in some areas reduced the inter-lamellar cohesion, so that a decrease in microhardness was found.

Beschreibung: The evolution of micro-damage and deformation of each phase in the composite plays a pivotal role in the clarification of deformation mechanism of composite. However, limited model and mechanical experiments were conducted to reveal the evolution of the deformation of the two phases in the tungsten fiber reinforced Zr-based bulk metallic glass composite. In this study, quasi-static compressive tests were performed on this composite. For the first time, the evolution of micro-damage and deformation of the two phases in this composite, i.e. , shear banding of the metallic glass matrix and buckling deformation of the tungsten fiber, were investigated systematically by controlling the loading process at different degrees of deformation. It is found that under uniaxial compression, buckling of the tungsten fiber occurs first, while the metallic glass matrix deforms homogeneously. Upon further loading, shear bands initiate from the fiber/matrix interface and propagate in the metallic glass matrix. Finally, the composite fractures in a mixed mode, with splitting in the tungsten fiber, along with shear fracture in the metallic glass matrix. Through the analysis on the stress state in the composite and resistance to shear banding of the two phases during compressive deformation, the possible deformation mechanism of the composite is unveiled. The deformation map of the composite, which covers from elastic deformation to final fracture, is obtained as well.

Beschreibung: In this investigation, the effect of hot extrusion process has been studied on the microstructure and tensile properties of aluminum matrix composite containing different amounts (10, 15, and 20 wt pct) of Al 4 Sr intermetallic phase. Microstructural examinations assessed by scanning electron microscopy revealed that hot extrusion breaks large Al 4 Sr particles and reduces their length tremendously. It was also found that although the addition of Al 4 Sr content in the composite reduces ultimate tensile strength and elongation values, hot extrusion improves tensile results significantly. Remarkable result of this study was concerned with significant improvement in the toughness of hot-extruded Al-10 wt pct Al 4 Sr composite in which elongation values raised up to 22 pct. Therefore, optimum amount of Al 4 Sr intermetallic in the composite was found to be 10 wt pct. Fractographic examinations revealed that hot extrusion encourages ductile mode of fracture by introducing homogeneous distribution of fine dimples on the fracture surface of the composites.

Beschreibung: The surface properties of 40CrMnMo7 steel have been increased by ionic nitriding thermo chemical treatment followed by laser quenching. Nitriding treatment increases the wear resistance and decreases the wear coefficient, guaranteeing an uniform, though shallow, hardening depth. Combining nitriding with laser quenching has been shown to increase of the hardening depth, while retaining the advantages obtained by the first treatment. In this work the effect of laser quenching parameters on microstructure and mechanical properties of 40CrMnMo7 steel have been investigated and predictive models developed suitable for further optimization of the process. DoE has been exploited to reduce the number of experiments and for evaluating by statistic methods the optimized process parameters.

Beschreibung: The evolution of local d -spacings between lattice planes of bainitic ferrite in a high strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)), has been determined to calculate the thermal expansion behavior. For this purpose, in-situ continuous cooling tests have been carried out in a high-energy synchrotron X-ray diffractometer. The results indicate thermal anisotropy in the bainitic ferrite planes.

Beschreibung: Influence of nitrogen content on low cycle fatigue life and fracture behavior of 316LN stainless steel (SS) alloyed with 0.07 to 0.22 wt pct nitrogen is presented in this paper over a range of total strain amplitudes ( ± 0.25 to 1.0 pct) in the temperature range from 773 K to 873 K (500 °C to 600 °C). The combined effect of nitrogen and strain amplitude on fatigue life is observed to be complex i.e. , fatigue life either decreases/increases with increase in nitrogen content or saturates/peaks at 0.14 wt pct N depending on strain amplitude and temperature. Coffin–Manson plots (CMPs) revealed both single-slope and dual-slope strain-life curves depending on the test temperature and nitrogen content. 316LN SS containing 0.07 and 0.22 wt pct N showed nearly single-slope CMP at all test temperatures, while 316LN SS with 0.11 and 0.14 wt pct N exhibited marked dual-slope behavior at 773 K (500 °C) that changes to single-slope behavior at 873 K (600 °C). The changes in slope of CMP are found to be in good correlation with deformation substructural changes.

Beschreibung: Multimaterial fabrication such as joining of steel and aluminum is currently prominent in a variety of industries. Friction stir welding is a novel solid-state welding process that causes good joint strength between steel and aluminum. However, the phenomenon contributing significant strength at the interface is not yet clear. In the present study, the interface of the friction stir lap-welded aluminum and coated steel sheet having joint strength maximum (71.4 pct of steel base metal) and minimum, respectively, under two parameter combinations, i.e. , 1000 rpm 50 mm min −1 and 500 rpm 100 mm min −1 , was exclusively characterized by X-ray diffraction, transmission electron microscopy (TEM), concentration profile, and elemental mapping by electron-probe microanalysis. A TEM-assisted EDS study identifies the morphologies of large size Al 13 Fe 4 and small size Fe 3 Al-type intermetallic compounds at the interface. The diffusion-induced intermetallic growth (thickness) measured from a backscattered image and concentration profile agreed well with the numerically calculated one. The growth of these two phases at 1000 rpm 50 mm min −1 is attributed to the slower cooling rate (~3.5 K/s) with higher diffusion time (44 seconds) along the interface in comparison to the same for 500 rpm 100 mm min −1 with faster cooling rate (~10 K/s) and less diffusion time (13.6 seconds). The formation of thermodynamically stable and hard intermetallic phase Al 13 Fe 4 at 1000 rpm and travel speed 50 mm min −1 in amounts higher than 500 rpm and a travel speed of 100 mm min −1 results in better joint strength, i.e. , 71.4 pct, of the steel base metal.

Beschreibung: X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were used to investigate the microstructures and orientation relationships (ORs) of Nb-16Si-22Ti-2Al-2Hf-(2,17)Cr alloys (hereafter referred to as 2Cr and 17Cr alloys, respectively). The mechanical properties of the two alloys at room and/or high temperatures were compared. The 2Cr alloy comprised Nb SS and (α + β)-Nb 5 Si 3 phases, while the 17Cr alloy consisted of Nb SS , (α + β)-Nb 5 Si 3 and Laves Cr 2 Nb phases with a C15 structure. The β-Nb 5 Si 3 and Laves Cr 2 Nb phases exhibited variable ORs with respect to the Nb SS phase. The Laves Cr 2 Nb phase was found to play a negative role on the fracture toughness at room temperature and on the compressive strength at temperatures from 1523 K to 1623 K (1250 °C to 1350 °C). The fracture toughness and the compressive yield strength at 1623 K (1350 °C) both decreased from 14.4 to 10.3 MPa m 1/2 and from 300 to 85 MPa, respectively, when the nominal Cr content increased from 2 to 17 at. pct. Finally, the fracture modes of these typical Nb SS /Nb 5 Si 3 and Nb SS /Nb 5 Si 3 /Cr 2 Nb microstructures under bending and compression conditions at room and high temperatures were investigated and discussed.

Beschreibung: Microstructural variations between the bond line and the base alloy of welds have been reported in various nickel-base superalloys that have found their applications as structural materials in aero- and land-based engines. This microstructural variation occurs within 50 to 100  μ m of majority of welds. Thus, in order to characterize the strength variations between the weld and the base alloy, mechanical testing at micron level is required. This paper presents the use of a newly developed microtensile testing system for an in situ micro-mechanical testing of a powder metallurgy nickel-base superalloy, RR1000 performed in a focused ion beam scanning electron microscope.

Beschreibung: The effect of thickness and annealing temperature on magnetic properties of ultrathin γ-Fe 2 O 3 films with MgO buffer layer grown on silicon substrate is investigated. The saturation magnetization and coercive force of samples at room temperature increase with increasing of annealing temperature, and decrease as annealing temperature is above 873 K (600 °C). The saturation magnetization of samples decreases with increasing of the thickness of γ-Fe 2 O 3 at room temperature. The samples with 3 to 4 nm thick γ-Fe 2 O 3 annealed at 873 K (600 °C) show saturation magnetization of about 400 emu/cm 3 , which is close to the bulk value of ~390 emu/cm 3 within the error range.

Beschreibung: A class of low-alloy steels often referred to as SA508 represent key materials for the manufacture of nuclear reactor pressure vessels. The alloys have good properties, but the scatter in properties is of prime interest in safe design. Such scatter can arise from microstructural variations but most studies conclude that large components made from such steels are, following heat treatment, fully bainitic. In the present work, we demonstrate with the help of a variety of experimental techniques that the microstructures of three SA508 Gr.3 alloys are far from homogeneous when considered in the context of the cooling rates encountered in practice. In particular, allotriomorphic ferrite that is expected to lead to a deterioration in toughness, is found in the microstructure for realistic combinations of austenite grain size and the cooling rate combination. Parameters are established to identify the domains in which SA508 Gr.3 steels transform only into the fine bainitic microstructures.

Beschreibung: Annealing of as-cast Mg-Al alloys (3, 6, and 9 wt pct Al) and Mg-Zn alloys (1.5, 4.0, and 5.5 wt pct Zn) was performed at 603 K and 673 K (330 °C and 400 °C) for 1, 2, 4, and 8 hours to systematically investigate the variation of second phase fraction and solute concentration profile in Mg matrix using SEM image analysis and EMPA-WDS, respectively. To calculate second phase fraction and solute concentration profile, a diffusion model considering the moving boundary was constructed, and the experimental results were successfully explained by the model. The expansion of model to ternary Mg-Al-Zn alloy is also described. This model can be used for the optimization of homogenization process of the Mg alloys.

Beschreibung: Intercritically annealed 10 pct Mn steel has been shown to exhibit an excellent combination of strength and ductility due to the plasticity-enhancing mechanisms of mechanical twinning and strain-induced martensite transformation occurring in sequence. This mechanical behavior is only achieved for a multi-phase microstructure obtained after annealing within a specific intercritical temperature range. A model for the selection of the optimal intercritical annealing temperature was developed to achieve a high strength-ductility balance for 10 pct Mn multi-phase steel. The model considers the room temperature stacking fault energy and the thermodynamic stability of the retained austenite.

Beschreibung: The effects of M 23 C 6 ( M  = Cr, Fe) on the high-temperature performance of the NiCrFe-7 welding rods and weld metals were studied by high-temperature tensile tests and microstructure analysis. M 23 C 6 at the grain boundaries (GBs) has a cube-on-cube coherence with one grain in the NiCrFe-7 weld metals, and the adjacent M 23 C 6 has the coherence relationship with the same grain. The grain with a coherent M 23 C 6 has a Cr-depletion region. The number and size of M 23 C 6 particles can be adjusted by heat treatment and alloying. There are two temperatures [ T E1 : 923 K to 1083 K (650 °C to 810 °C) and T E2 : 1143 K to 1203 K (870 °C to 930 °C)] at which the GBs and grains of the NiCrFe-7 welding rod have equal strength during the high-temperature tensile test. When the temperatures are between T E1 and T E2 , the strength of the GBs is lower than that of the grains, and the tensile fractures are intergranular. When the temperatures are below T E1 or over T E2 , the strength of the GBs is higher than that of the grains, and the tensile fractures are dimples. M 23 C 6 precipitates at the GBs, which deteriorates the ductility of the welding rods at temperature between T E1 and T E2 . M 23 C 6 aggravates ductility-dip-cracking (DDC) in the weld metals. The addition of Nb and Ti can form M X ( M  = Ti, Nb, X = C, N), fix C in grain, decrease the initial precipitation temperature of M 23 C 6 , and mitigate the precipitation of M 23 C 6 , which is helpful for minimizing DDC in the weld.

Beschreibung: With the use of today’s computer softwares, phase diagrams can readily be plotted with a wide choice of variables on the axes. Hillert defined a true phase diagram as a diagram where each point uniquely defines the stable phases. He also showed that not all choices of axis variables give true phase diagrams. In this note we will demonstrate that although the rules stated by Hillert are necessary they are not sufficient to have true phase diagrams.

Beschreibung: Ce 0.9− x Gd 0.1 Er x O 1.9− x /2 (0 ≤  x  ≤ 0.1) (EGDC) powders were successfully synthesized with a fast and facile cellulose-templating method for the first time and characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The samples were calcined at a relatively low calcination temperature of 773 K (500 °C). The sintering behavior of the calcined EGDC powders was also investigated at 1673 K (1400 °C) for 6 hours. Calcined Ce 0.9− x Gd 0.1 Er x O 1.9− x /2 (0 ≤  x  ≤ 0.1) powders and sintered Ce 0.9− x Gd 0.1 Er x O 1.9− x /2 (0 ≤  x  ≤ 0.1) pellets crystallized in the cubic fluorite structure. It was found that the relative densities of the sintered EGDC pellets were over 95 pct for all the Er contents studied. Moreover, the effect of Er content on the ionic conductivity of the gadolinium-doped ceria (GDC, Ce 0.9 Gd 0.1 O 1.90 ) was investigated. The highest ionic conductivity value was found to be 3.57 × 10 −2  S cm −1  at 1073 K (800 °C) for the sintered Ce 0.82 Gd 0.1 Er 0.08 O 1.91 at 1673 K (1400 °C) for 6 hours.

Beschreibung: The powder diffraction patterns of spherical nanocrystals made of five different fcc metals were generated using atomistic models within a Molecular Dynamics simulation. Static and dynamic effects are interpreted and discussed within the framework of two different approaches, respectively, based on (1) a Reciprocal Space and (2) a Direct Space representation of diffraction. Chosen elements display a wide range of properties, especially related to material stiffness and elastic anisotropy, so to deeply challenge interpretation paradigms. The effect of the shape on static and dynamic features is also addressed.

Beschreibung: Detailed transmission electron microscopy examinations verified that α ′-martensite formed in micron-sized pillars is nearly dislocation free, surprisingly different than its counterpart in bulk samples, which usually contains a high dislocation density. Furthermore, the martensite was found to nucleate at the intersection between two packets of stacking faults in this low stacking fault energy material. A corresponding mechanism for the nucleation and growth of martensite in micron-sized pillars was proposed.

Beschreibung: Effect of tungsten on transient creep deformation and minimum creep rate of reduced activation ferritic-martensitic (RAFM) steel has been assessed. Tungsten content in the 9Cr-RAFM steel has been varied between 1 and 2 wt pct, and creep tests were carried out over the stress range of 180 and 260 MPa at 823 K (550 °C). The tempered martensitic steel exhibited primary creep followed by tertiary stage of creep deformation with a minimum in creep deformation rate. The primary creep behavior has been assessed based on the Garofalo relationship, \( \varepsilon = \varepsilon_{\text{o}} + \varepsilon_{\text{T}} [1-\exp (-r^{\prime} \cdot t)] + \dot{\varepsilon }_{\text{m}} \cdot t \) , considering minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) instead of steady-state creep rate \( \dot{\varepsilon }_{\text{s}} \) . The relationships between (i) rate of exhaustion of transient creep r ′ with minimum creep rate, (ii) rate of exhaustion of transient creep r ′ with time to reach minimum creep rate, and (iii) initial creep rate \( \dot{\varepsilon }_{\text{i}} \) with minimum creep rate revealed that the first-order reaction-rate theory has prevailed throughout the transient region of the RAFM steel having different tungsten contents. The rate of exhaustion of transient creep r ′ and minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) decreased, whereas the transient strain ɛ T increased with increase in tungsten content. A master transient creep curve of the steels has been developed considering the variation of \( \frac{{\left( {\varepsilon - \varepsilon_{\text{o}} } \right)}}{{\varepsilon_{\text{T}} }} \) with \( \frac{{\dot{\varepsilon }_{\text{m}} \cdot t}}{{\varepsilon_{\text{T}} }} \) . The effect of tungsten on the variation of minimum creep rate with applied stress has been rationalized by invoking the back-stress concept.

Beschreibung: Thermodynamic calculations based on the CALPHAD method are nowadays often applied in the design of new materials due to increasing demands on shorter lead times for development. However, such calculations rely heavily on the assessed thermodynamic descriptions, which in turn rely on the amount and quality of available experimental data, especially for binary and ternary sub-systems. The ternary Co-Cr-C system is an extremely important subsystem to, e.g. , multi-component cemented carbide grades, such as W-Co-Cr-M-C (M = Ti,Ta,Nb,V,Zr,Hf), as well as Cr-containing Co-base alloys. In the case of the Co-Cr-C system, there is a lack of reliable data on the solubility of Co in Cr-carbides. Therefore, the present work concerns an experimental study of the solubility of Co in all three of the Cr-based carbides, i.e. , Cr 23 C 6 , Cr 7 C 3 , and Cr 3 C 2 . This was done by synthesizing appropriate samples in the M 7 C 3 +M 23 C 6 +liquid and M 7 C 3 +M 3 C 2 +graphite three-phase fields. The results show that a recent thermodynamic description of the Co-Cr-C system is unable to reproduce the experimentally determined solubilities. Therefore, the present study provides important input for future alloy development and improvement of the thermodynamic description of the Co-Cr-C system.

Beschreibung: The nugget formation of resistance spot welding (RSW) on dissimilar material sheets of aluminum and magnesium alloys was studied, and the element distribution, microstructure, and microhardness distribution near the joint interface were analyzed. It was found that the staggered high regions at the contact interface of aluminum and magnesium alloy sheets, where the dissimilar metal melted together, tended to be the preferred nucleation regions of nugget. The main technical problem of RSW on dissimilar metal sheets of aluminum and magnesium alloys was the brittle-hard Al 12 Mg 17 intermetallic compounds distributed in the nugget, with hardness much higher than either side of the base materials. Microcracks tended to generate at the interface of the nugget and base materials, which affected weld quality and strength.

Beschreibung: The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to Δ T p ~ 245 K (245 °C), where a mixture of α -Ni dendrite, Ni 3 B dendrite, rod eutectic, and precipitates was obtained. If Δ T p 〈 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni 3 B and L → Ni/Ni 3 B); if Δ T p ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni 23 B 6 , Ni 23 B 6 → Ni/Ni 3 B, and L r → Ni/Ni 3 B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as L r → Ni/Ni 3 B; it also showed that this velocity decreases with increasing Δ T r , which can be explained by reduction of the residual liquid fraction and decrease of Ni 23 B 6 decomposition rate.

Beschreibung: In a C-Mn steel without Nb, the mechanism of hot ductility loss and recovery has been understood. The specimens were solution treated at 1673 K (1400 °C), subsequently cooled to 1273 K (1000 °C) at a rate of 1 or 20 K/s and finally held at the temperature. At a rate of 1 K/s, the segregation concentration of sulfur at prior austenite grain boundaries decreases gradually with holding time. At the rate of 20 K/s, the segregation concentration of sulfur shows a convex profile in a time versus segregation concentration plot. Such segregation behaviors of sulfur are deeply related to the MnS reaction during cooling to 1273 K (1000 °C) or holding at the temperature. The high-temperature intergranular fracture observed in this steel is due to the sulfur segregated at the grain boundaries. The recovery of hot ductility results from the combination between the decrease in sulfur segregation concentration governed by the MnS reaction and the overall decohesion at the interface of the MnS particles which act as a strong sink of the free sulfur tending to segregate to the grain boundaries.

Beschreibung: Effects of Mn addition (17, 19, and 22 wt pct) on tensile and Charpy impact properties in three austenitic Fe-Mn-C-Al-based steels were investigated at room and cryogenic temperatures in relation with deformation mechanisms. Tensile strength and elongation were not varied much with Mn content at room temperature, but abruptly decreased with decreasing Mn content at 77 K (−196 °C). Charpy impact energies at 273 K (0 °C) were higher than 200 J in the three steels, but rapidly dropped to 44 J at 77 K (−196 °C) in the 17Mn steel, while they were higher than 120 J in the 19Mn and 22Mn steels. Although the cryogenic-temperature stacking fault energies (SFEs) were lower by 30 to 50 pct than the room-temperature SFEs, the SFE of the 22Mn steel was situated in the TWinning-induced plasticity regime. In the 17Mn and 19Mn steels, however, α ′-martensites were formed by the TRansformation-induced plasticity mechanism because of the low SFEs. EBSD analyses along with interrupted tensile tests at cryogenic temperature showed that the austenite was sufficiently deformed in the 19Mn steel even after the formation of α ′-martensite, thereby leading to the high impact energy over 120 J.

Beschreibung: As-quenched martensite was pre-tempered at 623 K and 923 K (350 °C and 650 °C), and then it reverted to austenite by intercritical annealing at 998 K (725 °C) in a Fe-2Mn-0.3C alloy. Pre-tempering at 623 K (350 °C) accelerates austenite formation, while pre-tempering at 923 K (650 °C) significantly retards it. It is proposed that austenite nucleation is accelerated by increasing the number density and particle size of cementite during tempering, whereas austenite growth is retarded by Mn enrichment in cementite during tempering at high temperature, leading to opposite effects of pre-tempering on reversion kinetics.

Beschreibung: Effect of carbon on the microstructure and mechanical properties of 0.011 and 0.032 pct carbon dual-phase steels was investigated. r m value was increased to 1.52 at around 400 MPa tensile strength level through the optimal design in the steel chemistry and proper control of phase transformation during continuous galvanizing cycle. The isolated martensite particles are expected to increase the strength but are expected not to be desirable for the deep drawability.

Beschreibung: In this study, a serial sectioning technique was employed in order to visualize the three-dimensional (3D) structure, and to accurately simulate the mechanical and thermal behaviors of SiC particle-reinforced Al composites. Sequential, two-dimensional (2D) optical images of the microstructure were acquired after polishing, and then reconstructed to develop 3D geometries for microstructural analyses and finite element modeling. Experimental compressive and thermal expansion tests were performed for comparison with the finite element method results. The Young’s modulus and thermal expansion coefficient of the composite, predicted using the 3D microstructure-based finite element analyses, were in good agreement with the experimental results. Furthermore, the 3D microstructure-based finite element model showed anisotropic thermal expansion behavior that was previously disregarded in the other models used in this study. Therefore, it was confirmed that the combined approach of serial sectioning and finite element modeling provides a significant improvement over 2D and 3D unit-cell modeling.