Showing papers in "Materials Transactions in 2020"

An Atomistic Modeling Study of the Relationship between Critical Resolved Shear Stress and Atomic Structure Distortion in FCC High Entropy Alloys — Relationship in Random Solid Solution and Chemical-Short-Range-Order Alloys —

Abstract: 1Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan 2Department of Physics, Pabna University of Science and Technology, Pabna-6600, Bangladesh 3Department of Applied Physics, Xi’an University of Technology, Xi’an 710054, China 4Center for the Element Strategy Initiative for Structural Materials (ESISM), Kyoto University, Kyoto 606-8501, Japan

Effect of Cobalt-Content on Mechanical Properties of Non-Equiatomic Co–Cr–Ni Medium Entropy Alloys

Abstract: Non-equiatomic high entropy alloys (HEAs) and medium entropy alloys (MEAs) are expected to have the potential to exhibit good mechanical properties due to abundant composition designs compared to equiatomic alloys. It has been reported that an equiatomic CoCrNi MEA shows better strength-ductility balance than CoCrFeMnNi HEA, and there is a possibility that the mechanical properties can be further improved by changing chemical composition. Among the constituent elements, cobalt (Co) has the effect of decreasing stacking fault energy (SFE). In this study, we clarified the effect of Co-content on mechanical properties of non-equiatomic Co­Cr­Ni MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Cox(CrNi)(1001x) (x = 20 (Co20), 40 (Co40), 60 (Co60) at%) MEAs were processed to very high plastic strains by high-pressure torsion (HPT) and subsequently annealed under proper conditions to obtain FCC single-phase and uniform fine grain sizes. Mechanical properties of the specimens with fully recrystallized microstructures were characterized by tensile tests at room temperature. Their deformed microstructures at different tensile strain levels were observed by electron microscopy. The result of the tensile tests showed that the work-hardening rate was enhanced with increasing the Co-content although early fracture before reaching plastic instability condition occurred in Co60. Planar slip of dislocations and deformation twinning were observed in Co20 (SFE = 30mJ/m2), while, in addition to them, deformation-induced martensitic transformation to HCP 3⁄4-martensite was observed in Co40 having lower SFE (SFE = 10mJ/m2), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of 3⁄4-martensite greatly increased in Co60 (SFE = 0mJ/m2) compared with Co40, and early fracture occurred due to stress concentration at intersects between martensite and grain boundaries. The present results suggested that the mechanical properties of the present materials could be effectively designed by controlling the SFE. [doi:10.2320/matertrans.MT-MK2019004]

Work-hardening mechanism in high-nitrogen austenitic stainless steel

Abstract: The remarkably high work-hardening rate in high-nitrogen austenitic stainless steels is generally believed to be due to the promotion of dislocation accumulation by nitrogen addition. However, analysis of dislocation accumulation behavior by the modified Williamson-Hall/ Warren-Averbach method reveals that no difference to the increment of the dislocation density during deformation exists between austenitic steels with and without nitrogen. Since cross slipping is markedly suppressed in high-nitrogen steels, the moving dislocations are back-stressed by the planar dislocation arrays. This induces the deformation resistance and the high work-hardening rate. [doi:10.2320/matertrans.H-M2020804]

Characterization of Growing Dendrites in CrMnFeCoNi High-Entropy Alloy by Time-Resolved and In-Situ Tomography

Abstract: Time-resolved tomography (4D-CT) and X-ray diffraction (XRD) were combined to observe growing dendrites and to measure their crystallographic orientation in a CrMnFeCoNi high-entropy alloy with an FCC structure. The evolution of the dendritic grains cooling at 0.083K/s was reconstructed using 200 projections over a 180° rotation every 4 s from 4D-CT and a phase field filter. The voxel size was a 6.5μm cube. Simultaneously, the crystallographic orientations of the dendritic grains were measured by XRD. The dendrite arms grew preferentially along the ©100a direction, corresponding with typical FCC alloys. The specific solid­liquid interfacial area, which was normalized by the total volume, was evaluated as a function of solid fraction. The interfacial area reached a maximum at a solid fraction of 0.55. The interfacial area was compatible with the reported values of Al­Cu and Mg­Sn alloys. The secondary arm spacing was on the same order of magnitude as the spacing of conventional alloys. Overall, it appears that solidification in this high-entropy alloy can be analyzed by using models developed for binary or pseudo-binary alloys. [doi:10.2320/matertrans.MT-MK2019006]

Development of a New Titanium Powder Sintering Process with Deoxidation Reaction Using Yttrium Metal

Abstract: Oxygen (O) contamination in titanium (Ti) is difficult to control using conventional Ti powder-metallurgy technologies, owing to the strong affinity between Ti and O. In this study, we developed a new sintering process that can remove O from Ti by placing Ti green and yttrium (Y) in molten salt. This study demonstrates that the O concentration in Ti can be reliably controlled in the range of 200­2000 ppm O by varying aY in the Y/Y2O3 equilibrium at 1300K in NaCl­KCl (l), such that the sintering reaction of Ti powder simultaneously proceeds. Furthermore, it is also shown that the O concentration in Ti can be reduced to 30­60 ppm O in YCl3 (l) in the sintering process, when the Y/YOCl/YCl3 equilibrium is employed. This study demonstrates the feasibility of a new sintering process that can control the O concentration in Ti to approximately 30­2000 ppm O. The process ensures economical rationality because the cost of Y metal is negligibly small in recent years. By developing this process, inexpensive high-O-concentration Ti powder can be applied for fabricating the desired low-O-concentration Ti products. [doi:10.2320/matertrans.MT-M2019340]

Molecular Dynamics Simulation of Nucleation from Undercooled Melt of Nickel–Aluminum Alloy and Discussion on Polymorphism in Nucleation

Abstract: Nucleation from undercooled melt of Ni­Al alloy is investigated by molecular dynamics (MD) simulation. Multiple nucleation of NiAl nuclei with B2 structure appears from undercooled melt of Ni­50 at%Al, which forms a fine microstructure of B2-NiAl. On the other hand, stepwise phase transition happens from undercooled melt of pure Ni, which is known as Ostwald’s step rule. That is, body-centered-cubic (BCC) phase appears first from the undercooled melt and then face-centered-cubic (FCC) nucleation occurs from the inside of previously existing BCC nucleus. Origin of the polymorphism in stepwise nucleation of Ni and the preferential nucleation of B2-NiAl from melt of Ni­Al alloy is discussed on the basis of classical nucleation theory. [doi:10.2320/matertrans.MT-M2019353]

Effects of Size and Crystallinity of CaCu3Fe4O12 on Catalytic Activity for Oxygen Evolution Reaction

Abstract: Effects of size and crystallinity of a quadruple perovskite oxide, CaCu3Fe4O12, were investigated for its catalytic activity for the oxygen evolution reaction (OER). Pristine CaCu3Fe4O12 powder sample was synthesized under high-pressure and high-temperature conditions of 8GPa and 1000°C, and a portion was treated by ball milling for 120min to obtain a powder sample with smaller particle size. The specific surface area significantly increased from 0.38 to 10.30m2g11 by ball-milling, leading to increased OER activity. However, it was found that the milling did not improve the OER activity efficiency in proportion to that expected from the increase in specific surface area as determined by BrunauerEmmett-Teller analysis of adsorption/desorption isotherms measured with nitrogen gas because the crystallinity was lowered by ball milling, which suppressed the catalytic activity. This study provides important information on how to achieve the best OER catalytic performance in terms of both size and crystallinity. [doi:10.2320/matertrans.MT-M2020147]