Enhanced Strength and Plasticity of CoCrNiAl0.1Si0.1 Medium Entropy Alloy via Deformation Twinning and Microband at Cryogenic Temperature

The synthesis of lightweight yet strong-ductile materials has been an imperative challenge in alloy design. In this study, the CoCrNi-based medium-entropy alloys (MEAs) with added Al and Si were manufactured by vacuum arc melting furnace subsequently followed by cool rolling and anneal process. The mechanical responses of CoCrNiAl0.1Si0.1 MEAs under quasi-static (1 × 10−3 s−1) tensile strength showed that MEAs had an outstanding balance of yield strength, ultimate tensile strength, and elongation. The yield strength, ultimate tensile strength, and elongation were increased from 480 MPa, 900 MPa, and 58% at 298 K to 700 MPa, 1250 MPa, and 72% at 77 K, respectively. Temperature dependencies of the yield strength and strain hardening were investigated to understand the excellent mechanical performance, considering the contribution of lattice distortions, deformation twins, and microbands. Severe lattice distortions were determined to play a predominant role in the temperature-dependent yield stress. The Peierls barrier height increased with decreasing temperature, owing to thermal vibrations causing the effective width of a dislocation core to decrease. Through the thermodynamic formula, the stacking fault energies were calculated to be 14.12 mJ/m2 and 8.32 mJ/m2 at 298 K and 77 K, respectively. In conclusion, the enhanced strength and ductility at cryogenic temperature can be attributed to multiple deformation mechanisms including dislocations, extensive deformation twins, and microbands. The synergistic effect of multiple deformation mechanisms lead to the outstanding mechanical properties of the alloy at room and cryogenic temperature.

Superior Strength and Ductility of 304 Austenitic Stainless Steel with Gradient Dislocations

Materials with designed gradient nanograins exhibit unprecedented mechanical properties, such as superior strength and ductility. In this study, a heterostructured 304 stainless steel with solely gradient dislocation structure (GDS) in micron-sized grains produced by cyclic-torsion processing was demonstrated to exhibit a substantially improved yield strength with slightly reduced uniform elongation, compared with its coarse grained counterparts. Microstructural observations reveal that multiple deformation mechanisms, associated with the formation of dense dislocation patterns, deformation twins and martensitic phase, are activated upon straining and contribute to the delocalized plastic deformation and the superior mechanical performance of the GDS 304 stainless steel.

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Microstructure Evolution and Recrystallization Temperature Change of Cold-Rolled Fe–19Mn–0.6C Twinning-Induced Plasticity Steel during Annealing

Ultra-high twinning-induced plasticity (TWIP) steel is receiving increasing attention in the automobile industry. Self-designed Fe–19Mn–0.6C TWIP steel was subjected to reveal the relationship between microstructures, which were related to recrystallization starting/ending temperature and cold rolling. The results indicated that initial deformation twins, secondary deformation twins, and nano-twins were successively generated in rolled TWIP steel with the increase of cold rolling, deformation twins, and dislocations, as well as with the elongation of grains. The elements remained uniformly dispersed rather than agglomerated in the twin crystals and grain boundaries. The recrystallization starting temperature changes of TWIP steel were 500–525, 400–425, 400–415, and 400–410 °C at cold rolling deformations of 25%, 50%, 75%, and 88%, respectively. Furthermore, the obtained corresponding recrystallization ending temperature changes were 580–600, 530–550, 520–540, and 500–520 °C, respectively. The linear relationship between cold deformation and hardness suggests that cold rolling can increase dislocation density and thus facilitate improving the hardness of TWIP steel.

Shock deformation microstructures in xenotime from the Spider impact structure, Western Australia

ABSTRACT The rare earth element–bearing phosphate xenotime (YPO4) is isostructural with zircon, and therefore it has been predicted that xenotime forms similar shock deformation microstructures. However, systematic characterization of the range of micro structures that form in xenotime has not been conducted previously. Here, we report a study of 25 xenotime grains from 10 shatter cones in silicified sandstone from the Spider impact structure in Western Australia. We used electron backscatter diffrac tion (EBSD) in order to characterize deformation and microstructures within xenotime. The studied grains preserve multiple sets of planar fractures, lamellar {112} deformation twins, high-angle planar deformation bands (PDBs), partially recrystallized domains, and pre-impact polycrystalline grains. Pressure estimates from micro structures in coexisting minerals (quartz and zircon) allow some broad empirical constraints on formation conditions of ~10–20 GPa to be placed on the observed microstructures in xenotime; at present, more precise formation conditions are unavailable due to the absence of experimental constraints. Results from this study indicate that the most promising microstructures in xenotime for recording shock deformation are lamellar {112} twins, polycrystalline grains, and high-angle PDBs. The {112} deformation twins in xenotime are likely to be a diagnostic shock indicator, but they may require a different stress regime than that of {112} twinning in zircon. Likewise, polycrystalline grains are suggestive of impact-induced thermal recrystallization; however, in contrast to zircon, the impact-generated polycrystalline xenotime grains here appear to have formed in the solid state, and, in some cases, they may be difficult to distinguish from diagenetic xenotime with broadly similar textures.