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The microstructure, room-temperature deformation mechanisms, and weldability of the medium-entropy alloy (MEA) NiCoVAl 0.2 have been investigated. Defect-free welded joints have been produced with good mechanical properties, indicating excellent weldability. The microstructure consists of f.c.c. grains containing many b.c.c. Ni 2 VAl (Ni 26.1 Co 33.5 V 27.8 Al 12.6) and few σ-type precipitates (Ni 15.5 Co 21.3 V 62.7 Al 0.5). Decreasing the aging temperature from 1100 °C to 900 °C, produced a higher area fraction of Ni 2 VAl precipitates (8 → 25%), much smaller L2 1 precipitates (7.2 → 1.4 μm), finer grains (17.1 → 2.4 μm), and presence of σ-type precipitate, resulting in simultaneous improvements in both the strength and ductility for both the thermo-mechanically treated (TMT) and electron beam welded (EBWed) MEA. The TMT MEA shows an excellent combination of strength (YS∼993 MPa, UTS∼1478 MPa) and ductility (30.3%), while the EBWed MEA shows slightly lower strength (YS∼822 MPa, UTS∼1194 MPa) and significantly reduced ductility (12.3%), i.e. a YS, UTS, and ductility of 83%, 81%, and 41%, respectively, of those of the TMT MEA. With increasing strain, both low-angle grain boundaries and geometrically-necessary dislocation (GND) densities increased. The TMT MEA has a higher GND density (1.6 × 1015 m−2) than the 9.1 × 1014 m−2 of EBWed MEA after strained to fracture. For both the TMT and EBWed MEA aged at 900 °C, the deformation was accommodated by dislocation slip, dislocation looping around precipitates, and nano-twinning. However, for both the TMT and EBWed MEA aged at 1100 °C, more abundant deformation twinning and an fcc→hcp shear transformation appeared, which may originate from a lower stacking fault energy due to differences in the f.c.c. matrix composition. [ABSTRACT FROM AUTHOR]

Effects of austenitizing temperature on the microstructure and mechanical property of a high-strength PM high-speed steel were systematically investigated using a series of heat treatments in the temperature range of 1070–1180 °C. Results showed that the optimal austenitizing temperature to obtain the desired combination of high compressive strength of 3838 MPa and high ductility of 29.7%, the optimal austenitizing temperature was 1180 °C. A maximum strength of 3870 MPa and a fracture strain of 22.7% was obtained by austenitized at 1120 °C. It is evident that the size distribution and carbide contents of both VC and Fe 3 W 3 C can be significantly changed by heat treatment and triple tempering resulted in slight decline in the amount of cracked carbides. Fractography showed that more broken particles of Fe 3 W 3 C carbide was observed than that of the VC particles in fractured specimens. [ABSTRACT FROM AUTHOR]

The microstructures and deformation mechanisms of the f.c.c. medium-entropy alloy (MEA) (NiCoCr) 76 (Ni 6 AlTi) 3 has been analyzed after various thermo-mechanical treatments. The solutionized, single-phase MEA, which had a grain size of 93.4 ± 31.9 μm, was cold-rolled (CR) to a 80% thickness reduction after which it showed both a high yield strength (YS) of 1539 MPa and a high ultimate tensile strength (UTS) of 1602 MPa, but an elongation to failure, ε, of only 16%. The CR MEA was then subjected to one of two heat treatments: (1) the CR MEA was recrystallized at 1100oC for 24 h, which produced a single-phase material with 85 ± 61.7 μm grain size that exhibited a much lower YS and UTS of 364 MPa and 747 MPa, respectively, but a much greater ε of 73%; and (2) the MEA was aged at 700oC for 24 h, which produced a fine-grained (1.1 ± 0.9 μm) material and a high volume fraction (0.35) of 12 nm dia. L1 2 nanoparticles that exhibited an excellent combination of strength and ductility, viz., YS∼1501 MPa, UTS∼1651 MPa, elongation ∼26%. The grain boundary strengthening and precipitation strengthening were together estimated to account for ∼58% of the YS in the latter material. After deformation, the recrystallized MEA contained numerous stacking faults and a Taylor lattice structure containing domain boundaries and microbands, while the aged MEA exhibited numerous stacking faults and sheared particles. The density of low-angle grain boundaries (LAGBs) in the recrystallized MEA increased by 100 times to 2.2 × 10−1 μm/μm2 while the density of CSL Σ3n boundaries decreased by 71% to 7.8 × 10−3 μm/μm2 after deformation. In contrast, the density of LAGBs in the aged MEA increased by 5 times to 2.6 μm/μm2 while the density of CSL Σ3n boundaries decreased by 57% to 0.6 μm/μm2 after deformation. [ABSTRACT FROM AUTHOR]

The coarsening behaviors of γ′ particles in Ni 28 Co 28 Fe 21 Cr 15 Al 4 Ti 4 high-entropy alloys and their influences on mechanical properties were analyzed. The lattice constant of the matrix decreased with the increase in aging time due to the precipitation of L1 2 nanoparticles. The increase in annealing time led to a continuous growth of both face-centered cubic grains and L1 2 particles. Both yield strength and microhardness exhibited age hardening, which varied with the holding time at 800 °C. When the holding time was in the range of 0–24 h, the yield strength and hardness increased with the aging time and dislocation cutting through particle mechanism dominates during deformation. When the holding time exceeded 24 h, the strength and hardness gradually decreased with the increase in aging time and Orowan dislocation looping was the dominant strengthening mechanism. Upon the aging at 800 °C for 24 h, the age hardening reached a peak, which provided an excellent combination of yield strength (855 MPa) and ultimate tensile strength (1031 MPa). The age hardening was related to the precipitation and ripening of spherical L1 2 nanoparticles. The ripening rate of L1 2 precipitates was calculated to be 0.31 × 10−27 m3/s. Our results can guide the designs of tough high-entropy alloys for various applications and heat treatments for a good combination of mechanical properties. • The precipitation of L1 2 nanoparticles in investigated HEA decreased the lattice parameter of face-centered cubic matrix. • Both ripening of L1 2 nanoparticles and grain size were analyzed and the growth rate was determined. • Remarkable age hardening behaviors of the tensile mechanical properties with the annealing time were observed. • The age-hardened HEA exhibited the desired combination of yield strength (855 MPa) and ultimate tensile strength (1031 MPa). [ABSTRACT FROM AUTHOR]

The weldability, microstructures, and mechanical properties of two L1 2 -nanoparticle-strengthened medium-entropy alloys (MEAs) Ni 43.4 Co 25.3 Cr 25.3 Al 3 Ti 3 and Ni 42.4 Co 24.3 Cr 24.3 Al 3 Ti 3 V 3 (at.%) are explored after electron beam welding (EBW). Strong yet ductile defect-free joints were produced with coarse columnar grains (118–245 μm) in the fusion zones, which were larger than the equiaxed grains in the heat-affected zones (15.6–22.3 μm) and in the base materials (4.6–5.6 μm). Both EBWed MEAs showed high yield strengths (838–858 MPa), high ultimate tensile strengths (1416–1420 MPa), and good fracture strains of 20–21 % at 77 K, which are 66∼83 %, 84–89 %, and 57–81 %, respectively, of those of the respective thermo-mechanically treated (TMT) MEAs. The V-doping improved the cryogenic mechanical properties of the TMT MEA while not influencing those of the EBWed MEA. High back-stress hardening contributes to over 50 % of the cryogenic strength. Both EBWed MEAs exhibited abundant dislocation networks, stacking faults, and nanoscale deformation twins after fracture, producing a high strain hardening rate and good ductility. [ABSTRACT FROM AUTHOR]

Shape memory alloys are emerging functional materials in various fields due to their unique mechanical properties. In this work, plate-impact experiments have been performed to study the dynamic behavior of near-equiatomic NiTi shape memory alloys with different initial phases, i.e., the single austenite (B2) and the mixed phase of austenite and martensite (B19′). A Doppler pin system and soft-recovery cylinder are employed to in-situ probe the macroscopic response and catch the shocked sample for postmortem characterizations, respectively. The observed bilinear behavior of the Hugoniot data, combined with the X-ray diffraction analysis of recovered samples, suggests that the B2 austenite undergoes a martensitic transformation at ∼5 GPa. Such a phase transition affects not only the spall damage but also the deformation mechanism of NiTi alloys. Specifically, it is identified that, in association with the B2 to B19' transformation, the spall strength is reduced by ∼7% and the austenite twinning disappears in the recovered NiTi sample. These results thus indicate that the deformation and fracture behaviors of NiTi shape memory alloys under shock compression are strongly coupled with the martensitic transformation. [ABSTRACT FROM AUTHOR]