Search

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]

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 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]