A comparison of thermo-mechanically-treated and electron-beam-welded strong, ductile medium-entropy alloy: Microstructural evolution and deformation mechanisms.

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]