Mechanism of microstructure evolution and spheroidization in ultrafine lamellar CoCrFeNi(Nb0·5/Ta0.4) eutectic high entropy alloys upon hot deformation

We report the hot deformation induced microstructure evolution and spheroidization in ultrafine lamellar CoCrFeNiNb0.5 and CoCrFeNiTa0.4 eutectic high entropy alloys (EHEAs) comprising of FCC solid solution and Co2Ta-type Laves phase up to 1123 K (0.7Tm) under compression. The CoCrFeNiNb0.5 EHEA retained high yield strength (σy) of 1334–1820 MPa exhibiting fracture strain (εf) of 5.7–12.5% at room temperature (RT)-723 K and strain hardening up to 1932–2241 MPa. Whereas, CoCrFeNiTa0.4 EHEA exhibited strain hardening with σy = 1403–1847 MPa and εf = 11.1–15.6% for the same temperature range. A gradual decrease of σy occurred at above 973 K exhibiting strain softening without failure even at εf > 50%. The Kocks-Mecking plot revealed the stage-III hardening followed by an inflection pointing to the onset of dynamic recrystallization at 1023 K and 973 K for Nb0.5 and Ta0.4, respectively. Whereas the flow softening occurred due to the flow localization, buckling, breakdown of the lamellar colonies, and spheroidization upon hot deformation. The microstructural coarsening and lamellar instabilities involve termination migration, cylinderization, edge spheroidization and boundary splitting. The superior specific yield strength of EHEAs has been observed up to 1023 K in dry laboratory conditions than that of other HEAs and high temperature alloys.