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 (0.7T(m)) under compression. The CoCrFeNiNb0.5 EHEA retained high yield strength (sigma(y)) of 1334-1820 MPa exhibiting fracture strain (epsilon(f)) 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 sigma(y) = 1403-1847 MPa and epsilon(f) = 11.1-15.6% for the same temperature range. A gradual decrease of sigma(y) occurred at above 973 K exhibiting strain softening without failure even at epsilon(f) > 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 Nb-0.5 and Ta-0.4, 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.

Automated summary

This paper reports the microstructure evolution and spheroidization of ultrafine lamellar high entropy alloys (EHEAs) under hot deformation. The experiments on CoCrFeNiNb0.5 and CoCrFeNiTa0.4 alloys at different temperatures reveal their different yield strengths and strain hardening behaviors, as well as deformation softening at high temperatures. In addition, the microstructural coarsening and lamellar instabilities during the evolution process are characterized.