Hardness and microstructural evolution of CoCrFeNi high-entropy alloys during severe plastic deformation

In this study, the evolution of hardness and microstructure in a CoCrFeNi high-entropy alloy processed by high-
pressure torsion was investigated. An initial increase in hardness was attributed to the rapid increase in low-
angle grain boundaries (LAGBs), accompanied by a high density of dislocations. With continuous straining,
some LAGBs evolved into high-angle grain boundaries and completed the grain boundary strengthening. A
remarkable increase in hardness was achieved in the final stage of hardness enhancement, primarily due to the
formation of numerous twins within nanograins, which were mainly formed by the emission of Shockley partial
dislocations emerging from grain boundaries. Furthermore, a twin trailing the stacking fault was found in the
nanograin, and a critical stress for twin nucleation was estimated to be about 2.16 GPa. Defects with low-energy
configurations are closely correlated with the thermal stability of nanocrystalline CoCrFeNi alloys. The defects
within nanograins yielded an average strain of about 1.1 % and the corresponding strain energy was identified to
about 17 J g $^{-1}$.