Inverse grain-size-dependent strain rate sensitivity of face-centered cubic high-entropy alloy

Abstract It is well documented that the strain rate sensitivity (m) increases at refined grain size for face-centered cubic (FCC) metals and alloys. Through a series of nanoindentation testing, however, we experimentally demonstrated a striking departure from conventional FCC metals that CoCrFeMnNi high entropy alloy (HEA) with FCC lattice structure exhibits monotonously decreased m as grain size reduced from ∼30.3 μm to 7.2 nm. Moreover, the apparent activation volume v*, which generally shows an opposite trend of m, exhibited the identical decreasing trend with reduced grain size as that of m. Such an unusual trend of m and its correlation with v* in the FCC HEA alloys can be understood by a distinct deformation-mechanism-transitions and unique dislocation morphology evolution that differs from conventional FCC metals.