Abstract: The activation volume (V^*) and strain-rate sensitivity exponent (m) of CoCrFeMnNi and Al_0.5CoCrFeMnNi high entropy alloys (HEAs) with various grain sizes (ranging between 2.4 and 356 µm) were measured at different strain rates and strain levels at room temperature. As the strain rate decreased, the plastic strain decreased, and the grain size increased, V^* increased. The enhanced solid-solution strengthening by addition of aluminum decreased. V^* The Hassen plot was modified to capture the grain-size dependence of V^* by considering the grain-size dependence of the dislocation density. As the plastic strain increased, the strain rate decreased, and the grain size decreased, m decreased. The behavior of m could be quantitatively predicted by using equations derived for the grain-size-dependent V^* and flow stress. The difference in the grain-size dependence of m between conventional face centered cubic (FCC) metals and FCC HEAs over the nanograin size range could not be explained in terms of a relatively large Hall-Petch slope of the FCC HEAs compared with that of the FCC conventional metals, but was explainable in terms of a substantially higher probability of activation of grain-boundary diffusion-controlled grain-boundary-sliding mechanism at nanograin sizes in the FCC conventional metals than in the FCC HEAs.

Key words: Grain size, High entropy alloys, Activation volume, Strain-rate sensitivity, Solid solution