Grain Size Dependence of Activation Volume and Strain Rate Sensitivity in FCC High Entropy Alloys

We study the strain rate sensitivity (SRS) and activation volume of face-centered cubic (FCC) high-entropy alloys (HEAs) across a wide range of grain sizes. An equiatomic CoCrFeMn HEA was synthesized using a newly developed mechanically driven method, Surface Mechanical Alloying and Consolidation (SMAC), followed by heat treatment at 1100 °C to produce second-phase-free alloys with grain sizes spanning from the nanocrystalline to the micrometer regime. Nanoindentation was employed to measure hardness, SRS, and activation volume. Unlike conventional FCC metals, but similar to body-centered cubic (BCC) metals, the SRS of FCC HEAs increases with increasing grain size. This unusual trend is attributed to the strong solid-solution effect: lattice distortion introduces energy fluctuations that act as additional, closely spaced barriers to dislocation activation. As a result, the increase in activation volume with grain size is orders of magnitude smaller than in traditional FCC metals. At the same time, the flow stress in FCC HEAs shows a much stronger grain-size dependence, reflected in their substantially higher Hall–Petch coefficients. Consequently, the influence of flow stress on SRS can outweigh that of activation volume, leading to a net increase in SRS with grain size. These findings, supported by both experiments and theoretical modeling, provide new insights into the mechanisms governing SRS in FCC HEAs and strengthen the understanding of how microstructure controls their mechanical behavior.