Effect of grain size on shape memory properties of Cr20Mn20Fe20Co35Ni5 high-entropy alloy

We investigated the effect of the grain size on the recovery strain of a non-equiatomic face-centered cubic (FCC) CrMnFeCoNi high-entropy alloy (HEA). A series of Cr20Mn20Fe20Co35Ni5 (Ni5) HEAs with grain sizes ranging from 14 to 730 mu m were produced by varying casting, rolling, and annealing conditions. As the grain size increased, the density of grain and twin boundaries decreased, while the volume fraction of stress-induced hexagonal close-packed (HCP) martensite increased from 5 to 31 %. The cast-and-annealed Ni5 HEA with a maximum grain size of 730 mu m exhibited a peak recovery strain of 4.7 % under tensile pre-straining followed by recovery heating. This is attributed to the low strain-hardening rate and high critical strain for plastic yielding, indicating that stress-induced martensitic transformation is the dominant deformation mode at the early stage of plastic deformation. In contrast, as-cast Ni5 HEA with a grain size of 670 mu m displayed more than two times lower recovery strain than the cast-and-annealed HEA despite the large grain size. This difference is attributed to compositional inhomogeneity, which promotes stress-induced martensite preferentially in Cr-, Co-, and Fe-rich dendritic regions, leading to a higher strain-hardening rate and a lower fraction of stress-induced martensite. These results suggest that both grain size and compositional homogeneity are important factors in the design of cast shape memory alloys within CrMnFeCoNi multicomponent alloy systems.