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

Abstract

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 μm 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 μm 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 μm 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.