Effect of Ni Content on the Phase Stability and Shape Memory Effect of Non-equiatomic CrMnFeCoNi High-Entropy Alloys

We investigated the effect of the Ni content on the face-centered cubic (FCC) phase stability and recovery strain of non-equiatomic CrMnFeCoNi alloys. From a well-known equiatomic CrMnFeCoNi high-entropy alloy, a series of Cr₂₀Mn₂₀Fe₂₀Co_40−xNi_x alloys (x = 0, 2, 4, 6, 8 at.%) are designed, and their shape memory effect is discussed. The alloys with moderate Ni content (6 ≤ x ≤ 8 at.%) are the FCC single-phase solid solutions, whereas the alloys with a less Ni (x ≤ 4 at.%) display a FCC + hexagonal close-packed (HCP) dual-phase structure due to thermally induced martensitic transformation. The replacement of Ni with Co strongly increased M_s and A_s of the Ni-lean CrMnFeCoNi alloys (x ≤ 6 at.%) by 31 K/at.% and 24 K/at.%, respectively. The pre-strained single-phase Cr₂₀Mn₂₀Fe₂₀Co₃₄Ni₆ alloy (x = 6 at.%) exhibited the highest recovery strain of 1.2% in the Cr₂₀Mn₂₀Fe₂₀Co_40−xNi_x system. This is attributed to the formation of stress-induced HCP martensite in a single orientation and low strain hardening compared with other dual-phase alloys. These results provide guidelines for designing new shape memory alloys in the CrMnFeCoNi and its derivative systems.