Physical Factors Controlling Large Shape Memory Effect in FCC ↔ HCP Martensitic Transformation in CrMnFeCoNi High-Entropy-Alloy Single Crystals

Abstract

A study was carried out on the effect of the level of external stresses, σex, and test temperature on the shape memory effect (SME), governed by the FCC ↔ HCP martensitic transformation, in single crystals of the Cr20Mn20Fe20Co34.5Ni5.5 (at.%) high-entropy alloy (HEA) along two different crystallographic orientations, i.e., [1¯23] and [011], under tensile strain. It was shown that the SME depends on the crystal orientation and the level of external stresses, σex, in the “cooling-heating” cycle under constant σex. In the “cooling-heating” cycle under constant σex, a maximum SME of 13.6 ± 0.2% was observed in [011]-oriented crystals at an external tensile stress of 150 MPa while in the [1¯23]-oriented crystals, a SME of 8.4 ± 0.2% was found under an external tensile stress of 170 MPa. In the “stress-strain” cycle, the maximum SME had similar values of 13–14% in studied orientations. General physical factors (the stress level of the FCC phase, short-range order, and change in the value of dislocation splitting in the external stress field) were established and ensured a large SME and its dependence on the crystal orientation in the Cr20Mn20Fe20Co34.5Ni5.5 HEA single crystals. For the studied orientations, a large SME in the FCC ↔ HCP MT was obtained for the first time.