Orientation-dependent superelasticity of a metastable high-entropy alloy

In the past decade, the concept of high-entropy alloys (HEAs) or multi-principal element alloys (MPEAs), which are composed of at least four principal elements, significantly expands the compositional space for alloy design. This concept can also be employed in the design of superelastic alloys to promote the development of this functional material field. Here, we report the orientation-dependent superelasticity of a metastable Fe-27.5Ni-16.5Co-10Al-2.2Ta-0.04B (at.%) HEA through in situ micropillar compression tests along & lang;001 & rang;, & lang;011 & rang;, and & lang;111 & rang; orientations. Our results show that considerable superelastic strains can be achieved along the three orientations in the metastable HEA via a reversible martensitic transformation. Thermoelastic martensite with thin-plate morphology was observed under cryogenic conditions. This work demonstrates that the maximum superelastic strains vary with different orientations, and the & lang;001 & rang;-oriented specimen shows the largest superelastic strain. The superelastic strains along specific orientations are compared with theoretical values calculated from the lattice deformation method and the energy minimization theory, respectively. The limited number of martensite variants under compression testing may be responsible for the discrepancy that exists in the experimental and the two theoretically predicted transformation strains. This study may provide a feasible strategy for the design of superelastic HEAs with specific orientation for applications in microsystems.