Reversible isotropic strain and huge volume change in the all-d-metal Heusler alloys Mn2NiTi

Abstract

All-d-metal Heusler ferromagnetic shape memory alloys such as Ni₂MnTi and Mn₂NiTi, composed entirely of transition metals, have attracted considerable attention for their significant volume change and substantial transformation latent heat, positioning them as ideal candidates for solid-state refrigeration and shape memory applications. However, the weak d-d covalent hybridization results in a broad martensitic transformation (MT) range and considerable hysteresis, complicating phase transformation control. We addressed these challenges by developing a dual-phase structure in Mn₅₀Ni_50-xTi_x alloys (x = 11.0 to 14.5) through systematic optimization of heat treatment conditions. The secondary γ phase provides nucleation sites, promoting MT, and serves as a buffer, absorbing local elastic energy and mitigating plastic deformation of the main phase during the MT. This adjustment reduced the transformation range to 23.9 K and decreased the hysteresis to just 14.0 K for the x = 14.5 alloy. This strategy was also applied to the x = 12.5 alloy, which undergoes MT near room temperature. In-situ digital image correlation (DIC) strain measurements indicate that strain distribution in the x = 12.5 alloy during thermally induced reversible MT is heterogeneous at the local level but isotropic at the macroscopic level. The reversible isotropic strain recorded was −6960 ppm, corresponding to a volume change of −2.06 % and an entropy change of 57.5 J kg⁻¹ K⁻¹, associated with MT. Our findings underscore the pivotal role of exploiting the unique dual-phase structure in all-d-metal Heusler alloys, providing a novel approach for applications in solid-state refrigeration and energy conversion.