Achieving ultrahigh elastocaloric cycling stability in Ni-Mn-Sn-based alloys by microstructure engineering

Developing superelastic shape memory alloys that integrate a large elastocaloric response with excellent cyclic stability is crucial for advancing solid-state elastocaloric refrigeration technology. In this study, first-principles calculations were utilized to clarify the preferential occupation of doped B atoms. The mechanism by which B alloying enhances the mechanical properties was investigated through elastic constants and differential charge density analysis. We further proposed an effective microstructure regulation strategy of suction casting combined with B alloying synergistically refines grains and introduces non-transforming secondary phases. This approach significantly improves both the mechanical performance and the cyclic stability of the elastocaloric effect in Ni-Mn-Sn alloys. A fine-grained (Ni43.5Mn46Sn10.5)98.5B1.5 alloy, featuring Mn2B precipitates at grain boundaries, achieved a large adiabatic temperature variation (|△Tad|) of 9.8 K upon unloading from a 3% compressive strain at 298 K. This excellent superelasticity and elastocaloric refrigeration performance were sustained across a wide temperature range from 288 K to 368 K. Moreover, the alloy exhibited exceptional elastocaloric stability with minimal degradation after 105 cycles under 600 MPa compressive stress, maintaining |ΔTad| from 9.8 K initially to 9.2 K after 105 cycles, outperforming most reported Ni-Mn-base and Ni-Fe-based ferromagnetic shape memory alloys.