Stable long-term elastocaloric effect in Si doped Ni-Mn-Ti polycrystalline alloys with a textured dual-phase microstructure

Abstract

Heusler-type Ni-Mn-Ti shape memory alloys have emerged as promising candidates for elastocaloric cooling applications owing to their colossal elastocaloric effect. However, simultaneously achieving high cooling capacity and long fatigue life in polycrystalline alloys remains challenging because of their inherent brittleness. In this study, we demonstrate a significant enhancement in the long-term elastocaloric performance of Ni-Mn-Ti polycrystalline alloys by developing a textured dual-phase microstructure. Through the combination of non-transforming Mn₂Ti precipitates induced by Si substitution for Ni and <001>_A oriented austenite matrix formed by directional solidification, exceptional cyclability over one million compressive loading-unloading cycles is achieved in a directionally solidified Ni₄₆Mn_32.5Ti_17.5Si₄ alloy, representing an improvement in the fatigue life of existing Heusler-type elastocaloric alloys by two or three orders of magnitude. Notably, even after this long-term cycling, a substantial adiabatic temperature variation of –12.7 K can still be obtained upon unloading from a moderate stress of 566 MPa. Such remarkable elastocaloric properties arise from the synergy of precipitate hardening and microstructure texturing, which not only significantly improves the fatigue life by enhancing resistance to dislocation motion and reducing stress hysteresis, but also enables a pronounced elastocaloric response under low driving stress.