Low critical stress induced large elastocaloric effect in Fe-doped Ni-Mn-Ti alloys with enhanced mechanical properties

The elastocaloric effect in shape memory alloys is primarily attributed to the release of latent heat accompanied by stress-induced martensite transformation, which endows them with significant potential in solid-state cooling technology. However, the progress in developing Ni-Mn-Ti-based elastocaloric alloys for durability and miniaturization applications is severely hindered by their poor mechanical properties and the high critical stress during martensite transformation. In this study, Fe alloying was employed to solve these two problems, and Ni_50-xMn₃₃Ti₁₇Fe_x (x = 0, 1, 2, 3, 4) alloys with <001> austenite orientation were successfully produced using directional solidification technology. The addition of Fe introduces a Fe-rich phase with high hardness and elastic modulus, thereby improving mechanical properties. Simultaneously, Fe alloying raises the phase transformation temperature and promotes the formation of numerous martensite domains, which serve as growth nuclei for further martensite development. This effectively reduces the critical driving stress required for stress-induced martensite phase transformation. Among these directionally solidified alloys, we found that the Ni₄₆Mn₃₃Ti₁₇Fe₄ alloy exhibits superior comprehensive performance with a |ΔT_ad/σ_cr| value of 0.33 K MPa⁻¹, which indicates that significant cooling effects can be achieved under low stresses. Meanwhile, the Ni₄₆Mn₃₃Ti₁₇Fe₄ alloy shows excellent mechanical properties including a compressive strength of up to 2250 MPa and a compressive strain of 78 %, enduring 1882 cycles with remarkably functional stability.