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- x Mn 33 Ti 17 Fe 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 Ni 46 Mn 33 Ti 17 Fe 4 alloy exhibits superior comprehensive performance with a | Δ T a d / σ c r | value of 0.33 K MPa −1 , which indicates that significant cooling effects can be achieved under low stresses. Meanwhile, the Ni 46 Mn 33 Ti 17 Fe 4 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. • Fe alloying enhances martensitic transformation by increasing energy difference between phases, achieving | ΔT ad /σ cr | = 0.33K MPa −1 in Ni 46 Mn 33 Ti 17 Fe 4 . • Fe raises transformation temperature, enabling martensite domains in austenite as growth nuclei, reducing critical driving stress. • Fe-rich phase with high hardness/modulus improves compressive strength (2250 MPa) and strain (78 %), maintaining stability over 1892 cycles.