Improved elastocaloric cooling performance in gradient-structured NiTi alloy processed by localized laser surface annealing

The coefficient of performance of material (COP_mat, the ratio of caloric effect to energy dissipation), a key figure-of-merit for solid-state refrigerants, is significantly degenerated by the inherent hysteresis of materials. In this work, a low-hysteresis NiTi refrigerant with gradient structures (grain sizes varying from ~ 10 nm to ~ 3500 nm) is fabricated for elastocaloric cooling by localized laser surface annealing on a severely-deformed substrate (50% thickness reduction). The obtained gradient-structured (GS) NiTi exhibits more than 83% improvement in COP_mat with a comparable adiabatic temperature change $\Delta T_{\text{ad}}$ compared to the homogeneous coarse-grained NiTi and extends the lower limit of operational temperature from above 283 K to 243 K. Furthermore, the large specific cooling capacity (~ 4.5 K/1%), narrow stress hysteresis (~ 60 MPa) and robust mechanical properties (high strength, high ductility and high stability) make the GS NiTi superior to most elastocaloric materials in refrigeration capability and efficiency. Such significantly enhanced cooling and mechanical performances of the GS NiTi originate from the unique gradient structure, which possesses a sound synergetic strengthening effect and an overall uniform phase transformation mode. The work proposes a promising strategy for optimization of thermomechanical performances of elastocaloric materials and demonstrates a great industrial potential of the GS NiTi in solid-state cooling.