Toward tunable mechanical behavior and enhanced elastocaloric effect in NiTi alloy by gradient structure

Tailoring properties of engineering materials to meet various application requirements is a long-standing challenge in materials science. Here, we demonstrate an unprecedented strategy to achieve highly tunable mechanical behavior and significantly enhanced elastocaloric effect in superelastic NiTi via gradient structure fabricated by laser surface annealing on a severely deformed matrix. The gradient-structured (GS) NiTi sheet is characterized by a nanocrystalline core sandwiched between two coarse-grained layers with grain-size gradients enabled by progressive annealing within the thickness due to the natural degradation of heat penetration. Tailorable martensitic transformation characteristics, which gradually change from a uniform mode with quasi-linear stress-strain response to a nucleation and growth mode with plateau-type superelasticity, are readily realized through tuning the grain-size gradient. Furthermore, the GS NiTi exhibits more than 50% and 130% improvement in elastocaloric cooling capacity and efficiency compared to traditional nanocrystalline and coarse-grained NiTi with homogeneous microstructures, respectively. Such significant performance breakthroughs are attributed to the strong synergetic strengthening between fine and coarse grains in the GS NiTi, which cannot be offered by the freestanding components. The unique strengthening mechanism is activated, even in the absence of plastic deformation, by the high mechanical incompatibility among heterogeneous domains and the resultant pronounced strain gradient accommodated by martensite variants. The work opens a novel avenue for fabricating bulk GS materials with desired mechanical properties and inspires the microstructure optimization in a wide range of ferroelastic materials for giant caloric effects.