Tunable quasi-linear superelasticity and improved elastocaloric effect of concentration-gradient NiTi alloys via phase-field engineering

The microstructure engineering of shape memory alloys (SMAs) with outstanding mechanical and elastocaloric performance is extremely crucial for environmentally friendly solid-state refrigeration, advanced morphing aircraft, and innovative artificial muscle technologies. In this work, the microstructure engineering of NiTi SMAs was systematically investigated using a phase-field approach. The emphases were on tailoring the Ni-concentration gradients and optimizing the proportion and morphological characteristics of Ni-rich and Ni-lean domains to enhance the mechanical performance and elastocaloric effect. The results show that NiTi SMAs have widely tunable and variable mechanical performance and elastocaloric effect. The phase-field simulations identified the Ni-concentration design schemes with outstanding mechanical performance and elastocaloric effect, such as the quasi-linear superelasticity, nearly free-hysteresis, low modulus E (22.9 GPa, which is 0.6 times the reported experimental value) with high elastic strain limit εe (3.4 %) or considerable adiabatic temperature change ΔTad (11.6 K) with high coefficient of performance of the material (COPmat = 525, which is 27 times the reported experimental value). This work highlights the significant potential of NiTi SMAs that are designed via concentration-gradient engineering as high-efficiency, solid-state refrigeration cores and highly flexible deformable structural materials.