Microstructure Evolution of Superelasticity and Elastocaloric Cooling Effect in NiTi Alloy

Abstract:

By conducting adiabatic cyclic loading tests on three types of NiTi alloys with different martensite contents, dislocation densities, and grain sizes, the intrinsic influence mechanisms of different microstructures on the superelasticity, deformation modes, and elastocaloric cooling effect during the deformation process of NiTi alloys were investigated. The results show that the presence of a high dislocation density, high martensite content, and small grain size can reduce the degree of superelastic functional degradation and the possibility of local uneven deformation in NiTi alloys. However, the elastocaloric cooling ability is weak. A smaller strain value results in superior superelasticity (minimum ε_residual=0.23%), but inferior elastocaloric cooling ability (maximum ?T_cooling=0.63 K). Completely eliminating dislocations and martensite, as well as increasing grain size, can achieve a significant elastocaloric cooling capacity (?T_cooling=25 K), but induces severe functional degradation (a drop from 25 K to 9.6 K, a decrease of 61.6%). Annealing at 400 ℃ for 15 min to tailor the dislocation density, martensite content and grain size results in good superelasticity, uniform deformation ability and a considerable elastocaloric cooling ability (?T_cooling=7.2 K), along with improved resistance to functional degradation.