Enhanced cyclic stability of NiTi shape memory alloy elastocaloric materials with Ni4Ti3 nanoprecipitates: Experiment and phase field modeling

In this work, a NiTi shape memory alloy (SMA) with excellent elastocaloric performance (with an ultrahigh coefficient of performance, i.e., COPmat of ∼46.5 and an adiabatic temperature change of ∼10.5 K) and good cyclic stability is prepared. A thermo-mechanically coupled and crystal-plasticity-based phase field model including both the descriptions of Ni4Ti3 precipitation and martensitic transformation (MT) is newly proposed to reveal the microscopic mechanism behind the cyclic stability of NiTi elastocaloric materials. The dependence of plasticity on the precipitate size is innovatively considered through a Hall-Petch-like relationship between the dislocation slip resistance and the distance between adjacent precipitates, and the pinning effect of dislocation on reverse MT is reflected by introducing an interaction energy. The elastocaloric effect (eCE) and its cyclic evolution of the single-crystal NiTi SMA systems containing Ni4Ti3 precipitates with different sizes are simulated. Combined with experimental observations and simulations, new insights are provided on the mechanism behind the enhanced cyclic stability of precipitation strengthened NiTi SMA elastocaloric materials. The results of this work can improve the valuable scheme and theoretical basis for the development of NiTi-based elastocaloric materials with outstanding eCE and good cyclic stability.