Achieving tunable graded functional properties of NiTi shape memory alloy: a phase field study

The functionally graded (FG) NiTi shape memory alloy (SMA) with both Ni-rich domain (exhibiting superelasticity, called as SE domain) and Ti-rich one (exhibiting shape memory effect, called as SME domain) was reported to exhibit a novel mechanical response with high displacement control accuracy. In this work, we first establish a phase field model to describe the formation of SE domain (initial state is fully austenitic) and SME domain (initial state is twinned martensite) within FG NiTi SMA. Then, we implement the proposed phase field model by finite element method to reveal the evolution of microstructure morphology contributing to the novel mechanical response of FG NiTi SMA. Besides, we investigate the influence of proportion and initial microstructure morphologies of SME domain on the novel mechanical response of FG NiTi SMA, which would save the cost and time of manufacturing as well as mechanical experiment. Our simulations show the possibility of improving displacement control accuracy and reducing dissipation by guiding the SE and SME domains for FG NiTi SMA, which can be controlled by processing parameters during manufacturing. It provides the avenue for obtaining the FG NiTi SMA actuators with excellent displacement control accuracy and low dissipation.