Effects of precipitate on the phase transformation of single-crystal NiTi alloy under thermal and mechanical loads: A molecular dynamics study

Molecular dynamics simulations have been carried out in this study to investigate the stress-induced and temperature-induced phase transformations in NiTi shape memory alloy in the presence of Ni4Ti3 precipitate. NiTi alloys form the most popular class of shape memory alloys that have many applications in various industries. Therefore, computational simulation of the material behavior can extend our understanding of the material properties and help the development of alloys with tailored properties. In the simulations, an Ni4Ti3 precipitate (with a rhombohedral crystal structure) was generated and embedded into the NiTi matrix (having a B2 crystal structure). The transformation temperatures were computed by applying a cooling-heating cycle starting from a high temperature. At the austenitic state (at a temperature above Af), a monotonically increasing uniaxial compressive force was applied to the models with and without precipitate and, as expected, both models exhibited superelastic behavior. Furthermore, similar models were made for different directions of the compressive force relative to the crystallographic directions to study the direction dependence of the mechanical response. For all the loading directions that phase transformation happened, a larger hysteresis occurred for the precipitated model compared to the pristine NiTi due to the hindrance of reverse phase transformation in presence of the precipitate. The results show that the precipitate has a pronounced effect on the superelastic behavior in <100 > loading directions.