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A molecular dynamics study on the effect of precipitate on the phase transformation in NiTi

One of the most important Shape Memory Alloys (SMAs) is NiTi (an almost equiatomic alloy of nickel and titanium) which has many applications in different areas like aerospace and biomedical industries. This popularity arises from its Shape Memory Effect (recovering pre-deformed shape when heated) and Superelasticity (reversible elastic behavior caused by phase transformation). It has been shown that the presence of Ni4Ti3 nanoprecipitates can have profound effects on the superelastic hysteresis loop, transformation temperatures and other characteristics of NiTi. In this study, the effect of the Ni4Ti3 precipitate on the stress-induced and temperature-induced phase transformations of NiTi is investigated using molecular dynamics (MD) simulations. In the simulations, an Ni4Ti3 precipitate was generated and embedded into the NiTi matrix. The transformation temperatures were computed by applying a cooling-heating cycle. At the austenitic phase, a monotonically increasing uniaxial compressive force was applied to the models with and without precipitate and, as expected, both models exhibited superelastic behavior at a temperature above Af. However, a larger hysteresis occurred for the precipitated model compared to the pristine NiTi due to the hinderance of reverse phase transformation in presence of the precipitate. Furthermore, similar models were made for different directions of the compressive load relative to the crystallographic directions to study the direction dependence of the response. The results show that the precipitate has a pronounced effect on superelastic behavior in 〈100〉 loading directions, while the results for other directions did not show noticeable difference.