Molecular dynamics simulation on the shape memory effect and superelasticity in NiTi shape memory alloy

Molecular dynamics (MD) simulations Ni-Ti based on a second nearest neighbor modified embedded-atom method potential were performed to study atomic-scale mechanical behavior and microstructural evolution of Ni-Ti alloy at different temperatures. By using four calculated characteristic transformation temperatures, the optimum simulated temperature ranges were determined. At temperatures lower than M-s, a thermally induced self-accommodating martensitic phase consisting of three variants formed. Each of the two variants was twin structures. In the subsequent loading process, the most favorable variant grew with the movement of interfaces among the variants, which produced remnants of the residual strain after unloading. After heating up to over A(f), the residual strain disappeared and the shape recovered to the original. The shape memory effect under austenite state was also performed, wherein the sample exhibited a typical stress-strain temperature curve and remained in a martensitic state after unloading. After heating at a high temperature, the martensite transformed back into its parent phase accompanied with the recovery of residual strain. The material exhibited superelastic behavior above A(f), and the critical transformation stress increased with increased temperature. Some parts of B2 structures remained at the end of the plateau stage and continued transforming into martensites during the hardening stage for both T > A(f) and M-s < T < A(f). The critical stresses for transformation satisfied the Clausius-Clapeyron relationship at T > M-s. (C) 2018 Elsevier B.V. All rights reserved.