Role of temperature and strain rate on the stress reversal in dynamic damage of monocrystalline NiTi alloy

Abstract

This study focuses on the temperature and strain rate dependences on tensile stress reversal in dynamic damage of single-crystal nickel titanium (NiTi) alloy. Molecular dynamics simulations are conducted to investigate the structural phase evolution and void evolution under specified temperatures (2000–3500 K). Interestingly, a transformation of one-two-one peak number is observed in the tensile stress curves. When the temperature (T) is lower than 2350 K, there is just one stress reversal, which is attributed to phase transition. On the contrary, a double-peak structure occurs when T ranges from 2350 to 2700 K. It is believed that the first stress reversal occurs through phase transition, but the second reversal is dependent upon void nucleation. When T is higher than 2700 K, the phase content is insignificantly changed, because the phase transformation has been almost accomplished during the relaxation process. Accordingly, the stress reversal takes place via void nucleation. In addition, it is found that the number of peaks changes from two to one as the strain rate increases from 3.0 × 109 s−1 to 2.0 × 1010 s−1. In such cases, the stress reversal is merely governed by phase transition. Since temperature softening effect is characterized by the constraint of phase transition, the void nucleation threshold decreases as the temperature increases. Contrarily, strain rate strengthening effect is represented by the intensification of phase transition, and then the void nucleation threshold would slightly increase by increasing strain rate.