Twinning and rotational deformation of nanocrystalline NiTi under shock loading

Abstract

Understanding the formation of twins and new grains (NGs) in B2 austenite NiTi alloys under shock loading is of significance and importance for its science insights and engineering applications. However, the formation of $\left\{112\right\}$ twin in austenite phase under shock loading is still controversial, and the NGs' evolution under shock loading is unclear. The Electron Backscatter Diffraction characterizations and x-ray diffraction analyses of the NiTi samples recovered from the shock experiments reveal that the main deformation modes include dislocations, twins, and new grains, etc. Similar phenomena are also obtained in our nonequilibrium molecular dynamics simulations for nanocrystalline NiTi (nc-NiTi) with limited-duration-pulse shock loading. Simulations confirmed that $\left\{112\right\}$ twins in austenite phase can be formed by successively gliding a displacement of $\frac{a}{3}$[111] on ($\overline{2}11$) plane. It can occur in both shock compression and release stages, in addition, the grain boundaries triple junction and the interaction of slip bands with different slip systems can serve as the nucleation of twins. Moreover, that the nanoscale rotational deformation leads to the formation of NGs is found in our simulation of nc-NiTi, without experiencing the conventional disorder-recrystallization-grain refinement stages in the corresponding region. Related to shear stress $\ensuremath{\tau}$, the shock loading velocity ${U}_{p}$ plays the key role in the formations of twins and NGs. These early successes may hope to get some insights into the deformation mechanism of NiTi under shock loading.