Molecular dynamics simulations on the effect of nanovoid on shock-induced phase transition in uranium nitride

The Angular-Dependent Potential (ADP) proposed by Tseplyaev et al. was used to study the structural behavior of uranium nitride (UN) under shock pressure by molecular dynamics (MD) simulations. Based on the calculations of shock velocity $U_S$ and particle velocity $U_P$, the results show that a pressure-induced phase transition of $\mathit{Fm}?3m\to R?3m$ structure in UN occurs at 35 GPa, and it agrees well with experimental results of 30–32 GPa. We also considered the effect of nanovoid on the phase transition of UN crystal from $Fm?3m$ to $R?3m$ structure. It is found that the pressure of phase transition decreases with the increasing nanovoid diameter. The phase transition takes place firstly around nanovoid, companied by the nanovoid collapsing, and then spreads to the void-free regions in the process of shock loading. Due to different stresses at different direction the spreading velocity of phase transition perpendicular to the direction of shock wave is observed to be far faster than the one parallel to the direction of shock wave.