Understanding the reaction initiation of energetic single crystals under external stimuli is a long-term challenge in the field of high energy density materials. Herewith, we developed an ab initio molecular dynamics method based on the multiscale shock technique (MSST) and reported the reaction initiation mechanism by performing large-scale simulations for the sensitive explosive benzotrifuroxan (BTF), insensitive explosive triaminotrinitrobenzene (TATB), four polymorphs of hexanitrohexaazaisowurtzitane (CL-20) pristine crystals and five novel CL-20 cocrystals. A theoretical indicator, t_initiation, the delay of decomposition reaction under shock, was proposed to characterize the shock sensitivity of energetic single crystal, which was proved to be reliable and satisfactorily consistent with experiments. We found that it was the coupling of heat and pressure that drove the shock reaction, wherein the vibrational spectra, the specific heat capacity, as well as the strength of the trigger bonds were the determinants of the shock sensitivity. The intermolecular hydrogen bonds were found to effectively buffer the system from heating, thereby delaying the decomposition reaction and reducing the shock sensitivity of the energetic single crystal. Theoretical rules for synthesizing novel energetic materials with low shock sensitivity were given. Our work is expected to provide a useful reference for the understanding, certifying and adjusting of the shock sensitivity of novel energetic materials.

Mechanical responses of nanoporous aluminum samples under shock in different crystallographic orientations (, , , and ) are investigated by molecular dynamics simulations. The shape evolution of void during collapse is found to have no relationship with the shock orientation. Void collapse rate and dislocation activities at the void surface are found to strongly dependent on the shock orientation. For a relatively weaker shock, void collapses fastest when shocked along the orientation; while for a relatively stronger shock, void collapses fastest in the orientation. The dislocation nucleation position is strongly depended on the impacting crystallographic orientation. A theory based on resolved shear stress is used to explain which slip planes the earliest-appearing dislocations prefer to nucleate on under different shock orientations.

Shock-induced plasticity and phase transition in single crystal lead are investigated by nonequilibrium molecular dynamics simulations. Under dynamic shock loading, the appearance of plasticity in materials precedes that of phase transition. Plasticity mainly causes two effects: one is that plasticity has a significant relaxation effect on shear stress, and the other is that deformation twinning serves as important nucleation sites for the phase transition. This twinning is caused by mutual impediments among different cross-slips and {111} slips. There are three main stages in the dynamic phase transition process of lead: fcc → bcc-like phase transition, plasticity, and hcp phase formation and growth. Moreover, phase transition has a more significant relaxation effect on shear stress, which relaxes the shear stress to a minimum value. The spall strength of lead decreases as the shock intensity increases, but its rate of decrease under different shock intensities is different. Plasticity, especially phase transition, would obviously result in a lower rate of decrease in spall strength.

In this study, we developed a theoretical model that explores material melting and crystallization kinetics at high pressures by combining the Kolmogorov-Johnson-Mehl-Avrami kinetics model with equations pulled from hydrodynamic and thermodynamic theories. Our model only requires two inputs: an equilibrium melting curve and an equation of state, both of which can be derived empirically through experiments. We use our model to investigate the melting and crystallization behaviors of aluminum at high pressures. At a constant homologous temperature, our model predicts that melting rates will increase, crystallization growth rates will decrease, and the minimum isothermal crystallization time will increase as pressure increases. Additionally, higher pressures also reduce the critical homologous temperature at which the isothermal crystallization time is at the minimum.