Inert Melting Curve and Thermal Melting Kinetics of β-HMX for 10 GPa ≤ P ≤ 40 GPa*

Non-reactive all-atom molecular dynamics simulations of solid-liquid phase coexistence were used to determine the pressure-dependent melting curve T_melt (P) and kinetics of melting for (010)-oriented β-HMX on the interval 10 GPa ≤ P ≤ 40 GPa. The study extends the melting curve for  P ≤ 5 GPa due to Kroonblawd and Austin (K-A) [Mech. Mater. 152, 103644 (2021)] to detonation pressures. A time dependent, layer-by-layer analysis of molecular displacements and 2D radial distribution functions is developed to track advancement of the melt front in quasi-1D phase coexistence simulations. The melting temperature is predicted to increase from 860 K at 1 GPa to ≈ 2280 K at 40 GPa and is found to follow the empirical Simon-Glatzel (S-G) function. Assessment of the extrapolation error when only low-P data are used in the S-G fit reveals that such extrapolations are fraught. Thermal melting rate coefficients on a given isobar were extracted (i) by fitting the Arrhenius rate-law form to overall crystal-zone melting times and (ii) by fitting to first-order kinetics for disintegration of individual layers in the crystal as melting advances at a given T and P. The melting rates increase exponentially with T on a given isobar but decrease significantly with increasing P. Comparisons of the predicted melting rates to characteristic time scales for pore collapse suggest that melting is slow and thus melting kinetics should be incorporated into future, high-fidelity mesoscale simulations that explicitly resolve hot spot formation and evolution.