Modeling Framework to Predict Melting Dynamics at Microstructural Defects in TNT-HMX High Explosive Composites

Many
high explosive (HE) formulations are composite materials whose
microstructure is understood to impact functional characteristics.
Interfaces are known to mediate the formation of hot spots that control
their safety and initiation. To study such processes at molecular
scales, we developed all-atom force fields (FFs) for Octol, a prototypical
HE formulation comprised of TNT (2,4,6-trinitrotoluene) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine).
We extended a FF for TNT and recasted it in a form that can be readily
combined with a well-established FF for HMX. The resulting FF was
extensively validated against experimental results and density functional
theory calculations. We applied the new combined TNT-HMX FF to predict
and rank surface and interface energies, which indicate that there
is an energetic driver for coarsening of microstructural grains in
TNT-HMX composites. Finally, we assess the impact of several microstructural
environments on the dynamic melting of TNT crystal under ultrafast
thermal loading. We find that both free surfaces and planar material
interfaces are effective nucleation points for TNT melting. However,
MD simulations show that TNT crystal is prone to superheating by at
least 50 K on subnanosecond time scales and that the degree of superheating
is inversely correlated with surface and interface energy. The modeling
framework presented here will enable future studies on hot spot formation
processes in accident scenarios that are governed by strong coupling
between microstructural interfaces, material mechanics, momentum and
energy transport, phase transitions, and chemistry.