Mechanical behavior of grain boundaries in molecular crystal cyclotetramethylene-tetranitramine (β-HMX)

Cyclotetramethylene-tetranitramine (HMX) is an energetic molecular crystal commonly used in plastic bonded explosives (PBX). Its detonation may be triggered by plastic deformation. Plasticity mechanisms have been explored by molecular simulations and have been incorporated into constitutive equations used in continuum models. However, the structure and mechanics of interfaces between HMX crystals, such as those occurring in PBX and in polycrystals, have not been explored to date. We evaluate the shear strength of several types of grain boundaries between HMX crystals at pressures of 0.1 MPa, 10 GPa, and 20 GPa and investigate the reconstruction associated with the formation of grain boundaries. Structural reconstruction is limited in all cases. Grain boundary sliding critical stress and sliding phenomenology are highly pressure-sensitive. In ambient conditions, grain boundary sliding takes place at low stress and without major structural disordering. At 20 GPa pressure, steric hindrance is sufficiently high, and all boundaries become amorphous during sliding. At 10 GPa pressure, some boundaries slide without major structural disordering or dislocation nucleation, while others become amorphous. Overall, the resolved shear stress for grain boundary sliding is smaller than or comparable to the lower range of Peierls–Nabarro stress for dislocation motion in the most active slip systems of the β-HMX crystal, which indicates that, at all pressures considered, grain boundaries undergo plastic activity before the crystal bulk.