Dislocation energy and line tension in molecular crystal cyclotetramethylene tetranitramine (β-HMX)

Abstract

Cyclotetramethylene tetranitramine (β-HMX) is an energetic molecular crystal often used in plastic bonded explosives. Its decomposition reaction may be triggered by plastic deformation. Efforts have been made in recent years to evaluate the mechanisms of plasticity in these crystals and to develop constitutive descriptions that can be used to represent plastic deformation on the microstructural level. In this work, we use atomistic simulations to evaluate the dislocation self-energy, core energy, and line tension in four slip systems previously identified as being the most active. The cores are compact and the anisotropic elasticity solution applies at distances from the dislocation line larger than approximately one Burgers vector. Core energies between 0.3 and 0.5 eV/Å result. The line tension varies rapidly when the character of the dislocation is modified due to the strong elastic anisotropy of the crystal, with maxima at approximately ±40° relative to the screw orientation. The line tension also varies from slip system to slip system. These quantities enter many models of elementary mechanisms of dislocation motion such as cross-slip, dislocation nucleation from stress concentrators, the strength of dislocation junctions and other dislocation structures, and the critical stress for the operation of Frank–Read dislocation sources. The data reported here can be used to evaluate the conditions in which these processes operate and as an input to dislocation dynamics simulations.