Dislocation cross slip in molecular crystal cyclotetramethylene tetranitramine (β-HMX)

Abstract

In this work, we explore the mechanism of cross-slip in the low symmetry molecular crystal cyclotetramethylene tetranitramine (β-HMX)—a secondary explosive. Cross-slip is well studied and understood in high symmetry crystals but virtually uninvestigated in molecular crystals. To this end, we use molecular simulations and observe that only screw dislocations with the [100] Burgers vector may cross-slip effectively. The process involves the (011), (010), (001), and (011¯) planes and takes place in both the positive and negative directions of dislocation motion in each of the respective slip systems. Resolved shear stresses larger than ∼0.6 of the critical resolved shear stress are necessary in at least two of the planes in order to activate cross-slip. The application of pressure does not prevent cross-slip from taking place. The phenomenon occurs at elevated pressures in the same slip systems as at zero pressure. However, due to the limited number of slip systems involved, cross-slip does not appear to be of central importance in β-HMX and, of course, remains relevant only as long as the dislocation-based mechanism of plasticity is not replaced by the shear localization mode, which becomes dominant at high pressure, under strong shock conditions.