Cross slip not key deformation in explosive molecular crystal

Examples of molecular crystals include everyday things, such as ice and sugar, and some less common things, like explosives. However, the atomic-level processes that take place in molecular crystals as they are deformed by stretching, bending or compressing are not well understood. Understanding deformation is important for safe use of explosives in military and civilian applications, because it can help predict the onset of explosions and aid in the design of new plastic-bonded explosives.

One non-planar deformation mechanism is called cross slip. The defects move around on planes of atoms – so called slip planes. The process of cross slip takes the defect from one plane of atoms to an intersecting plane of atoms, like a car turning a corner at an intersection. Khan and Picu studied cross slip in cyclotetramethylene tetranitramine (HMX), a high explosive widely used for military applications. They are the first to study cross slip in a molecular crystal, as well as one of the first to study the molecular-scale mechanisms of deformation in HMX.

Using molecular simulations, the authors found the cross-slip mechanism is present in HMX, but it is not a key contributor to deformation. Because this non-planar deformation is not of central importance, the authors determined slip in HMX is mostly planar. The low symmetry and weak interplanar bonding of the crystal also promote planar slip.

Author Catalin Picu said this work adds to the physical picture of plasticity in molecular crystals, whose current understanding is very sketchy. This work also contributes to deformation models, which can be used to design plastic-bonded explosives and predict their behavior. The authors will continue to investigate deformation mechanisms in molecular crystals for use in constitutive models that could be applied to plastic-bonded explosives design.

Source: “Dislocation cross slip in molecular crystal cyclotetramethylene tetranitramine (β-HMX),” by Mohammad Khan and Catalin R. Picu, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5114940 .