Homogeneous Dislocation Nucleation in Molecular Crystal Cyclotetramethylene-Tetranitramine (beta-HMX)

ZC Zhang and CR Picu, PROPELLANTS EXPLOSIVES PYROTECHNICS, 47, e202100381 (2022).

DOI: 10.1002/prep.202100381

The molecular crystal cyclotetramethylene-tetranitramine (beta-HMX) is the active ingredient in some broadly used plastic bonded explosives. Plasticity is thought to be essential for reaction initiation and detonation. In this work we use molecular simulations to investigate the conditions under which dislocations nucleate homogeneously in beta-HMX. We evaluate the resolved shear stress leading to homogeneous nucleation of dislocation loops in the planes currently considered to be most important for plasticity in this crystal, (011) and (101), and at pressures up to 20 GPa. The importance of homogeneous nucleation for plastic deformation is discussed in relation to the other plasticity mechanisms operating in HMX. It is concluded that homogeneous nucleation is unlikely to happen at pressures above 5 GPa, while at pressures below this threshold homogeneous nucleation competes with shear localization. The article also presents y-surfaces for the (011) and (101) planes, which are used here to interpret the nucleation process and critical stresses, and have broader importance in other contexts. A discussion of the procedure required to compute gamma-surfaces in molecular crystals is included.

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