Homogeneous Dislocation Nucleation in Molecular Crystal Cyclotetramethylene‐Tetranitramine (β‐HMX)

Abstract

The molecular crystal cyclotetramethylene‐tetranitramine (β‐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 β‐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 γ‐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 γ‐surfaces in molecular crystals is included.

Identifying Possible Slip Systems of Molecular Crystals via a Geometry-Based Procedure

“…Slip asymmetry is also discovered in other molecular crystals. 71,72 3.2. Monoclinic Molecular Crystals.…”

“…However, according to a previous study, due to asymmetry and anisotropy of the slip plane, though the critical shear stress for slipping along the (110)[001] and (110)[001̅] is both 0.3 MPa, those along (110)[11̅0] and (110)[1̅10] are 0.2 and 0.4 MPa, respectively, suggesting the complexity of molecular crystals. Slip asymmetry is also discovered in other molecular crystals. , …”

Insight into material behavior via surface free energy calculations for common energetic materials

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