Plasticity in molecular crystal cyclotetramethylene tetranitramine (β-hmx)

Abstract

May2025School of EngineeringMolecular crystal cyclotetramethylene tetranitramine (β-HMX) is the active ingredient in widely used plastic bonded explosives. Plasticity is believed to be essential for its reaction initiation and detonation. To explore the energetic cost associated with the relative gliding of crystal planes, we calculate γ-surfaces for the most active glide planes in β-HMX, the (101) plane and the (011) plane, with pressure up to 15 GPa. Stable stacking faults are observed on both glide planes, suggesting dislocation disassociation into partials takes place. Furthermore, the γ-surface of the (101) plane indicates twinning on the (101) plane. With increasing pressure, the values of γ-surfaces increase drastically, however, the topography of γ-surfaces remains the same. Homogeneous dislocation nucleation was found to be a relevant mechanism of plastic deformation in β-HMX. In this work, we conduct atomistic simulations to investigate the conditions under which dislocations nucleate homogeneously in the (101) and (011) planes at pressures up to 20 GPa. Critical resolved shear stresses (CRSS) for dislocation nucleation are reported. The competition between the homogeneous nucleation and other mechanisms of plastic deformation shows that homogeneous nucleation is less likely to happen at pressures above 5 GPa, while at pressures below this threshold, homogeneous nucleation competes with shear localization. Further, molecular dynamics simulations are performed to evaluate the dislocation velocity vs. resolved shear stress relation at pressures up to 20 GPa in several slip systems, which helps defining the strain rate sensitivity of the crystal. Based on this data and data from the literature, we establish a mechanism-based constitutive model for β-HMX crystals. The model captures the thermally activated and dislocation drag regimes for dislocation motion and, more importantly, the model is strongly pressure-dependent, and rate sensitive. An isotropic version of the model based of Reuss averaging is also presented. This model has the potential to be broadly applicable in the continuum modeling of HMX. Further we study conditions under which plastic deformation in HMX becomes non-crystallographic, particularly in situations such as pore collapse under shock loading, which is considered to be a key mechanism of detonation. We observe fluidization once the applied pressure and rate are above specific thresholds, and associate this transition with the concomitant fulfillment of two conditions, one dependent on the maximum shear stress and the other dependent on the deformation rate.Ph