Dislocation mobility and critical stresses at finite temperatures in molecular crystal cyclotetramethylene tetranitramine (β-HMX)

Abstract

Cyclotetramethylene tetranitramine (HMX) is an energetic material commonly found in plastic bonded explosives. Its crystallographic sensitivity to initiation and detonation has been linked to plastic slip in the various crystalline planes; shock directions which cannot accommodate substantial crystalline slip are expected to be more sensitive. In this work, we carry out constant stress-constant temperature molecular dynamics simulations of the motion of dislocations and characterize their behavior in the two most probable slip planes, (101) and (011), of this crystal. We evaluate critical thresholds for activating steady-state dislocation motion and the associated dislocation mobilities at various temperatures below 400 K. This information can be used to develop and calibrate larger scale models of plastic deformation in this material.

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

“…The constitutive behavior of shear bands has been characterized as a function of the applied pressure and temperature using molecular simulations [31]. Furthermore, it was observed that fast moving dislocations in β‐HMX lead to strain localization [10, 27], which is consistent with reports for other crystals [32]. In fact, high pressure inhibits dislocation motion and promotes localization [10], which is thought to become the dominant plasticity mechanism under shock conditions.…”

“…The loop is circular, which indicates that the mobility of all segments is approximately identical, independent of the angle between the Burgers vector and the dislocation line vector. The analysis of dislocation mobility reported in [27], which was performed using quasi-2D models with pre-existing dislocations indicates approximately iden-tical mobility for edge and screw dislocations moving in the negative direction. It also indicates stick-slip-type motion when loading is applied in the positive direction for both edge and screw.…”

“…The slip systems of β‐HMX have been identified based on results from indentation experiments [13, 25], the interpretation of plate impact experiments [26] and dedicated atomistic simulations [9, 27]. The most active slip planes are (101) and (011), each containing two slip distraction, i. e. [010] and for the (101) plane, and [100] and for the (011) plane.…”

“…Molecular packing indicates that the negative slip direction in the (011)[100] system is identical to the positive slip direction in the system. The Peierls‐Nabarro stress [9], the temperature dependent dislocation mobility [27] and the pressure sensitivity of the flow stress [10] of these systems have been determined.…”

An Anisotropic Elastoviscoplasticity Model of Thermomechanical Responses of Shocked β‐HMX and α‐RDX Single Crystals

“…The thermal expansion coefficients determined by X-ray powder diffraction are ð0:29; 12; 2:3Þ � 10 À 5 K À 1 along the (a, b, c) axes and dðln bÞ=dT ¼ 2:6 � 10 À 5 K À 1 [48,49]. Recent works from Picu's group used the molecular dynamics simulations to investigate the dislocation motion and identify the potentially active slip systems in β-HMX at 0 K and finite temperatures [50]. Through indentation experiments and chemical etching, Sheen et al [24] reported two operative slip systems in β-HMX, namely, (001) [100] and (101) [10 � 1], based on the monoclinic crystallographic axis.…”