Peierls-Nabarro stresses of dislocations in monoclinic cyclotetramethylene tetranitramine (beta-HMX)
A Pal and CR Picu, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 26, 045005 (2018).
HMX (cyclotetramethylene tetranitramine) is an energetic material which releases substantial amounts of energy upon decomposition. The role of defects and deformation in causing reaction initiation was discussed in the literature but remains insufficiently understood. In this work, we identify, using computational methods, the slip systems which are potentially active in beta-HMX and rank them in terms of their propensity for slip. To this end, we develop first a tentative ranking based on the degree of steric hindrance associated with slip. This is quantified using a geometric analog of the gamma-surface. Further, we use atomistic models to compute the Peierls-Nabarro (PN) stress for the motion of dislocations in the slip systems with smallest degree of steric hindrance. A complex mechanical behavior is observed, including strong slip asymmetry, twinning and cleavage. The five systems with the lowest PN stress are (011)011, (011)100, (101)010, (101)101 and (021)100. We conclude that the material has enough slip systems available for supporting a generalized state of plastic strain provided the twinning system (101)101 is taken into consideration and that the resolved shear stress is at least 260 MPa.
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