Structure and properties of dislocations and the twin boundary on (101) in β-cyclotetramethylene tetranitramine

Dispersion-corrected density functional theory calculations of the generalised stacking fault energy surface of the (101) plane in monoclinic beta-cyclotetramethylene tetranitramine (beta-HMX) indicate that a stacking fault with an energy of approximately 130 mi/m(2) at the translation vector t = 1/2[11 (1) over bar] is metastable. Using the energy of the metastable stacking fault and anisotropic linear elasticity theory, we have evaluated whether dislocations with total Burgers vectors b = [010], [10 (1) over bar] and [11 (1) over bar] will split into partial dislocations on (101). Our results suggest that it is not favourable for the [010) screw dislocation to split into 1/2 < 11 (1) over bar > partials but that the edge dislocation will split into partials separated by a narrow, 19.8 angstrom, stacking fault. Both screw and edge [11 (1) over bar] dislocations are predicted to split into two partials with splitting distances of about 28.7 and 43.4 angstrom, respectively. The screw and edge [10 (1) over bar] dislocation are also predicted to split into partials, with equilibrium distances of 16.3 and 43.2 angstrom, respectively. These results are consistent with recent analyses of Knoop indentation experiments in which the (101)[010) slip system is inactive whereas [10 (1) over bar] dislocations are glissile. Since twinning is an important deformation mechanism in beta-HMX, dispersion-corrected density functional theory calculations have also been used to compute the energy of the (101)[10 (1) over bar) twin boundary, for which we report a value of 163 mJ/m(2).