Molecular dynamics study of diffusionless phase transformations in HMX: β-HMX twinning and β-ɛ phase transition

We use molecular dynamics to study the mechanism of deformation twinning of β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX) in the P21/n space group setting for the twin system specified by K1=(101), η1=[101¯], K2=(101¯), and η2=[101] at T=1 and 300 K. Twinning of a single perfect crystal was induced by imposing increasing stress. The following three forms of stress were considered: uniaxial compression along [001], shear stress in the K1 plane along the η1 direction, and shear stress in the K2 plane along the η2 direction. In all cases, the crystal transforms to its twin by the same mechanism: as the stress increases, the a and c lattice parameters become, respectively, longer and shorter; soon after the magnitude of a exceeds that of c the system undergoes a quick phase-transition-like transformation. This transformation can be approximately separated into two stages: glide of the essentially intact {101} crystal planes along ⟨101¯⟩ crystal directions followed by rotations of all HMX molecules accompanied by N-NO2 and CH2 group rearrangements. The overall process corresponds to a military transformation. If uniaxial compression along [001] is applied to a β-HMX crystal which is already subject to a hydrostatic pressure ≳10 GPa, the transformation described above proceeds through the crystal-plane gliding stage but only minor molecular rearrangements occurs. This results in a high-pressure phase of HMX which belongs to the P21/n space group. The coexistence curve for this high-pressure phase and β-HMX is constructed using the harmonic approximation for the crystal Hamiltonians.