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

A Pereverzev, JOURNAL OF APPLIED PHYSICS, 134, 125105 (2023).

DOI: 10.1063/5.0171571

We use molecular dynamics to study the mechanism of deformation twinning of beta-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (beta-HMX) in the P2(1)/n space group setting for the twin system specified by K-1 = (101), eta(1) = 10 (1) over bar, K-2 = (101), and eta(2) = 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 eta(1) direction, and shear stress in the K-2 plane along the eta(2) 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 < 10 (1) over bar > 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 beta-HMX crystal which is already subject to a hydrostatic pressure greater than or similar to 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 P2(1)/n space group. The coexistence curve for this high-pressure phase and beta-HMX is constructed using the harmonic approximation for the crystal Hamiltonians.

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