The formation mechanism of twin type shear bands in β-HMX: molecular rotation and translation.

Context: Molecular dynamics simulations are performed to clarify the deformation mechanism of β-HMX crystal in the P 2 1 / n space group setting under uniaxial compression. Nanoscale shear bands whose internal structure is regular enough to form twin with parent structure were found under high strain rate loading in the [010] direction. These deformation twins are formed by the change of lattice orientation due to atomic translation under shear stress, with 2 3 ¯ 1 ¯ (or 23 1 ¯ ) twin planes and ⟨ 11 1 ¯ ⟩ (or ⟨ 1 1 ¯ 1 ¯ ⟩) twin directions. Molecular rotation can significantly reduce the activation barrier of twin systems; meanwhile, the change of lattice direction is accomplished by a serial of fractional translation steps. Our results implicate that these factors, such as decreasing the activation energy barrier of twin systems via molecular rotation and new twin systems introducing shear bands, should be considered via applying the crystal plasticity model to investigate the hot spot formation in energetic explosive crystals. Methods: All simulations were carried out with the MD code package LAMMPS. The non-reactive and flexible molecular force field proposed by Smith and Bharadwaj was adopted to simulate the uniaxial compression of the monoclinic β-HMX molecular crystal in the P 2 1 / n space group setting on (010) plane along [010] direction. In addition, the Shake algorithm was used to constrain all C-H bonds to the equilibrium length. Two methods, i.e., the Von Mises strain and the relative displacements of molecules, were applied to analyze the structure of twin type shear bands of β-HMX during compression. Visualization analysis for atomistic simulations was performed by using OVITO. [ABSTRACT FROM AUTHOR]