Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): a first-order structural phase transition

Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-s-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot-spots which control detonation initiation. We predict the formation of similar nanoscale shear bands in RDX subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core-softening of effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.