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. Using classical molecular dynamics, we predict the formation of similar nanoscale shear bands in the alpha-RDX crystal 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 the effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.

Automated summary

This article investigates the formation mechanism and thermodynamic properties of nanoscale shear bands in energetic molecular crystals. The study reveals that shear bands can be formed under shock and quasistatic conditions through shear strain localization, and exhibit a critical behavior associated with a first-order structural phase transition.