Lattice defects and dislocation mediated plasticity in cyclotrimethylenetrinitramine : a molecular dynamics study

Abstract

This thesis presents an investigation into lattice defects and their role in mechanical response of energetic molecular crystal cyclotrimethylenetrinitramine, commonly known as RDX. The investigations are performed using molecular dynamics simulations with a flexible molecular model for RDX. The initial part of the thesis deals with conformational stability of RDX crystals. Point defects, in the form of molecular conformational changes (conformational defects), are studied in RDX crystals under applied strain. The energetics of these defects are investigated and their role in local temperature fluctuations in the crystal is demonstrated.

This is followed by a study of dislocation mediated plasticity in RDX using large-scale molecular dynamics simulations. The nature and ease of plastic deformation in RDX crystals is studied by quantifying the critical stresses required for dislocation motion on multiple experimentally observed slip systems. Plasticity in RDX is found to be hindered owing to a low number of available slip systems and high stresses required for dislocation motion. Anisotropic mechanical response of RDX observed in shock and indentation experiments are explained in terms of these results. Plastic deformation mechanisms are found to be influenced by topological interactions between molecules. The investigations also provide numerical evidence for the existence of asymmetric motion of dislocations, a unique deformation mechanism reported for the first time.

Molecular crystals consist of a periodic arrangement of molecules in a lattice and find a wide variety of applications in pharmaceutical, electronic and energetic systems. In comparison to atomic crystals, the deformation mechanisms in molecular crystals are more complex due to the presence of a flexible molecule at a lattice site. Owing to their low symmetries and complex packing, molecular crystals are expected to have limited plasticity. Despite the acknowledged importance of mechanical response of molecular crystals in various applications, a fundamental understanding of the same is still developing.