Fundamental Measurements of Plasticity in Energetic Molecular Crystals

Plastic flow in crystalline solids most often occurs from the motion of dislocations. Accommodating general plasticity requires multiple slip systems, the ability to make new dislocations, and the existence of local stresses sufficient to move said dislocations. While the vast majority of plasticity studies are in atomic or ionic systems, the complex bonding in molecular crystals (exhibited by both pharmaceutical and energetic systems) opens up spaces in materials where it may be hard to make but easy to move, or hard to move and easy to make, the defects needed to accommodate plasticity. In this current study we examine the onset of plasticity in as-grown powder form (sub-mm) crystals and the hardness of these materials once plastic flow has been established using nanoindentation techniques. In systems ranging from RDX and HMX to PETN and CL-20 we find that the yield point phenomena appears when the maximum applied shear stress reaches between 2-7% of the elastic modulus of the material. The activation volume implied from a cumulative event analysis of approximately 20 yield-exhibiting indentations in each material is on the order of the molecular volume, but does not directly correspond to that across a broad range of space groups, suggesting that the process of nucleating dislocations in, in these molecular solids, more complicated than the atomistic processes often found in metallic or ionic systems. The hardness of these materials does not correlate well with the yield conditions, again showing the independence of the nucleation event and the subsequent mobility of the dislocations in crystals of limited slip condition. Finally, data where growth conditions were varied to create crystals of varying quality exhibited the same nominal activation volume, the same overall hardness, but different mean values for the yield point stresses, suggesting that the activation of plasticity processes in energetic molecular crystals can be directly correlated to defect density and these defects are not solely controlling the subsequent dislocation motion.