Collapse of helium-filled voids in extreme deformation: Dislocation mechanisms

The mechanisms responsible for the collapse of helium-filled bubbles during the passage of shock waves in monocrystalline copper are revealed. Both internal pressure (caused by pre-existing helium atoms) and bubble size are varied in molecular dynamics simulations to understand the atomistic scale deformation as they are subjected to shock compression at pressures of 48, 123, and 170 GPa, corresponding to particle velocities of 1.0, 2.0, and 2.5 km/s. Both empty and helium filled bubbles serve as dislocation sources, generating intense, localized plastic regions. There are distinct differences in the collapse of empty voids compared to He-filled bubbles, the former requiring less stress and generating a greater density of dislocations for a given shock strength. A generalized model for dislocation emission is proposed, where the inclusion of shear stress generated by the helium bubble increases the critical stress to generate dislocations at the defect surface, demonstrating the change in plastic deformation. • Shock compression studies on single crystal Cu with pre-existing voids and helium bubbles generate localized, increased shear dislocation loops • Varying size and internal pressure alters dislocation loop generation; as loops grow and interact, complex networks of sessile dislocations are formed • The threshold shock stress is calculated via both analytical and computational means and reflects a clear trend of decreasing stress with increasing radius is observed; additionally, the critical shock stress for emission of dislocations around helium filled bubbles is higher than that of empty voids.