Impact of a dense helium-bubble superlattice on the deformation of copper by twinning

Recent experimental work on helium-irradiated single-crystal (SC) and nano-twinned (NT) copper has shown that the formation of a helium bubble superlattice has a dramatic effect on the critical resolved shear stress of twin formation in SC copper pillars and twin migration in NT copper pillars. The mechanisms governing this dramatic change in the mechanical response of the material after helium irradiation are explained theoretically in this work. Atomistic simulations show that the presence of a helium bubble superlattice has a profound effect on twin nucleation and propagation, which are related to the mechanical responses of SC and NT copper respectively. Based on the simulations, we estimate that the bubble lattice can decrease the ideal twin nucleation stress compared to pure copper by approximately 50%. The modeled response of a step in a twin boundary to an applied shear stress leads to an increase in the critical resolved shear stress due to the bubble lattice. The computed increase is in qualitative agreement with experimental findings for NT copper.