The rupture and ejection of near-surface helium bubble in single crystal Cu under shock loading

Abstract

• The dynamic evolution process of near-surface He bubble under shock loading and subsequent release involves three stages: shock compression, equilibration, and expansion-rupture-ejection. • The plastic failure or melt of metal region surrounding the He bubble and the upper metal layer leads to the rupture and following ejection of the bubble. • Impact strength has significant effects on internal jet and He bubble evolution, showing that higher impact velocity results in stronger internal jet and more significant expansion and rupture of the bubble. • The size of helium bubble and the number ratio of He/vacancy also influence the dynamic response of the bubble, and the inherent mechanisms are explored. Using molecular dynamics simulation, we studied the dynamic behavior and microscopic mechanisms of near-surface helium bubble in single crystal Cu subjected to shock loading. The whole process of bubble evolution includes three stages: shock compression, equilibration, and expansion-rupture-ejection. When shock wave reaches the bubble, it is compressed and an internal jetting is formed, leading to the increases of velocity and internal pressure of the bubble. The internal pressure then reduces slightly due to the relaxation of helium atoms. After the reflected rarefaction wave reaching the bubble, the internal pressure greatly decreases to several GPa and the volume expands a lot. Because of the pressure gradient between the bubble and the free surface, the velocity of the bubble increases again, squeezing the upper thin metal layer continuously and finally making it plastically fail or melt. Then the bubble ruptures, ejecting helium atoms and some Cu clusters. Impact strength has significant effects on this dynamic process. Higher impact velocity leads to stronger jet and more significant expansion and rupture of the bubble. The latter is attributed to the microstructure evolution of the surrounding metal atoms and the upper metal layer, for which plastic failure or melt is observed at higher impact velocity. The influences of helium bubble size and number ratio of He/vacancy are also investigated. We find that the internal jet is stronger and temperature rise and local melting of metal is more obvious for larger bubble, resulting in easier failure of the metal layer and thus more remarkable expansion and rupture of the bubble. Higher number ratio results in weaker internal jet but stronger expansion and rupture of the bubble. This is because more helium atoms in the bubble for higher ratio can impact on the upper metal layer, leading to easier failure of the metal layer.