The spallation characteristics of polycrystalline aluminum with helium bubbles under a wide range of shock stresses

The spallation behavior of polycrystalline Al with helium (He) bubbles (poly Al–He) under unsupported shock loadings at a wide range of impact velocities was investigated via molecular dynamics simulations. We found that the microstructural features during shock compression and release processes for the poly Al–He are highly analogous to those observed in polycrystalline Al (poly Al), indicating that the bubbles studied here do not have a significant influence on the mechanical deformation before tension. During the tension process, the expansion-merging of He bubbles dominates damage accumulation and leads to the ultimate fracture of the metal, the same as that in a single crystal with He bubbles. The presence of grain boundaries (GBs) does not exhibit an apparent effect on the evolution of He bubbles, resulting in comparable expansion rates for the bubbles in different locations (i.e., near GBs or at grain interiors). Additionally, the nucleation of voids occurs subsequent to bubble expansion due to the much higher critical stress. Voids are preferentially nucleated on GBs when the material is solid and at liquid parts when the material is partially molten, demonstrating that GBs and melting can strongly facilitate void nucleation. However, He bubbles significantly impede void nucleation and growth, resulting in a much smaller quantity and volume of voids formed in the poly Al–He, compared to the poly Al. Furthermore, the critical stress for void nucleation and the spall strength of the metal matrix are reduced by He bubbles.