Coupling of spallation and microjetting in aluminum at the atomic scale

Nonequilibrium molecular dynamics simulations were carried out to explore the coupling behaviors of spallation and microjetting in single-crystal (SC) and nanocrystalline (NC) Al at the atomic scale. Both SC and NC models exhibited void collapse, serving as an indispensable element complementary to the classical ductile fracture mechanisms dominated by nucleation, growth, and coalescence. Two representative mechanisms---compressive collapse and spontaneous collapse---were uncovered, with a unique behavior in which a coalesced void also collapsed. It was also discovered that the spallation might either cause the microjet to disappear or accelerate fragmentation, with the disappearance effectuating a peculiar transformation from coexisting spallation and microjetting to pure spallation. The difference between SC and NC microjetting models residing in that grain boundary not only caused a larger peak velocity of the spike tip due to the inhomogeneous deformation but also restrained the Richtmyer-Meshkov instability growth to some extent owing to energy dissipation. The jet sheet fragmentation was attributed to three mechanisms: void nucleation, growth, and coalescence for the jet body; longitudinal necking induced by the tensile stress for the residual one-dimensional jet body; and transverse necking induced by the shear and tensile stresses for the jet head.