Molecular dynamics study on shock-induced spallation and damage evolution in nano-polycrystalline Ta: Internal grain size effect vs external shock intensity effect

Abstract

As a typical body-centered cubic material, Ta has both high strength and high temperature resistance and is, thus, widely applied in the field of high-energy physics. In this work, the spallation behavior and its underlying physical mechanism of nano-polycrystalline Ta was systematically studied by non-equilibrium molecular dynamics simulations, with special attention to the internal grain size effect vs external shock intensity. The results reveal that the grain size effects on void evolution, spallation strength, and corresponding mechanical and thermodynamic responses are different under different shock intensities. Under the piston velocity of 0.75 km/s, when the grain size decreases from 30 to 2 nm, the spallation mechanism switches from intergranular fracture (d ≥ 10 nm) to cavitation failure (d ≤ 5 nm), and the correlation between spallation strength and grain size also switches from an inverse Hall-Petch to a Hall-Petch relation at a critical grain size dc ∼10-20 nm. As the piston velocity increases to 1.5 or 1.8 km/s, a failure mode transition from classical spallation to micro-spallation is observed, leading to a significantly weakened grain size effect on the spallation strength. Through thermodynamic analysis, melting is detected in the tensile region, which is responsible for the micro-spallation. These results can help to understand the effects of internal grain size and external shock intensity on the spallation behavior of Ta and make a leap in the design of shock-resistant materials.