Spall failure of ultra-high purity single crystals

Spall failure of metals is governed by void nucleation, growth, and coalescence. Recent experimental observations suggest that under extreme conditions nano-void nucleation may be mediated by vacancy clustering in addition to the more traditional view of void nucleation at second phase particles. Here, we use molecular dynamics simulations as a tool to investigate the time-dependence associated with vacancy clustering in ultra-high purity single crystals. The time evolution of the vacancy cluster size statistics are investigated as a function of temperature, pressure, and vacancy concentration. It is shown that increasing the temperature and/or initial vacancy concentration can accelerate the incubation time for generating a cluster of a critical size. Since the critical stress to grow these nano-voids is strongly size-dependent, an interesting time-temperature-vacancy concentration coupling may arise in spall failure. We propose a continuum theory for this diffusion-fracture coupled process, which is shown to provide favorable agreement with our molecular dynamics simulations. This atomistically-informed theory of void nucleation may be integrated into multiscale frameworks of ductile fracture of metals to predict the macroscopic implications of these atomistic processes.