Molecular dynamics investigation of void evolution dynamics in single crystal iron at extreme strain rates

High strain rate deformation and fracture of materials is of interest for high velocity impact and penetration problems. In this work, we perform triaxial deformation of single crystal BCC iron at (1.5 x 10(8)-1.5 x 10(10)) s(-1 )volumetric strain rates with 300 K temperature and find that at very high volumetric strain rates, relaxation of tensile stress takes place first through the structural changes (phase transformation) and then via nucleation and growth of voids. While at lower volumetric strain rates, the nucleation and growth of voids is the only route to release the built up stress. High void nucleation events occur at volumetric strain rate of 1.5 x 10(10) s(-1) while average void growth rate is high at 1.5 x 10(8) s(-1 )volumetric strain rate. This suggests that the nucleation of voids is more preferred at 1.5 x 10(10) s (-1) volumetric strain rate while void growth is more preferred at 1.5 x 10(8 )s(-1) volumetric strain rate to accommodate the applied strain. The evolution of individual void volume fraction takes place through the discrete jumps indicating coalescence events. All the nucleated voids do not grow with equal rates and hence do not make significant contribution to the overall void volume fraction. Size distribution of voids follows exponential distribution in the region where nucleation and growth processes of the voids contribute most to the overall void volume fraction while power law function describes the void size distribution in the coalescence dominated regime of the overall void volume fraction. A framework to compute overall void volume fraction in terms of the individual voids is presented. The results can be useful to develop the fracture models at high strain rates to describe the damage evolution at continuum length scales.