Atomic-scale modeling of the void nucleation, growth, and coalescence in Al at high strain rates

Abstract

Molecular dynamics (MD) simulations in single-crystal (SC) and nanocrystalline (NC) aluminum (Al) were performed using Large-scale Atomic/Molecular Massively Parallel Simulator software (LAMMPS) to investigate the nucleation, growth, and coalescence of voids at the micro-scale under isotropic and triaxial tension. The effects of temperature, strain rate, initial pressure, and grain size were examined. Compared with experimental results, the Hugoniot curve of Al was predicted to validate the accuracy of the selected potential function. Based on the relationship between the tensile stress and void volume fraction, the progressive micro-damage procedure was divided into four stages: atomic disorder, nucleation, growth, and coalescence of voids. These are characterized by the void volume fraction history and the cross-sectional internal and external snapshots of the void distribution. The strain rate had a more significant effect on the micro-fracture than temperature. In addition, an initial pressure of 1000 MPa impeded the time of initial void nucleation and growth. Intergranular, transgranular, and intragranular fractures occurred in NC Al but only intragranular fracture appeared in the SC Al. The void volume fraction of NC Al was independent of grain size (6 and 60 nm), which affected the void number inside the grains. Furthermore, the parameter identification of the nucleation and growth (NAG) model was achieved using an improved genetic optimal algorithm. Thus, the relationships between tensile stress, void volume fraction, and NAG parameters were explored.