Molecular dynamics investigation of loading orientation effect on dynamic behaviors of void in aluminum

The micro-damage mechanisms in ductile metals play a very important role in the spallation process. Nonequilibrium molecular dynamics (NEMD) is used to examine the effect of loading orientation on void evolution in single crystal (SC) and nanocrystalline (NC) aluminum under isotropic tensions. Besides void nucleation, growth, and coalescence, a brand-new phenomenon, void collapse, is also discovered in the late growth stage, and the collapse behavior is confirmed by the atomic trajectory tracing approach (ATTA). The loading orientation has a significant impact on the maximum void number, maximum void surface area, and void collapse velocity in the SC model but not in the NC model; while the void volume and its fraction that can capture the void collapse behavior are unaffected by the loading direction in the SC and NC models. In the SC model, the loading orientations [−110][001][110] and [100][010][001] make void collapse hardest and simplest. Meanwhile, with the help of the slicing method, it is discovered that the damage mechanism in the NC model is dominated by intracrystalline, intercrystalline, and transcrystalline fractures, with the second being the directional fracture along the grain boundary, whereas it belongs to the intracrystalline fracture in the SC model. In addition, the material experiences the hardening and softening phases during the nucleation stage, which are distinguished by the maximal tensile stress.