Molecular dynamics simulations on the intergranular crack propagation of magnesium bicrystals

As an important micro-structure, grain boundary plays a significant role in the micro-structure evolution and mechanical property of metallic materials. In this study, comprehensive molecular dynamics simulations were performed to investigate the propagation mechanisms of intergranular cracks along [11-00] symmetric tilt grain boundaries in magnesium bicrystals under tensile loading conditions. The effects of grain boundary misorientation angle, temperature and solid solution were considered. The results show: At low temperature, most of the cracks with different misorientation angles propagate by the brittle cleavage mechanism, which generally agrees with the theoretical predictions based on the Rice’s model. With increasing the temperature, the crack propagation mechanism changes, on the one hand, from brittle cleavage to void nucleation, growth and coalescence; on the other hand, the local plastic deformation becomes favorable near the crack tip, due to the dislocation emission and nanograin nucleation via the split of original grain boundary, which relaxes the stress concentration and shields the crack tip. Also, the brittle-to-ductile transition is observed due to the effect of temperature. By adding the alloying elements of Al, Ca, Zn, it is seen that the strength of the grain boundary is enhanced. This work provides insights into the understanding of the ductility of magnesium and its alloys.