Abstract:

The dominant deformation mode at low temperatures in magnesium and its alloys is generally regarded to be twinning because of the hcp crystal structure. More recently, the phenomenon of a “loss” of twins has been reported in microcompression experiments on magnesium single crystals, while significant plasticity and hardening occurred due to six active pyramidal slip systems. These results pointed to an intriguing deformation pattern of magnesium single crystal at the nanoscale, namely no deformation twins are present in the microcompression deformed specimens. In this work, the deformation behaviors in magnesium single crystal under the c-axis tension and compression are investigated by molecular dynamics simulations. Under c-axis tension at low temperature, twinning is indeed found to be the main deformation mechanisms, which are consistent with the related experimental results. However, simulations of c-axis compression show that the pyramidal slip dominates under compression loading. No compression twins are observed in simulations at different temperatures for different loading and boundary conditions. This is explained by an analysis of the lattice structure of twins in pure magnesium, revealing that the change of the strain energy caused by the lattice rotation may play a more important role on the plastic deformation mechanisms at the nanoscale. Our theoretical and simulation results provide a unifying interpretation of recent microcompression experiments on magnesium (0001) single crystals.