Molecular Dynamics Study On The Plastic Deformation Mechanism Of Single Crystal Magnesium With Initial Defects

Deformation mechanism of magnesium and its alloy has always been an important topic in our group. It has becoming a popular trend in material science to map nano-scale crystal behavior into macro-scale behavior of material. Reports show that temperature, loading rate, geometry, surface condition and initial defects of crystal are factors which influence twinning nucleation. For long time, there is no satisfactory explanation for the size effect in hcp crystals. This thesis focuses on the affection of initial defects to c-axis compression of magnesium single crystal.(1) Using Molecular Dynamics method, magnesium single crystal models with different vacancy density are constructed. Periodic direction is [1210]. Compressive loadings are applied in [0001] direction on six models. The simulation shows that the vacancy will not disappear after the relaxation, and the vacancy density will keeps almost the constant. In addition, the strength of model with vacancy density less than density 0.05%, will not decrease with the increasing of vacancy density. Twinning is also not observed in compressive condition.(2) Using Molecular Dynamics method, magnesium single crystal models with different shape of initial defects are constructed, and the periodic boundary condition is applied in [1210] direction. Square, rectangular and triangular surface defects with different sizes and also various aspect ratios are studied. Under compressive loadings, dislocation nucleation position is affected by both shape and dimension of the surface defects. For instance, dislocation nucleation is less prone to happen in surface with the increase of the size of the rectangular defect. There is no deformation twinning is observed in compressive simulation. It is still the pyramidal dislocations and basal plane stacking fault that dominate the plastic deformation in magnesium.(3) Using Molecular Dynamics method, magnesium single crystal models with multiple rectangular defects are constructed. We focus on the interaction between the adjacent defects, and their effect on the overall plastic deformation. Through the deformation simulation of models with two and three adjacent rectangular defects, it is found that the initial dislocation is less likely to nucleate on the surface defect when the defects are closer. There is no deformation twinning in compressive simulation. It is still the pyramidal dislocations and basal plane stacking fault that dominate plastic deformation in magnesium.