Atomistic Simulation Of Deformation Behavior And The Micro-structural Evolution In Magnesium Single Crystal

Magnesium and its alloys have attracted attention in recent years as lightweight for the transportation industry due to their low density and relatively high specific strength. However, the application of magnesium alloys is restricted due to the limited forming capacity and the poor corrosion resistance. From the mechanical point of view, magnesium and its wrought alloys show a pronounced direction-dependence of plastic yielding and work hardening, as well as different yielding behavior in tension and compression, the so-called strength differential effect. This feature is due to the crystal structure of magnesium, which is a hexagonally close-packed (hcp) structure. Compared to face-centered cubic (fcc) or body-centered cubic (bcc) metals, the number of slip systems allowing plastic deformation in magnesium is limited. It is generally agreed that mechanical behavior of magnesium is sensitive to twinning and slip. Nevertheless, deformation mechanisms acting on magnesium and its alloys are sophisticated and still a subject to be discussed. Therefore, the deformation mechanism is the key rule of magnesium and its alloys in rencent years.Because magnesium single crystal and magnesium alloys have the same or similar structures and deformation characteristics, it is an effective way to investigate the deformation behavior of magnesium and its alloys by researching the behaviors of magnesium single crystal. In this thesis, molecular dynamics simulatin at a certain strain rate is applied to investigate deformation mechanism in magnesium single crystal. The tension and the compression are applied along the c-axis direction and the shear stress is applied perpendicular to the c-direction at different temperatures. Furthermore, the influence of other defects in magnesium single crystal on the twinning behavior is discussed. The main contents and results including:1. Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under c-axis extension and compression at different temperatures. For extension, the {1012} twin and the pyramidal slip are found to be the main deformation mechanisms under the c-axis tension in magnesium single crystal. For compression, the<c+a> pyramid slip is found to be the main deformation mechanism, instead of the twin.2. Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under shear at different temperatures. These simulation results indicate that stacking fault and phase transformation are the main deformation mechanisms under shear. The new phase shares the same close-packed plane with the original hcp lattice.3. The influence of the point defect、the linear defect and the plane defect on the movement of {1012} twin in magnesium single crystal at different temperatures are investigated by molecular dynamics simulation. The simulation results indicate that the vacancy and the linear defects have no distinctly influence on the movement of twin. For the plane defect, at ultralow temperatures (5K), it will impede the {1012} twin movement; at low and room temperatures (150K-350K), the plane defect impedes twin movement when the twinboundary moves near it. However, when the twin boundary pass through the head of this line defect, the effect of this line defect hindering the twin boundaries movement can be ignored; at elevated temperatures (350K-580K), the impeding effect of this line defect on the twin movement can be ignored.