Molecular Dynamics Simulations Of Deformation Mechanism Of Nanotwinned Magnesium With Different Orientations

The excellent comprehensive mechanical properties of Nanocrystalline (NC) metals are due to its unique plastic deformation mechanism. A large number of experimental tests, numerical simulation and theoretical analysis show that the interaction between dislocation and twin boundaries is the intrinsic reason for the macroscopic properties of the nanocrystalline twin metals, which is essentially different from that the interaction between dislocation and grain boundaries in polycrystalline. In fact, the microstructural features of twin (such as the twin sheet thickness, the length of the twin, the orientation of twin boundary, etc.), external loading, and stress state will directly affect the initial dislocation emission in the deformation process, thereby significantly influence the reaction between dislocation and twin boundary. Therefore, molecular dynamics simulations are applied in this paper to investigate the deformation mechanisms of the {1012}〈1011〉nanotwinned magnesium with different orientations under the uniaxial (tensile and compressive) and shear loadings. The main results are as follows:1. When the orientation angle is small, the deformation mechanisma of nanotwinned magnesium with different orientations under tension are the twin boundary migration caused by twinning dislocation slip and the face centered cubic phase transformation caused by basal dislocation slip. With the increase of the angle, the shear stress component increases, and the deformation mechanism is the migration of grain boundaries and the detwinning, no phase transformation occurs. In the range of 9nm-17nm, the twin boundary spacing has little effect on the deformation mechanisms of nanotwinned magnesium when the orientation is 10°. Furthermore, the temperature has no distinct influence on the deformation mechanisms of different oriented {1012}〈1011〉nanotwinned magnesium under tension. With the increase of the temperature, the twinning dislocation is easy to be activated at a smaller strain, while the influence of the temperature for the basal dislocation emission is very small. Moreover, the yield strength of the model decreases gradually with the increase of temperature, but the flow stress changes little.2. Deformation mechanisms of {1012}〈1011〉nanotwinned magnesium with different orientations under compression are mainly manifested as twin boundary migration caused by twinning dislocation slip and the detwinning caused by twin boundary migration. The emission of basal dislocations can only be observed when the angle is small (below 10 degrees). The linear elastic stage only appears when the strain is less than 1.2%, and with the increase of the angle, the linear elastic stage become short. In the range of 9nm-17nm, the twin boundary spacing has little effect on the deformation mechanisms of nanotwinned magnesium when the orientation is 10°. The temperature has no distinct influence on the deformation mechanisms of different oriented nanotwinned magnesium under compression. With the increase of temperature, the basal dislocation is easier to be activated, and. detwinning and basal dislocation emission will occur at a smaller strain.3. The plastic deformation mechanisms of {1012}〈1011〉nanotwinned magnesium under shear are twin boundary migration and detwinning caused by twinning dislocation nucleation and moving along the twin boundaries. In the range of 7 nm to 15 nm, the twin boundary spacing has little effect on the mechanical properties of the model. The temperature and strain rate have significant influence on the mechanical properties of the {1012}〈1011〉nanotwinned magnesium under shear. As the temperature increases, the yield strength decreases and the flow stress is also reduced. But the flow stress becomes more and more stable with the increase of temperature. With the increase of strain rate, the yield strength of the model decreases, but the flow stress don’t change distinctly.