Molecular Dynamics Study On The Tensile Deformation Of Cu And γ-TiAl Alloys

Material properties do not completely depend on their chemical composition. Microstructures including the configuration of dislocations and grain boundaries have decisive effect on the mechanical properties of metallic materials in general. Existence of grain boundaries in polycrystalline metallic materials leads to complicated mechanic behavior due to the effect of atomic structures and orientation of the boundaries. Consequently, it is important to preview and improve the mechanical properties of polycrystalline metallic materials that studying on the atomic structures and plastic deformation behaviors of grain boundaries. Employing molecular dynamics method, the present thesis discusses two important issues, the effect of grain boundaries on the tensile deformation mechanisms and the mechanical properties of Cu and γ-TiAl alloys.The simulation results of tensile deformation process of Cu bicrystals shows that the peak tensile stress is different with various orientation angles. The peak tensile stress decreased with the increase of the orientation angle among the boundaries which orientation angle is greater than 90°. There are dislocation emissions or not indicate the difference of deformation mechanisms of Cu bicrystals with various orientation angles. Fracture mechanisms of Cu bicrystals are that micro-cracks nucleate at the grain boundary and propagate along the boundary.Polycrystalline cell of γ-TiAl alloy was built up by using the voronoi algorithm, and tensile deformation process was simulated by molecular dynamics method. The results indicate that tensile stresses of Polycrystalline γ-TiAl alloy present a process of elastic stage, yield stages, plastic deformation stage and fracture stage, which is agree with the tensile deformation process of macroscopic γ-TiAl alloy. The true stress-true strain curve of Polycrystalline cell shows obvious sensitivity with the strain rate, that is, yield stresses and peak stresses increase with the tensile strain. Anti Hall-Petch relationship was shown in the tensile deformation process of Polycrystalline cell with different grain size. Sliding and rotation of the grain boundaries first occurred at the early stage of tensile deformation of the Polycrystalline cell, and which played an essential role in the deformation stage with few dislocation nucleate at the grain boundaries. When the true tensile strain was up to 0.203, the dislocation density quickly increased at the boundaries, indicating that dislocation movement is getting to the main deformation mechanisms of Polycrystalline γ-TiAl alloy with the gradual increase of the tensile strain.