Molecular Dynamics Simulations Of Nano-void Shrinkage In γ-TiAl And Amorphous Crystallization Mechanism Of Ti₃Al

As one of the most development potential of high temperature alloy material,γ-Ti Al alloy has high oxidation resistance, high melting point, low density and excellent high temperature strength, and has become the focus of research in low density alloys. Compared to γ-Ti Al, the deformation process of Ti3 Al amorphous alloys is more complicated. The strength, the hardness as well as the elastic limit of them are very different from the γ-Ti Al alloy. Therefore, it is very important to study the deformation of the γ-TiAl crystalline alloy and Ti3 Al amorphous alloys.In this thesis, molecular dynamics simulation was performed to study the γ-Ti Al single crystal with void in uniaxial compression load. The mechanism of the spherical nano-void shrinkage was analyzed, and the effects on the mechanical behavior of the specimen size, void volume fraction, and strain rate were also investigated. The results show that during the whole compression process, the nano-void undergoes both elastic deformation and plastic deformation. In the elastic deformation period,the nano-void shrinks linearly along compression direction while it expands linearly along the directions perpendicular to the compression direction. While in the plastic deformation period, the nano-void collapses from all directions at the beginning of the plastic process. It is found that the stacking faults’ slipping in the crystal during plastic period is the main deformation mechanism of the void shrinkage: dislocation structures initially start to nucleate around the equator area of the void, then partial dislocations and planar faults constantly propagate, finally the stacking faults and partial dislocations swap through the entire system and the void shrinks. Void volume fraction and strain rate have significant effect on the yield strength and Young’s modulus of the sample, and the specimen size is only effect on the yield strength.The crystallization behavior of Ti3 Al amorphous alloy under uniaxial compression load is also studied in this thesis. In the process of the simulation, we consider the effect of different cooling rates on the formation ability, and the effect of different strain rates on the crystallization behavior of Ti3 Al amorphous alloy, the mechanism of stress crystallization is also analyzed. The results show that the Ti3 Al liquid alloy can be changed into a crystal structure of long range order during the cooling process in the condition of the cooling rate is 1×1012K/s. As the cooling rate increases gradually, Ti3 Al liquid alloy can be changed into disordered amorphous structure, and the atoms distribution of disordered structure is more homogeneous when the cooling rate is 1×1014K/s. During the compression process, the larger thestrain rate is, the more easily the Ti3 Al crystallization in the process of deformation.In the process of stress crystallization, the crystallization process of Ti3 Al amorphous alloy is divided into two periods: initial nucleation and crystal growth. The shear deposition in the amorphous matrix is the main crystallization mechanism, which is accompanied by the surface diffusion process of a single atom by amorphous to the initial nucleation.