The Simulation And Research For A Part Of Micro-factors Influencing Recrystallization And Grain Growth In AZ31Mg Alloy

Phase-field model is a physical model in form of Ginzburg-Landau expressions based on classical thermodynamics and dynamics theory with some phenomenological parameters. In the phase field model, a set of field variables as a function of time and space are introduced to describe transient state in the system, in order to provide relevant information about microstructure evolution.An improved phase field model is established in present paper based on our previous work. A local free energy density expression for recrystallization was suggested in the model by introducing composition variables and a stored energy term in it. The model can be used to simulate microstructure evolution in real space-time applied in industry range. Micro-factors which affect the recrystallization and grain growth process of AZ31Mg alloy were studied by simulation in different parameter values in the model.From the simulation results, it is shown that the coupling item coefficient K1in local free energy function has no effect on the size of the grain boundary range, and only the ordering parameter gradient term, K2, has the determing effect on it. Moreover, both the coupling item coefficient K1and the gradient term K2have important effects on the grain boundary energy though the K1plays a more important role.By changing the value of stored energy as a micro-factor, we have studied its influence on the process of recrystallization and grain growth in AZ31Mg alloy. The simulation results show that, at a certain temperature, both the average grain size and the growth rate decrease with the increase of stored energy. When recrystallization temperature is elevated from250℃to300℃, the stored energy has a significantly increasing effect on the recrystallization and grain growth. However, when the temperature is further up from300℃to400℃, the effect of the stored energy changes little, and the increment of average grain growth rate becomes small.The paper has also investigated the influence of boundary energy as a micro-factor on the process of recrystallization and grain growth in ZA31Mg alloy system. A critical value of boundary energy was found by present study and the value is0.33J/m2. The average grain size and grain growth velocity increase with increasing grain boundary energy when the boundary energy is larger than the critical value in the alloy system and the effect of grain boundary energy on recrystallization behaviors can be neglect when grain boundary energy is less than it.In addition, the phase field model was adapted to simulate grain growth in a nano-structural polycrystalline system to research the abnormal grain growth in nano-structure AZ31Mg alloy. The simulation is intended to provide new ways to control the microstructure of nano-structural materials for improving their plasticity and ductility. By studying the simulation result we found that, the expected grains mixture microstructure with partial abnormal grain growth might be obtained by controlling three key micro-factors of interface energy, strain restored energy and interface mobility. If there is a small group of specially orientated nano-size grains in the original nano-structure with local low grain boundary energy or local high strain energy or local high interface mobility, the microstructure with scattered a few enormous grains in the nano-structural matrix can be achieved in order to improve the ductility of nano-structure materials while maintaining their high strength.Finally, the influence of Al atom diffusion in the AZ31alloy on grain boundary segregation and grain growth process is examined in present paper. The simulation conclusions are as following:the grain microstructure represented by the ordering parameter is consistent with that formed according to the Al composition field, which proves that the construction of the local free energy density function is correct. Moreover, if the value of diffusion mobility is large, the segregation concentration of Al composition at the grain boundary is severe. When the diffusion mobility is large enough, it has little impact on the average grain size. However, if the diffusion mobility is less than a critical value, reducing the diffusion mobility will lead to a significant increase of the average grain size. The segregation of Al composition is smooth along straight grain boundary but fluctuates along bending grain boundary. The distribution of Al atom within the grain is more homogeneous and the grain boundary segregation is more significant during recrystallization if the diffusion mobility is large.