A Reserch On The Influence Of Microcosmic Mechanism Such As Stored Energy, Nucleation Etc On Recrystallization Of AZ31Magnusium Alloy

Recrystallization is a major method to refine grain size and to control mechanical properties of alloys. AZ31is a popular applicable magnesium alloy but with low plasticity at room temperature and weak dispersion strengthening by precipitates compared to aluminum alloys so that recrystallization is more important to the alloy to tailor its properties. Therefore, AZ31magnesium alloy is selected as the target material to study here because recrystallization can provide a proper microstructure to enhance the performance for the alloy. A phase field model has been established to simulate microstructure evolution in recrystallization of alloys. Phase-field simulation of recrystallization in industrial space and time scale has been realized, and the simulation results are consistent of well with experimental measurements. The relationships between recrystallization grain size and main micromechanisms were studied by simulation.Phase-field models to simulate microstructure evolution in solid state were discussed. The effect of different values of simulation parameters on features of the grain boundaries in the phase field model were investigated systematically. A new conception of grain boundary range was suggested. The gradual variation range of the order parameter cross a grain boundary corresponds to the boundary range, and its physical meaning is found as the range of grain boundary energy distribution across the boundary. The range is also corresponding to the segregation range of alloying elements around the boundary.A format of f(ηide,ηjre)=D(ηide)2(1-(ηjre)2) is suggested to express stored energy by cold deformation in present model for simulating recrystallization in alloys. The mechanisms of cold deformation on the stored energy and on the deformation grain morphology were examined both in simulation and literature review. The common experimental phenomenon, that there is a peak at the critical cold strain on the recrystal grain size as a fuction of cold deformation, was proved by our simulation results. A theoretical explain to the mechanism of the peak was also established by discussion on the simulation.Value of parameters B1re, B1de B2re and B2de were determined according to boundary energy and the free energy difference between the order parameters equal to1and equal to zero. Values of A, Al and A2were determined by numerically fitting the free energy density fuction with the experimental curve given in the thermodynamic software THERMOCALC. It is shown that the gradient parameter decides the size of the grain boundary range but boundary energy is determined by both the gradient parameter and the free energy coupling parameter. The effects of the boundary range value on microstructure features after recrystallization were examined by simulation. The simulation results are consistent of well with reported experimental measurements when the boundary range has the value of1.18μm.L is the coefficient related to the grain boundary mobility and may be expressed as the form of Arrhenius formula. And activation energy in the Arrhenius formula was found being the segregation activation energy of zinc whose difference with matrix atoms in size is the largest among the alloying elements in the alloy. In order to obtain the constant Lo, which is unconcerned with temperature, the experimental grain size measurements at400℃as a function of recrystallization time were used as the target curve in simulation.The recrystallization processes of AZ31magnesium alloy during a temperature range of210℃to400℃were simulated by means of the new model. The simulated results show that recrystallization grain size increaces with stored energy. Influence of the stored energy is obvious at initial stage of recrystallization, descends with annealing time. The nucleation process of recrystallization is realized for the first time by the model based on the theory that nucleation in recrystallization process is bulging of some original grain boundaries in deformed polycrystalline. The physical feature of the nucleation is indicated by a high speed release of stored energy at the initial stage of recrystallization. It is found that the boundary nucleation rate has little influence on recrystallization when the rate is greater than a small critical value. Recrystallization grain size increases with the original grain size after cold deformation but decreases with aspect radio of the original deformation grains. Grain growth of a cold deformed alloy during the annealing process was exposed automatically into two stages by present modeling:recrystallization and thermal growth. The theoretical time to finish recrystallization found by simulation is only about2/3of that defined by textbooks and this showes that the textbook definition of recrystallization was based on experimental so that the definition is not accurate with some thermal growth stage in it.