An experimental and theoretical investigation of the effect of second-phase particles on grain growth during the annealing of hot-rolled AZ61 magnesium alloy

The mechanical properties of a magnesium alloy strongly depend on its grain structure. It is desirable to minimize grain growth during the post-forming annealing treatment, thereby minimizing the loss in strength while regaining ductility. In the present research, the annealing of the hot-rolled AZ61 Mg alloy at different temperatures for different times was performed to reveal the kinetics of grain growth, as affected by the precipitation or dissolution of second-phase particles. Three approaches, i.e., experimental, analytical modeling and atomistic simulation were taken and the results were compared. The predictions made from the analytical model and Monte Carlo simulation were both in acceptable agreement with experimental results in terms of the resultant grain sizes. However, the Monte Carlo simulation showed advantages over the analytical model. It was found that with increasing annealing temperature and holding time, the fraction of second-phase particles reduced, which strongly affected the kinetics of grain growth, limiting grain size and grain size homogeneity. The average grain sizes and the largest grain sizes were both taken as the characteristic parameters of the as-annealed microstructure. The results pointed out the importance of choosing an appropriate combination of annealing temperature and time in order to retain second-phase particles during annealing not only for preventing unrestricted grain growth but also for avoiding grain size inhomogeneity.