Grain Growth in Multiple Scales of Polycrystalline AZ31 Magnesium Alloy by Phase-Field Simulation

A multiscale phase-field model was established on the assumption of an isotropic single-phase system to simulate the realistic spatiotemporal process of grain growth for polycrystalline Mg-Al-Zn alloy AZ31, especially to determine the mechanisms for unique nanostructure evolution. The expression of the local free energy density function was improved according to different driving forces. The grain boundary range and grain boundary energy were studied in each scale to determine the correct gradient and coupling parameters, respectively. It is shown that the grain boundary energy in nanoscales is lower down to about half that in the micron scale, the time exponent n in the kinetic equation is varied from 5 to 2 from the nanograins to the micrograins, and the grain growth rate in nanoscale is much slower in an order of magnitude than that in the micron scale. These findings can be proven by the limited experimental results in the literature. Simulations expose that the solute atoms like to segregate at the grain boundaries much more severely in nanostructure than that in conventional microstructure, and this may be the reason why nanostructure shows a low boundary mobility to result in a strange low grain growth rate at up to an initial long annealing time.