MICROSTRUCTURE EVOLUTION OF NANOCRYS-TALLINE AZ31 MAGNESIUM ALLOY BYPHASE FIELD SIMULATION

Abstract

A phase field model has been established to investigate grain growth of nanocrystalline AZ31 Mg alloy under realistic spatial-temporal scales.Most previous phase field models are limited to grain growth at micron scale.A set of rules as following has been proposed to determine the real physical value of all parameters in this new model.The expression of local free energy density function is modified due to the different initial state of grain growth process at nanoscale.The grain boundary range and grain boundary energy are studied to determine the correct gradient and coupling parameters,respectively,where the term of grain boundary range is to explain the physical backgrounds of the order parameter gradients at grain boundary and the diffusion grain boundary.The mobility constant of grain boundary for this model is originated by fitting a group of grain size from experimental results and then the values of grain boundary mobility at different temperatures are calculated by the Arrhenius equation combined with this mobility constant.The study aims especially to find out the mechanisms for nano-structural evolution by comparing the simulated results with experimental results in the literature and simulated results in micron scale.It is shown that the grain boundary range will cover two adjacent grains in nanoscale polycrystalline and the grain boundary energy is lower down to about a half than that in micron scale polycrystalline.It is found that the grain growth rate at nanoscale is slower than that at the micron scale,and these simulated results can be proved by the experimental results in the literature.Simulations expose that solute atoms would like to segregate at the grain boundaries more severely in nano-structure than in micron-structure,and this may be the reason why nano-structure shows a lower boundary mobility to result in a strange low grain growth rate in the first stage.It is found that the grain size fluctuation is more intensely in nano-sized grains than that in micron-sized grains by the quantitative analysis of the mixed degree of grains size in nano-structure and micron-structure in the models.