Evolution of microstructural length scales during solidification of magnesium alloys

The microstructure of magnesium alloys usually presents a sixfold symmetry that is not amenable to the usual characterization of fourfold symmetric alloys. A more systematic and quantitative description can be achieved through Fourier analysis of the spatial distribution of the microstructure. Through this analysis, the microstructures obtained from phase-field simulations reveal self-affine regions with different scaling dimensions and crossover regions between them. The time evolution of these spatial scales and the analysis of local microstructural events reveal that isolated liquid pools are created through the merging of dendrite branches that follow the universal dynamics of merging predicted by Aagesen et al., and proceed to contract following the dynamics dictated by late stage cooling and back diffusion. Our results are consistent with new experiments that reveal a correlation between the dependence on cooling rate of the length scales associated with the spacing of main branches, and the distribution and average size of constituent phases that form during late stages of solidification.