Dendritic Microstructure in Directional Solidification of Magnesium Alloys

We demonstrate morphological transitions in Mg-Al alloy dendritic microstructure as the cooling conditions change during steady state and transient directional solidification. The effect of temperature gradient on the transition is investigated numerically using two-dimensional phase field simulations. The six-fold symmetry of Mg alloys leads to very different dendrite morphologies than those encountered in alloys exhibiting four-fold surface tension anisotropy. In particular, we find that at high temperature gradients primary dendrites become columnar in the direction of thermal gradient. In contrast, in the regions where surface energy anisotropy is dominant, primary stalks cross at 60-degree angles that characterize hexagonal crystal structure. Our modelling observations are compared to new Mg-Al experiments.