Growth crystallography and in-situ imaging of nucleation and growth dynamics of Al8Mn5 solidifying in AZ magnesium alloys

Many magnesium-aluminium-based alloys contain a small manganese addition to improve corrosion resistance. However, this introduces Al-Mn intermetallics which add complexity to the phase transformations. Here we study the crystal growth of the most common Al-Mn phase, Al8Mn5, in Mg-Al-Zn-Mn-based magnesium alloy solidification by combining electron microscopy of the faceted growth crystallography with in-situ synchrotron X-ray imaging of the Al8Mn5 nucleation and growth dynamics. Three Al8Mn5 morphologies, equiaxed, rod and plate, are shown to all come from cyclic twinned growth associated with the pseudo-cubic symmetry of rhombohedral Al8Mn5. X-ray imaging revealed Al8Mn5 nucleated throughout the freezing range and grew with α-Mg dendrites over a wide temperature range by divorced eutectic solidification. This occurs because, in these alloys, Mn solute has little influence on the solute undercooling of growing α-Mg dendrites, although strongly affects the constitutional supercooling with respect to the Al8Mn5 liquidus. Rod/plate growth of Al8Mn5 is shown to be promoted by slow cooling rates and by divorced eutectic solidification. The findings provide new insights into the conditions that cause large, deleterious rods and plates, and explain why there is a large variation in Al8Mn5 growth morphology after solidification.