Creep behavior and Microstructures in Grain Boundary Strengthened Mg-Al-Ca Thixomolded® Alloys

The microstructures of thixomolded (R) (TM) Mg-Al-Ca alloys consist of alpha-Mg and eutectic compounds along grain boundaries. Misch metal (Mm) addition to TM Mg-Al-Ca alloys makes precipitates within alpha-Mg matrix and their number density and size depend on heat-treatment conditions. The small addition of Mm can keep the network-like grain boundary covering and the improvement of microstructure stability during creep. On the other hand, excessive Mm addition causes the deterioration of creep strength. The grain boundary coverage decreases with increasing Mm content due to the formation of coarse spherical Al-Mm based intermetallic compounds. Creep strength is significantly affected by both of the grain boundary coverage and the morphology of eutectic compounds along grain boundaries.