Phase transformation and creep of Mg-Al-Ca based die-cast alloys

The microstructure and microstructural stability of die-cast AC53 (Mg-5Al-3Ca) and AXJ530 (Mg-5Al-3Ca-0.15Sr) have been investigated in detail by transmission electron microscopy (TEM). Both alloys have an as-cast microstructure of alpha-Mg with (Mg, Al)(2)Ca (dihexagonal C36) eutectic at grain boundaries. During aging at 573 K, the C36 phase transforms to Al2Ca (cubic C15) phase. These two phases have a crystallographic orientation relationship Of (0001)(C36)//{111}(C15) and [2110](C36)//[011](C15), and the transformation from C36 to C15 occurs by a shear-assisted process. Despite this change in the phase constitution, the network structure of the intermetallic compound(s) surrounding alpha-Mg gains is fairly stable, morphologically, even after prolonged exposure at elevated temperature. In the alpha-Mg matrix phase, precipitation of Al2Ca was observed after aging for 360 ks at 573 K. The precipitates are disc-shaped with a habit plane of {111}(C15)//(0001)(alpha). AXJ530 shows higher creep resistance than AC53. The dislocation substructure that evolved during creep deformation was investigated in both alloys, and the basal and non-basal slip of a-dislocation and other slip modes of a+c dislocations were observed. The relationship between creep properties and microstructure is discussed.