HRTEM Characterization of an Age-Hardened Mg-Ca Binary Alloy

A Mg-2.8 mass％ Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg2Ca phase (with C14 structure), which precipitated in the primary α-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg2Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {1010}α columnar plane of the α-Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the α-Mg matrix for aging time of 3 h, which corresponded to the peak-aged condition. Furthermore, during the coarsening process, misfit dislocations were introduced on the planar surface of the precipitates, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.