A Mg–2.8 massÿ 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 ÿ–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)ÿ basal plane and sides that are parallel to the {1010}ÿ columnar plane of
the ÿ–Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the ÿ–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.