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

Abstract

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.

Fig. 4 HRTEM image of the Mg–2.8 mass% Ca alloy aged at 423 K for 3 h, taken with B = [11-20]_α (a). The C14–Mg₂Ca precipitate was taken with B = [1-21-3]_α (b).