The three-dimensional morphology and thickness of an Al 2 Ca Laves phase with a C15 crystal structure, which precipitated within the primary ¡-Mg grain of a Mg5Al1.5Ca alloy that had been over-aged at 523 K for 100 h, were investigated using high-resolution transmission electron microscopy. The C15Al 2 Ca precipitate exhibits a hexagonal plate-like morphology, with a planar surface parallel to the (0001) ¡ basal plane and the sides of the hexagonal plate parallel to the f11 20g ¡ second columnar plane of the ¡ matrix. A typical coffee bean contrast was clearly visible around the precipitate, which is indicative of the coherent precipitation of the C15Al 2 Ca phase with respect to the ¡-Mg matrix. The thickness of the Al 2 Ca precipitate, which corresponds to six layers of the (111) C15 plane composed of Ca atoms, was evaluated as approximately 1.5 nm.
The coarsening kinetics of a fine C15Al 2 Ca Laves phase with a plate-like morphology precipitated within the primary ¡-Mg grains were investigated for Mg5Al1.5Ca alloy aged at 523 K. The Al 2 Ca precipitate coarsened as its coherence was retained in the aging time below 300 h, and a quantitative relationship was obtained between precipitate length (l) and aging time (t) as l £ t 0.23 . However, the ¡/C15 coherent interface changed into a semi-coherent interface by introduction of misfit dislocations on the planar surface of the precipitates in the aging time above 300 h, which resulted in the promotion of Al 2 Ca coarsening. The Al 2 Ca phase was assumed to precipitate through a nucleation and growth mechanism rather than spinodal decomposition, and its coarsening was explained using the terrace-ledge-kink mechanism. The result shows that the aspect ratio of the Al 2 Ca precipitates was predominantly determined by the aging temperature, and it decreased at higher aging temperatures.
A Mg2.8 mass% Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg 2 Ca phase (with C14 structure), which precipitated in the primary ¡-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg 2 Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001) ¡ basal plane and sides that are parallel to the {-1010} ¡ columnar plane of the ¡-Mg matrix. The coarsening Mg 2 Ca precipitate retained coherency with the ¡-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 Mg 2 Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.
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