Three-Dimensional Morphology of C15–Al<sub>2</sub>Ca Precipitates in a Mg–Al–Ca Alloy

The three-dimensional morphology and thickness of an Al2Ca Laves phase with a C15 crystal structure, which precipitated within the primary α-Mg grain of a Mg–5Al–1.5Ca alloy that had been over-aged at 523 K for 100 h, were investigated using high-resolution transmission electron microscopy. The C15–Al2Ca precipitate exhibits a hexagonal plate-like morphology, with a planar surface parallel to the (0001)α basal plane and the sides of the hexagonal plate parallel to the {1120} second columnar plane of the α matrix. A typical coffee bean contrast was clearly visible around the precipitate, which is indicative of the coherent precipitation of the C15–Al2Ca phase with respect to the α-Mg matrix. The thickness of the Al2Ca precipitate, which corresponds to six layers of the (111)C15 plane composed of Ca atoms, was evaluated as approximately 1.5 nm.