Three-Dimensional Morphology of C15-Al2Ca 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 alpha-Mg grain of a Mg-5Al-1.5Ca alloy that had been over-aged at 523K 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) basal plane and the sides of the hexagonal plate parallel to the {11 (2) over bar0}(alpha) second columnar plane of the alpha 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 alpha-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.