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

Abstract

The three-dimensional morphology and thickness of the C15–Al2Ca Laves phase, which precipitated within the primary α-Mg grains, were investigated for the Mg–5Al–1.5Ca alloy over-aged at 523 K for 100 h using the high-resolution transmission electron microscopy (HRTEM). The C15–Al2Ca phase precipitated with a hexagonal plate-like morphology along the (0001)α basal plane of the α-Mg matrix phase, where the sides of the hexagonal plates were parallel to the second columnar plane {1120}α of the α matrix. The typical coffee bean contrast was clearly visible around the precipitates, indicative of coherent precipitation of the C15–Al2Ca phase. The thickness of the C15–Al2Ca precipitates, which corresponds to the six layers of (111)C15 plane composed of Ca atoms, was evaluated to be approximately 1.5 nm.