Three-Dimensional Morphology of C15–Al₂Ca Precipitates in a Mg–Al–Ca Alloy

Abstract

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.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 193–197.

HRTEM image of the C15–Al₂Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [11-20]_α.