No abstract

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.

No abstract

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.