A novel basal precipitate plate derived from a Mg2Ca Laves phase in a Mg−Nd−Ca alloy

Abstract

The basal precipitate is a category of the key precipitation strengthening phase in Mg-based alloys. In this study, a novel basal plate designated as γ″′ in a Mg−Nd−Ca alloy is investigated by high-angle annular dark-field scanning transmission electron microscopy. It is found that the γ″′ phase has a hexagonal structure (a = 0.604 nm and c = 1.008 nm) and a ternary composition. Structurally, it can be described as an alternate arrangement of the γ″ plate and the (0001) monolayer atomic layer. Particularly, different with the conventional γ″ phase, the γ″′ phase has an extraordinary origin and is proved to be derived from a Mg2Ca C14 Laves phase, which is formed earlier in β1 chains in the shape of “bubble”. This study contributes to provide a new understanding of the formation mechanism of γ-type basal precipitates.