On the multiple orientation relationship of the Mg/γ-Mg₁₇Al₁₂ precipitation system

The six orientation relationships (ORs) found in the Mg/γ-Mg₁₇Al₁₂ precipitation system were summarized and systematically interpreted based on the atomic structure of the precipitate γ-Mg₁₂Al₁₇ and the invariant deformation element (IDE) model for diffusional phase transformations. It was found that the pseudo-twinning relationship between the six ORs is a reflection of the pseudo-twinning relationship between the close- or near-close-packing planes (), (11), (033), (411) and () in the precipitate γ-Mg₁₂Al₁₇. As a result, the Pitsch–Schrader OR is the starting point for the other five ORs. Multiple morphologies, growth directions and habit planes could be rationally interpreted by the IDE model. This implies that a multiple orientation relationship between the variants of precipitates is favourable in order to minimize the gross energy of precipitation systems in which the matrix has a simple structure while the precipitate has a complicated structure, such as Mg/γ-Mg₁₂Al₁₇, Mg/δ-Zn₂Zr₃ and Mg/η-MgZn₂ couples.