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

The six orientation relationships (ORs) found in the Mg/gamma-Mg17Al12 precipitation system were summarized and systematically interpreted based on the atomic structure of the precipitate gamma-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 ((8) over bar(7) over bar(7) over bar), ((4) over bar 11), (033), (411) and (8 (7) over bar(7) over bar) in the precipitate gamma-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/gamma-Mg12Al17, Mg/delta-Zn2Zr3 and Mg/eta-MgZn2 couples.