Multi-orientation relationship and {112}_γ pseudo-twin relationship of γ-Mg₁₇Al₁₂ precipitates in an AZ91 Mg-Al alloy

The multi-orientation relationships and {112}_γ pseudo-twin relationship of three types of γ-Mg₁₇Al₁₂ precipitates (bcc) formed in an aged AZ91 Mg-Al alloy were studied using TEM. Keeping a Burgers, Crawley and Porter relationship with the Mg matrix (hcp) respectively, these precipitates were lath-, hexagonal prism- and rod-shaped; and a {112}_γ pseudo-twin relationship was formed both between the same kinds of precipitates and between different kinds. The formation mechanisms of the multi-orientation relationship and {112}_γ pseudo-twin relationship were discussed in terms of the transformation crystallography, and the remarkable difference in density of the three precipitates was explained based on the misfits between individual precipitate and the matrix.