Morphology and crystallography of β-(Mg₁₇Al₁₂) precipitate in an AZ91 magnesium-aluminum alloy

Morphology and crystallography of continuous precipitates formed in an aged AZ91 alloy were studied using TEM. Four kinds of β-(Mg₁₇Al₁₂) precipitates with different morphologies and crystallographgies were found, with a majority of the precipitates being the first, or lath-shaped one. While keeping a Burgers orientation relationship (O.R.) with the matrix and taking the base plane (0001)_α as its habit plane, the lath-shaped precipitate grows parallel to [7, 2, 9, 0]_α in the base plane. All the crystallographic measurements of the lath-shaped precipitates are in excellent agreement with the predictions from the invariant line strain model. In a Crawley O.R. with its matrix, the second precipitate is an hexagonal prism-shaped one, with its axis perpendicular to the base plane and all its prism planes are parallel to the common planes {1100}_α // {110}β. A few short rod-shaped (third) precipitates having Porter O.R. with the matrix were also observed. A new O.R. associated with an equiaxial precipitate was occasionally observed. The formation mechanisms of the different morphologies and crystallographies were discussed.