Morphology and crystallography of β-Mg2Si precipitation in Al-Mg-Si alloys

The exothermic reaction occurring after beta" precipitation in Al-Mg-Si alloys has been investigated by means of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The reaction peak height observed in DSC increases with increasing excess Mg or decreasing excess Si content from Al-Mg2Si quasi-binary composition. It was confirmed that this reaction peak arises from the precipitation of cuboid beta-Mg2Si particles related to the matrix with the cube-cube orientation relationship. At the later stage of ageing, thin beta-Mg2Si plates nucleate at the cuboid beta-phase particles. The orientation relationship of them is rotated 45degrees or 18.4degrees in the further aged specimens about the direction normal to the {001}(Al) parallel to {001}(beta) habit plane from the cube-cube relationship. The changes in morphology and the crystallography of the beta-Mg2Si precipitates with the progress of ageing can be explained qualitatively in terms of coherency and interface energy of the precipitates.