Morphology and Crystallography of β-Mg₂Si Precipitation in Al-Mg-Si Alloys

Abstract

The exothermic reaction occurring after β^′′ 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 β-Mg₂Si particles related to the matrix with the cube-cube orientation relationship. At the later stage of ageing, thin β-Mg₂Si plates nucleate at the cuboid β-phase particles. The orientation relationship of them is rotated 45^° or 18.4^° in the further aged specimens about the direction normal to the {001}_Al||{001}_β habit plane from the cube-cube relationship. The changes in morphology and the crystallography of the β-Mg₂Si precipitates with the progress of ageing can be explained qualitatively in terms of coherency and interface energy of the precipitates.