Transmission electron microscopy study of the β′ phase (Al-Mg-Si alloys) and QC phase (Al-Cu-Mg-Si alloys): ordering mechanism and crystallographic structure

The β′ and QC rod-shaped precipitates, observed in the Al-Mg-Si and Al-Cu-Mg-Si alloys, respectively, are studied by transmission electron microscopy (TEM). Superstructure images and microdiffraction patterns confirm the likeness between the β′ and QC phases. The microdiffraction patterns are compared with computed ones in the aim to refine a previous structural model based on the existence of a latent disordered phase QP constituted by sub-unit triangular clusters. The space group of the β′ and QC phase is found to be hexagonal P6̄2m. Besides its smaller hexagonal parameter (a = 6.7 nm for QC and a = 0.71 nm for β′), the QC structure differs from the β′ one due to the Cu atoms contained in addition to the Si atoms in the triangular sites of the clusters. The phase transition of QC (or β′) from the latent phase QP (or βP) is concluded to be displacive. The complete phase transition (QP → QC → Q) or (βP → β′ → B′) during the elaboration of the materials is discussed.