On the crystal structure of the metastable β'-phase in an Al-Mg-Si alloy using symmetry analysis and transmission Electron microscopy

Metastable beta'- phase forms in Al -Mg -Si alloys upon the decomposition of the supersaturated solid solution. It consists of rod -shaped semi -coherent precipitates aligned along ( 001 ) Al directions. The most reported crystal structure for the beta'- phase in the literature is an hexagonal structure, but which nevertheless remains hypothetical. In this study, the beta' crystal structure is revisited using transmission electron microscopy (TEM) combined for the first time with group theory. The obtained results lead to two possible symmetry groups for the beta'- phase, namely: G p = 4 m 3 m 2 an d G p = 4 2 m which are consistent with quadratic or cubic crystal systems, respectively. Both point 2 m m groups lead to the intersection point group H 16 = G M boolean AND G beta ' = 4 2 m dictating the needle -like morphology with 3 2 m m variants developed in each grain of the aluminium matrix. These results are consistent with the TEM observations. Based on these results, a space group for the beta'- phase is proposed and a parallel is made with the those given here and there in the bibliography.