The crystal structure of the β′ phase in Al–Mg–Si alloys

The crystal structure of β′, one of the metastable phases formed during precipitation hardening of Al–Mg–Si (6xxx) alloys, has been determined using electron diffraction (ED). With the aid of high-resolution electron microscopyimages and the composition estimated from energy dispersive X-ray analysis, an initial model for the structure of the β′ phase was obtained. The data from digitally recorded ED patterns were then used for a least-squares refinement of the atomic parameters of the model with a software program package developed in Delft (MSLS), taking into account dynamic scattering. The β′ structure has a composition of Mg9Si5. The unit cell is hexagonal, space group P63/m, with unit cell parameters a = 0.715 nm and c = 1.215 nm. The composition and structure were confirmed by ab initio calculations.