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

The crystal structure of β″, one of the strengthening phases in the commercially important Al–Mg–Si alloys, is determined by use of high resolution electron microscopy (HREM) and electron diffraction (ED). A trial structure was established from exit wave phase reconstructed HREM images. A least-square refinement of the model coordinates was done using data from digitally recorded ED patterns. A recently developed computer program (MSLS) was applied, taking into account dynamic scattering. The atomic unit cell contains two units of Mg5Si6. It is C-centred monoclinic, space group C2/m, a=1.516±0.002 nm, b=0.405 nm, c=0.674±0.002 nm, β=105.3±0.5°. The atomic packing may be regarded as a hard ball packing using clusters, the clusters being (1) centred tetragons of Mg atoms and (2) so-called twin icosacaps where Mg atoms are centred above and below pentagonal rings of four Si atoms an one Mg atom. A growth related stacking fault in the structure is explained by a deficiency of Mg atoms. A model for the β″/Al interface is given.