Aberration-corrected HAADF-STEM investigations of precipitate structures in Al-Mg-Si alloys with low Cu additions

Precipitates in a lean Al–Mg–Si alloy with low Cu addition (~0.10 wt.%) were investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Most precipitates were found to be disordered on the generally ordered network of Si atomic columns which is common for the metastable precipitate structures. Fragments of known metastable precipitates in the Al–Mg–Si–(Cu) alloy system are found in the disordered precipitates. It was revealed that the disordered precipitates arise as a consequence of coexistence of the Si-network. Cu atomic columns are observed to either in-between the Si-network or replacing a Si-network column. In both cases, Cu is the center in a three-fold rotational symmetry on the Si-network. Parts of unit cells of Q′ phase were observed in the ends of a string-type precipitates known to extend along dislocation lines. It is suggested that the string-types form by a growth as extension of the B′/Q′ precipitates initially nucleated along dislocation lines. Alternating Mg and Si columns form a well-ordered interface structure in the disordered Q′ precipitate. It is identical to the interface of the Q′ parts in the string-type precipitate.