HAADF-STEM study of β ′-type precipitates in an over-aged Al–Mg–Si–Ag alloy.

Coarse, rod-shaped precipitates growing along ⟨100⟩Al directions in an Al–1.0 wt% Mg2Si alloy with 0.5 wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62 m (189) and composition Al3Mg3Si2Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg9Si5 phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates [ABSTRACT FROM AUTHOR]