Scanning transmission electron microscopy investigation of an Al–Mg–Si–Ge–Cu alloy

Abstract

Precipitation in a Mg-rich Al–Mg–Si–Ge–Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to ⟨001⟩_Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al–Mg–Si–Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. These structural units were often arranged in a locally ordered fashion, and in some cases the projected structure possessed and overall point symmetry. The amount of strain on the surrounding matrix was found to vary depending on the width of the precipitate cross section.

Keywords:

• aluminium alloy
• precipitation
• Al–Mg–Si alloy
• HAADF STEM
• aberration correction

Acknowledgements

RB, SJA, CDM and RH were supported by Hydro, Steertec Raufoss and the Research Council of Norway through project no. 176816/I40. RB would like to thank Drs. Matthew Weyland and Christian Maunders for expert advice on use of the FEI Titan³ 80-300 FEG-TEM. The FEI Titan³ 80-300 FEG-TEM was funded via the Australian Research Council infrastructure grant LE0454166.