Aberration-corrected scanning transmission electron microscopy study of β′-like precipitates in an Al-Mg-Ge alloy

Precipitates in an Al-Mg-Ge alloy similar to the β′ phase in Al-Mg-Si alloys were investigated using qualitative and quantitative aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). The needle-shaped β′-Ge precipitates are coherent with the Al matrix along the needle direction which is parallel to a 〈0 0 1〉Al direction, as well as one direction in the cross-section plane that is parallel to a 〈1 0 0〉Al direction. This is linked to a smaller lattice parameter in the needle cross-section plane than that of coarser, less coherent β′ precipitates in Al-Mg-Si alloys, despite Ge having a larger atomic radius than Si. Quantitative HAADF STEM results show that the nominal Ge columns of the β′-Ge precipitates are not fully occupied by Ge, the intensity of these columns being consistent with an Al concentration of 30 ± 10%, or a vacancy concentration of 20 ± 10%. The coherent interface is atomically smooth, but with matrix columns containing Ge spaced periodically along the interface.