Precipitation in Two Al-Mg-Ge Alloys

Two Al-Mg-Ge alloys with compositions Al-0.87Mg-0.43Ge (at. pct) and Al-0.59Mg-0.71Ge (at. pct) were investigated and compared using high-resolution transmission electron microscopy, annular dark-field scanning transmission electron microscopy, and nano-beam electron diffraction. The alloys contained fine needle- and lath-shaped precipitates after aging at 473 K (200 °C) for 16 hours, which produced hardnesses similar to those measured in comparable Al-Mg-Si alloys. The β″ phase was not observed. Instead, hardness was achieved by β′-like and disordered precipitates in the Mg-rich alloy, and U1-like and disordered precipitates in the Ge-rich alloy. In all cases, the fine precipitates had structures containing an ordered near-hexagonal network of Ge atoms with a = b ≈ 0.4 nm, which could be visualized directly in annular dark-field mode. The network is very similar to the recently discovered Si network that relates all precipitate structures in the Al-Mg-Si alloys. The orientation of the precipitate unit cells and the Ge network relative to the Al matrix differed from what has been observed for β′ and U1 in the Al-Mg-Si system.