An alloy with aluminium as its base element is heat treated to form a multitude of precipitate phases known from different classes of industrial alloys: Al-Cu(-Mg), Al-Mg-Si-Cu, and Al-Zn-Mg. Nanometer-sized needle-shaped particles define the starting point of the phase nucleation, after which there is a split in the precipitation sequence into six phases of highly diverse compositions and morphologies. There are several unique effects of phases from different alloy systems being present in the same host lattice, of which we concentrate on two: the replacement of Ag by Zn on the Ω interface and the formation of combined plates of the θ' and C phases. Using atomically resolved scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy, we investigate the formation mechanisms, crystal structures and compositions of the precipitates.
The grain boundaries of a fibrous Al-Mg-Si-Cu alloy have been investigated with Transmission Electron Microscopy. The compositions have been mapped by Energy Dispersive X-ray Spectroscopy. The alloy has been aged for 12 hours at 155°C after solution heat treatment and is in a slightly underaged condition. The precipitates nucleated on the high angle grain boundaries are coarse, while the precipitates on the low angle grain boundaries are smaller and more numerous. The precipitates on both types of grain boundaries has been identified as Q'-type. Copper is segregated to both the low and high angle grain boundaries. The effect of this segregation will be discussed with regards to the corrosion properties of the alloy.
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