Early-stage precipitation in Al–Zn–Mg–Cu alloy (7050) Original Research Article

GP zone and metastable η′ formation during the early-stage precipitation of a 7050 Al alloy aged at 121 °C has been investigated using transmission electron microscopy, together with 3-dimensional atom probe analysis (of the chemistry and morphology) of individual precipitates. Small Mg-rich clusters (GPI zones) are found in the alloy after short ageing times at 121 °C, together with larger GPI zones (Zn/Mg=1.0). Zn-rich η′ platelets are seen to form mainly between 30 and 240 min ageing, coexisting with larger GPI zones. A significant fraction of 〈1 1 0〉 elongated clusters have also been observed over this period of ageing. The dominant mechanism for η′ formation at this stage is shown to be by transformation of small GPI zones, via these elongated clusters and not by nucleation on larger zones.