Abstract
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.
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