Influence of Mg and Li content on the microstructure evolution of AlCuLi alloys during long-term ageing

The evolution of microstructure during long-term ageing (LTA) up to 3000 h at 85 °C following a T8 heat treatment has been characterised in three AlCuLiMg alloys of variable Mg and Li content. A combination of transmission electron microscopy, differential scanning calorimetry and small-angle X-ray scattering has been used to evaluate the evolution of the different phases. In the T8 condition all three alloys contain a high density of T1 precipitates together with S precipitates along the dislocations introduced by pre-stretching. In addition, the alloy with highest Li contains δ′ precipitates. During LTA this last alloy experiences a significant complementary precipitation of δ′. The two other alloys experience a much more limited microstructure evolution, which appears to be linked to the formation and subsequent evolution of metastable phases (GP and GPB zones) in the regions between T1 platelets. The evolution of the alloys' strength is shown to be well correlated to their microstructure.

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