Influence of Mg and Li content on the microstructure evolution of Al−Cu−Li 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 Al−Cu−Li−Mg 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.