The influence of Cu/Li ratio on precipitation in Al–Cu–Li–x alloys

The impact of the Cu/Li ratio on the sequence and kinetics of solid-state precipitation is studied for two recently developed Al–Cu–Li–Mg–Ag alloys: AA2198 and AA2196. A quantitative evaluation of the alloy microstructure is carried out using small-angle synchrotron X-ray and neutron scattering, conventional and aberration-corrected (scanning) transmission electron microscopy and differential scanning calorimetry. It is shown that small modifications to alloy chemistry profoundly change the phases formed during natural ageing: Cu-rich clusters in the Li-lean alloy and the δ′(Al3Li) phase in the Li-rich alloy. Despite this difference in early ageing, the peak aged microstructures of the two alloys are similar, dominated by high-aspect-ratio T1 plates with a thickness of 1.3 nm and a diameter up to 50 nm. However, the incubation time for nucleation and saturation size after growth of the precipitates is found to be significantly different between the two alloys. Small amounts of other phases from the binary Al–Cu sequence are also observed. Mechanisms of formation of the T1 phase are discussed in view of these experimental results.