Effect of cold compression on precipitation and conductivityof an Al–Li–Cu alloy
Transmission electron microscopy has been used to investigate the effect of increasing the degree of deformation applied by cold compression on the ageing kinetics and electrical conductivity response of an Al–Li–Cu alloy containing Mg and Ag. When cold compressed greater than 3%, the increased dislocation density accelerates the widespread precipitation of the T1 phase resulting in an enhanced age hardening response. The lengthening rate of T1 precipitates is also reduced in this cold compressed condition owing to the reduced local solute supersaturation, a result of the widespread precipitation of T1 plates. Cold compression by less than 3% does not increase the age hardening response, and the precipitation of GP zones/θ″ appears to be suppressed. Precipitation of the T1 phase is also not significantly enhanced compared with that of the more than 3% cold compressed conditions. The anomalous decrease in electrical conductivity is associated with the nucleation and growth of the T1 phase. Strain fields around T1 precipitates combined with the increased volume fraction of  T1 are thought to be the cause of the anomalous conductivity behaviour.
The influence of Cu/Li ratio on precipitation in Al-Cu-Li-x alloys
Abstract 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.
Coarsening of θ′ plates in al-cu alloys—I. experimental determination of mechanisms
Abstract The coarsening of θ′ plates has been studied in alloys of different copper concentration (2 wt% to 5wt%) and with various types of aging (isothermal aging, aging after reversion or after reversion followed by a plastic deformation). The mean thickness of precipitates always grows rather independently of initial conditions whereas the mean diameter may have various evolutions (decrease, increase or stabilization) according to the types of aging. Comparison between the different experimental results shows two processes of coarsening: 1. (i) a shape evolution when the aspect ratio of precipitates is high: 2. (ii) a size evolution when the aspect ratio is low. due to the size effect, which is the effect taken into account in classical theories of coarsening (L.S.W.). Observed laws of growth of mean sizes of precipitates are however different ( ~ t 1 5 ) from those predicted by these theories.