Precipitation characteristics in two Al-Li-Cu-Mg-Ag(-Zr) alloys with high Cu/Li ratio has been investigated by means of Vickers hardness, tensile test, transmission electron microscopy and optical microscopy.
In the artificially aged condition, $T_1$ phase was the primary strengthening phase, with smaller amount of S' phase. The δ' phase was observed only at overaged condition in the case of no-stretch condition. A rapid increase of yield stress of the alloy during the initial stage of aging is due to the rapid precipitataion of $T_1$ phase because of high Cu/Li ratio. The strength of the alloys has been greatly improved on applying a stretching treatment. This is because the stretching treatment greatly enhances the nucleation rate of $T_1$ phase, resulting in a more uniform and fine distribution of $T_1$ plates. The stretching treatment however reduces the nucleation rate of S' phase. It is because the effective Cu content for S' precipitation decreases due to a drastic increase of the nucleation rate of $T_1$ phase.
The strength of the alloys continuously increases with the holding time during the natural aging treatment. The refined δ' precipitates form by homogeneous nucleation during or right after water quench. The precipitation of fine δ' particles is mainly responsible for the initial strengthening. Further strengthening was however closely related with the precipitation of Cu rich G.P.zones which are observed at a late stage of natural aging. The stretch treatment tends to reduce the strength of the alloys during the initial stage of natural aging. This is probably because the stretch treatment reduces the vacancy concentration in the matrix.
No definite relationship could be established on the effect of Cu/Li ratio on the mechanical properties because of limited scope of experiments. Microstructural analysis however indicated that the alloy with small Cu/Li ratio tends to have comparatively higher nucleation rate of δ' phase. Difference in the nucleation rate of $T_1$ phase could not be definitively determined.
