Precipitation characteristics in two Al-Li-Cu-Mg-Ag(-Zr) alloys has been investigated by means of Vickers hardness, tensile test and transmission electron microscope. The principal precipitates were δ', $T_1$, and S' phase within the composition range in the present work. Reverting technique has been employed in order to estimate the contribution of each type of precipitates to the total strengthening effect. δ' phase plays an important role in early stage of aging condition. $T_1$ and S' phase plays an important role in early stage of aging condition. $T_1$ and S' phase have been known that nucleation rate is slow. The contribution of $T_1$ and S' phases to the total strength approaches to the maximum in the vicinity of peak aged condition.
The strength of alloy has been greatly improved on applying a stretching treatment, but its magnitude has been independent of the Cu content. The stretching treatment greatly enhanced the nucleation rates of the $T_1$ phase, resulting in a more uniform distribution and higher volume fraction. Thickness and average diameter of the $T_1$ phase was observed to decrease in the stretching condition. Precipitation of S'phase was increased by the stretching treatment but the effects are somewhat smaller as compared to that of $T_1$ phase.
The strength of the alloy has been increased with increasing of the solution treatment temperature. The grain size was not affected by the solution treatment temperature. Increasing the solution treatment temperature resulted in high exess vacancy concentration. It is considered that high concentration of the exess vacancy induced the accelerated precipitations of $T_1$ and S' phase.
