The effects of Ag and Li/Cu ratio on the precipitation and mechanical properties of three different Al-Li-Cu-Mg-Ag-Zr alloys has been investigated using electron microscopy, optical microscopy and tensile testing.
In the artificially aged condition, $T_1$ phase was the primary strengthening phase with S' phase. The δ' phase was observed in the case of high Li/Cu ratio condition.
The strength of alloy has been greatly improved on adding Ag. This is because the Ag addition greatly enhances the precipitation of $T_1$ phase and especially S' phase. But the addition of Ag results in a reduction in ductility and toughness, associated with the equilibrium phase along grain boundaries.
A long time exposure at low temperature (120℃) for normally aged specimens indicated a significant reduction of toughness. Its reduction is large for alloy without Ag, primarily associated with the precipitation and growth of equilibrium phase along grain boundaries, whereas it is comparatively small for alloy with Ag.
At low Li/Cu ratio alloy the δ' phase was nearly absent and the $T_1$ and S' phase was greatly increased. This results in improved strength and ductility compared with high Li/Cu ratio alloy.
The stretching treatment increased the strength of alloy because of increased precipitation of $T_1$ and S' phase.
The homogenization temperature was found to affect the grain size of the alloys and their strength. The grain size tended to be smaller in the case of lower homogenization temperature. This was turned out to be due to a presence of larger amount of primary solidification compounds as compared to that for higher homogenization temperature. The result of EDAX analysis indicated that these primary solidification compounds contain a significant amount of Cu. Therefore the alloy processed at lower homogenization temperature tends to show a lower strength due to a loss of strength agent to the primary compound.
