Effect of silver on precipitation response of Al-Li-Cu-Mg alloys

The precipitation response of silver bearing and silver free Al-Li-Cu-Mg alloys has been monitored using hardness and electrical conductivity measurements. The silver bearing alloy displayed enhanced age hardening with an anomalous decrease in conductivity. Transmission electron microscopy was used to investigate the evolution of microstructure in these alloys. The presence of small amounts of silver promoted the formation of GP zones and Al₂Cu (θ') precipitates, and greatly enhanced the precipitation of T ₁. The anomalous decrease in conductivity was associated with the precipitation of T₁ phase. The initial age hardening in silver bearing alloy was due to δ', GP zones and θ'. Precipitation of T₁, δ' and small amounts of θ' and S' were responsible during the later stages of aging. Precipitation of θ' was not observed in the silver free alloy. In the overaged condition, dissolution of δ' was more prominent in the silver free alloy.