Effects of a small addition of magnesium and silver on the precipitation of T1 phase in an Al4%Cu1.1%Li0.2%Zr alloy

A transmission election microscopy study has been carried out on Al 4%Cu-1.1%Li-0.2%Zr alloys with and without 0.4% of magnesium and silver to provide understanding of the role of added magnesium and silver in the precipitation of T1 phase which is the major strengthening phase. Specimens were solution treated, water quenched and subsequently aged at 180 °C for various times. Single addition of silver had no effect on T1 precipitation while magnesium addition induced a refinement of the distribution of T1 plates. In the magnesium-free alloys, T1 plates nucleated only on the dislocation loops which surrounded incoherent dispersoid particles containing zirconium. In the magnesium-bearing alloys, on the contrary, octahedral voids and monolayer Guinier-Preston (GP) zones on {111} matrix planes were newly revealed at early stages of aging and were found to act as nucleation sites for T1 phase in addition to the dislocation loops mentioned above. Silver addition to the magnesium-added alloy caused an increase in the distribution density of the GP zones and hence a further increase in the density of T1 plates. Both elements were shown to be enriched in the GP zones together with copper.