The precipitation microstructure of the γ' (AlAg2) intermetallic phase was examined in aluminium-silver-copper alloys. The microstructure developed in an Al-90Cu-0.90at%Ag alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the γ' phase formed assemblies with a two-dimensional, open arrangement of precipitates. Each such assembly contained two variants of the γ' phase alternately arranged to form a faceted elliptical unit. The θ' (Al2Cu) phase formed on these assemblies after further ageing. Each assembly was formed via repeated precipitation of the γ' phase on dissociated segments of a single dislocation loop. This faceted elliptical assembly has not been previously reported for the γ' precipitate. The difference between the precipitation behaviour of the γ' phase in Al-Ag and Al-Ag-Cu alloys was attributed to copper modifying the as-quenched defect structure of the matrix. The formation of faceted elliptical γ' phase assemblies clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys.

Precipitate Assemblies Formed on Dislocation Loops in Aluminium-Silver-Copper Alloys. J.M.Rosalie, L.Bourgeois, B.C.Muddle: Philosophical Magazine, 2009, 89[25], 2195-211