Precipitate assemblies formed on dislocation loops in   aluminium-silver-copper alloys
  The precipitation microstructure of the \gamma' (AlAg2) intermetallic phase has been examined in aluminium-silver-copper alloys. The microstructure developed in an Al-0.90at.%Ag-0.90at.%Cu alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the \gamma' phase formed as aggregates with a two-dimensional open assemblies. Each such assembly contained two variants of the \gamma' phase alternately arranged to form a faceted elliptical unit. The \theta' (Al2Cu) phase formed on these assemblies after further ageing. The faceted elliptical assembly morphology has not been previously reported for the \gamma' precipitate. The change in precipitation behaviour was attributed to copper modifying the as-quenched defect structure of the matrix. This precipitation morphology clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys. 
Julian M. Rosalie, Laure Bourgeois and Barrington C. Muddle
cond-mat.mtrl-sci