  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.
