  A detailed study has been carried out on $\gamma^\prime$ (AlAg$_2$)
precipitates in Al-Ag and Al-Ag-Cu alloys to reconcile the conflicting reports
on chemical ordering and stacking faults in this phase. High angle annular dark
field scanning transmission electron microscopy and convergent beam electron
diffraction show no indication of chemical ordering on alternate basal planes
of $\gamma^\prime$ precipitates in alloys aged at 473 K for 2-23 h.
Precipitates were visible as Ag-rich regions with 1-13 fcc$\rightarrow$hcp
stacking faults, corresponding to $\gamma^\prime$ platelets with thicknesses
ranging from 0.69-6.44 nm. There were no systematically absent thicknesses.
Growth ledges with a riser height equal to the $c$-lattice parameter (0.46 nm)
were directly observed for the first time. Genuine stacking faults within the
precipitates were extremely rare and only observed in thicker precipitates. In
precipitates with 1-3 stacking faults there was also substantial Ag in the
surrounding fcc layers of the matrix, indicating that Ag strongly segregated to
the broad, planar precipitate-matrix interfaces. This segregation is
responsible for previous reports of stacking faults in $\gamma^\prime$
precipitates. The results indicate that the early stages of $\gamma^\prime$
precipitate growth are interfacially controlled.
