On chemical order and interfacial segregation in γ′ (AlAg2) precipitates

A detailed study has been carried out on γ′ (AlAg2) 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 γ′ precipitates in alloys aged at 473 K for 2-23 h. Precipitates were visible as Ag-rich regions with 1-13 face-centred cubic (fcc) → hexagonal close-packed stacking faults, corresponding to γ′ platelets with thicknesses ranging from 0.69 to 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 γ′ precipitates. The results indicate that the early stages of γ′ precipitate growth are interfacially controlled.