On chemical order and interfacial segregation in $\gamma^\prime$ (AlAg$_2$) precipitates

TL;DR

This study investigates the chemical ordering and stacking faults in  precipitates of Al-Ag alloys, revealing no chemical order, rare stacking faults, and significant Ag segregation at interfaces, clarifying previous conflicting reports.

Contribution

It provides detailed microscopy evidence showing the absence of chemical ordering and the nature of stacking faults in  precipitates, clarifying their growth mechanisms.

Findings

No chemical ordering on basal planes.

Stacking faults are rare and mostly in thicker precipitates.

Ag segregation at interfaces influences stacking fault observations.

Abstract

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.