Precipitation of the θ'(Al₂Cu) phase in Al-Cu-Ag alloys

Precipitation of the θ' (Al₂Cu) phase was examined in Al-(1.75-x) at.%Cu-x at.%Ag alloys. In alloys containing trace Ag the θ' precipitates formed T- or cross-shaped arrays of sympathetically-nucleated arrays similar to those reported in binary Al-Cu alloys. However, in alloys with equal atomic levels of copper and silver the θ' plates formed at specific sites on dislocation loops alongside preexisting γ' (AlAg₂) precipitates. The dislocation loops appeared to dissociate at these sites in a manner which would provide a stacking fault that was isostructural with the θ' precipitate. Energy dispersive X-ray (EDX) mapping showed that the dislocation loops were silver-enriched at the sites where the θ' phase eventually precipitated and that a silver atmosphere remained after precipitation of the θ' plates.