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

Abstract

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.