Nucleation of intermediate phases in aluminium-silver-copper alloys

The nucleation of intermediate phases in aluminium alloys containing silver and copper has been examined. When aged isothermally these alloying elements precipitate as the metastable phases θ′(Al₂Cu) and γ(Ag₂Al), forming on {100} and {111} habit planes respectively. This study focuses on determining the influence of the θ′ precipitate phase upon the nucleation of the γ phase. Alloys with equimolar amounts of copper and silver show a considerably more refined distribution of the γ phase than equivalent binary aluminium-silver alloys. In addition, the θ′ precipitate is seen in close association with one or more γ particles, with the silver-rich precipitate adjoining a growth ledge of the θ′ phase. This suggests that θ′, and in particular the growth ledges, play a role in the nucleation of γ.