Precipitate assemblies formed on dislocation loops in aluminium-silver-copper alloys

Abstract

The precipitation microstructure of the γ′ (AlAg₂) intermetallic phase has been examined in aluminium-silver-copper alloys. The microstructure developed in an Al-0.90at.%Ag-90at.Cu alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the γ′ phase formed assemblies with a two-dimensional, open arrangement of precipitates. Each such assembly contained two variants of the γ′ phase alternately arranged to form a faceted elliptical unit. The θ′ (Al₂Cu) phase formed on these assemblies after further ageing. Each assembly was formed via repeated precipitation of the γ′ phase on dissociated segments of a single dislocation loop. This faceted elliptical assembly has not been previously reported for the γ′ precipitate. The difference between the precipitation behaviour of the γ′ phase in Al-Ag and Al-Ag-Cu alloys was attributed to copper modifying the as-quenched defect structure of the matrix. The formation of faceted elliptical γ′ phase assemblies clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys.

Keywords:

• dislocation loops
• aluminium alloys
• dislocations
• HRTEM
• intermetallic phases
• precipitation
• TEM

Acknowledgements

One of the authors (JMR) gratefully acknowledges receipt of an Australian Research Council (APA) scholarship and support through the ARC Centre of Excellence for Design in Light Metals. A second (BCM) is an ARC Federation Fellow.