Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ' (Al2Cu) precipitates in Al-Cu-Ag alloys were examined using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ' (AlAg2) precipitates were present. These dislocation loops were enriched in silver prior to θ' precipitation. Coherent, planar interfaces between the aluminium matrix and θ' precipitates were decorated by a layer of silver of two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al-θ' interfacial energy. The lateral growth of the θ' precipitates was accompanied by the extension of this silver bi-layer, resulting in the loss of silver from neighbouring γ' precipitates and contributing to the deterioration of the γ' precipitate assemblies.