Silver segregation to \theta' (Al2Cu)-Al interfaces in Al-Cu-Ag alloys

Abstract

\theta' (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 {\gamma}' (AlAg2) precipitates were present. These dislocation loops were enriched in silver prior to \theta' precipitation. Coherent, planar interfaces between the aluminium matrix and \theta' 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-\theta' interfacial energy. The lateral growth of the \theta' precipitates was accompanied by the extension of this silver bi-layer, resulting in the loss of silver from neighbouring \gamma' precipitates and contributing to the deterioration of the \gamma' precipitate assemblies.