Presence of silver in the strengthening particles of an Al-Cu-Mg-Si-Zr-Ti-Ag alloy during severe overaging and creep

The results of a study aimed at investigating the presence of silver in the strengthening phases of age-hardenable AlCuMgSi alloys are presented. HR STEM observations were performed on Al-4Cu-0.55Mg-0.5Si-0.15Zr-0.1Ti-0.5Ag (wt%) alloy samples aged or crept at 200 °C to monitor the evolution of secondary phases over long times. Results showed that in the peak aged condition silver mostly segregated at the surface of some strengthening precipitates types. This occurred specifically at the particle/matrix interface of ?' phase while particles, matching the wide compositional range of the quaternary Q' phase were free from Ag. A third population of strengthening particles having the shape of a core-shell rod was also observed. They were characterized by a Mg- and Si-rich core and a Cu- and Ag-rich shell. The exposure at 200 °C for up to 570 h, caused an evident coarsening of Q' and ?' plate-like particles, and the Ag surface enrichment of the latter went progressively lost. On the contrary, silver concentrated inhomogeneously on the surface of the core-shell particles for which a relatively low coarsening rate was observed.