Coincidence doppler broadening study of the pre-precipitation stage of an Al-Cu-Mg-Ag alloy

The role of Ag in the pre-precipitation process of an Al-1.9at.%Cu- 0.3 at.% Mg-0.2at.% Ag alloy has been investigated in view of the interaction between the quenched-in vacancies and Ag atoms. Positron annihilation methods, coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopies, and transmission electron microscopy have been employed. In an as-quenched state, quenched-in vacancies V in the Al- 1.9at.%Cu-0.3at.%Mg~0.2at.%Ag alloy are associated with Ag atoms as well as Mg atoms (V-Mg-Ag) while V-Mg complexes are formed in the A1 1.9at.%Cu-0.3at.%Mg alloy. The recovery of the vacancies after 15s ageing at 180°C is suppressed by the existence of Ag atoms. These results indicate that the V-Mg complexes are stabilized by the associated Ag atoms, which promotes the co-clustering of Ag and Mg atoms by ageing rather than by the formation of dislocation loops.