Positron annihilation spectroscopy was used to study the effects of micro-alloying additions of Ag, to Al-4Cu and Al-4Cu-0.3wt%Mg alloys, upon the association of vacancies with solute elements. The results showed that, in the case of Al-Cu, the addition of Ag increased the effective activation energy – which controlled the formation of solute aggregates - from 0.32 to 0.61eV. In the case of Al-Cu-Mg, the activation energy was decreased from 0.65 to 0.22eV. In Al-Cu and Al-Cu-0.3wt%Ag alloys, the solute clusters or Guinier-Preston zones which formed after long-term aging below 70C did not contain vacancies. In the case of Al-Cu-Mg, the addition of Ag enhanced the formation of co-clusters which contained Mg and vacancies. The formation of Mg-rich aggregates was also enhanced by Ag during aging at 180C. This led to a better stabilization of the structure which was produced by artificial aging. The results were explained in terms of the tendency of Ag atoms to bind vacancies in Al-Cu, and vacancy-Mg complexes in Al-Cu-Mg.

Positron-Annihilation Study of the Aging Kinetics of AlCu-Based Alloys - Ag Microalloying. A.Somoza, A.Dupasquier, I.J.Polmear, P.Folegati, R.Ferragut: Physical Review B, 2000, 61[21], 14464-9