Secondary aging (microstructural evolution occurring at room temperature following short-term heating at temperatures above the metastable phase boundary of Guinier-Preston zones) was studied in an alloy with a high Cu/Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry showed that, upon secondary aging after 300 or 420s at 190C, hardening first took place at a rate which was 9 to 16 times slower than natural age-hardening. Secondly, vacancies which were slowly released by Cu-rich aggregates formed during heat treatment at 190C promoted the further formation of solute aggregates, with a time-dependent chemical composition. Thirdly, the thermal stability of the structures formed during secondary aging increased with increasing dwell-time at room temperature. Fourthly, solute aggregates which formed at 190C underwent structural reorganization, and perhaps a change in the composition, leading to species having a different thermal stability. The slow release of vacancies from Cu-rich aggregates was proposed to be one of the limiting factors of the hardening rate.

Secondary Aging in Al-Cu-Mg. M.Massazza, G.Riontino, A.Dupasquier, R.Ferragut, A.Somoza, P.Folegati: Philosophical Magazine Letters, 2002, 82[9], 495-502