The role of vacancy-solute association in the formation of coherent precipitates at room temperature and at 70C was studied by means of coincidence Doppler broadening spectroscopy. The formation of clusters and GP zones was monitored by differential scanning calorimetry. Two conditions were explored at low temperature: a) one-step aging after solution treatment and quenching and b) secondary aging (after solution treatment and quenching plus 120h at room-temperature plus 560s at 150C). The assessment of the average fractional concentration of solute in contact with the vacancies was obtained by analyzing the coincidence Doppler broadening data. The results showed considerable aggregation of Cu to vacancies at the very early stage of decomposition of the supersaturated solid solution; the contribution of Zn and Mg increased progressively with the formation of coherent precipitates. The preliminary exposure at 150C did not alter this sequence, although a reduction in the dissolution enthalpy observed during differential scanning calorimetry scans revealed retarded formation of coherent precipitates.

Vacancy-Solute Association in Coherent Nanostructures Formed in a Commercial Al-Zn-Mg-Cu Alloy. A.Dupasquier, R.Ferragut, M.M.Iglesias, F.Quasso: Physica Status Solidi C, 2007, 4[10], 3526-9