Vacancy-solute association in coherent nanostructures formed in a commercial Al-Zn-Mg-Cu alloy



The role of vacancy-solute association in the formation of coherent precipitates at room temperature (RT) and at 70 °C was studied by means of coincidence Doppler broadening (CDB) spectroscopy. The formation of clusters and GP zones was monitored by differential scanning calorimetry (DSC). 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 + 5 days at RT + 8 min at 150°C). The assessment of the average fractional concentration of solute in contact with the vacancies was obtained by analyzing the CDB data. The results show considerable aggregation of Cu to vacancies at the very early stage of decomposition of the supersaturated solid solution; the contribution of Zn and Mg increases progressively with the formation of coherent precipitates. The preliminary exposure at 150 °C does not alter this sequence, although a reduction in the dissolution enthalpy observed during DSC scans shows retarded formation of coherent precipitates. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)