On the age-hardening of an Al–Zn–Mg–Cu alloy: A vacancy perspective

 Positron annihilation spectroscopy was performed to study the microstructure of peak aged Al–Zn–Mg–Cu alloy. Applying artificial ageing at different temperatures, the solute content around vacancies is found to be increased compared to as-quenched state. The similarity of vacancy surroundings of all artificially aged states, except for over-aged ones, suggests that peak strength is caused not only by η′ precipitates but also by GPII zones. Moreover, η′ precipitates produced by artificial aging at 403 K are apparently coherent with the Al matrix.