Microstructural evolution of 7012 alloy during the early stages of artificial ageing

A study of the microstructural evolution of a commercial 7012 (Al–Zn–Mg–Cu) age-hardenable alloy following artificial ageing by high resolution and conventional transmission electron microscopy and positron annihilation lifetime spectroscopy is presented. At the early stages of decomposition, the microstructure included precipitation of either pre-precipitate solute clusters or Guinier–Preston zones and semicoherent η′ precipitates, with typical sizes between 1 and 10 nm. Quantitative information on the size, number density and morphology of the particles present in the microstructure was obtained. The results were correlated with those obtained using positron annihilation lifetime spectroscopy.