Hardening nanostructures in an AlZnMg alloy

The formation of nanoscale and sub-nanoscale solute aggregates (clusters, Guinier-Preston zones and precipitates) in an AlZnMg alloy (Al-2.1 at.% Zn-1.5 at.% Mg) has been followed by a combination of experimental techniques with the aim of correlating the properties of the aggregates with their thermal history. The choice of thermal treatments was guided by the results of mechanical and calorimetric characterizations, supported by transmission electron microscopy for the identification of the morphology of the aggregates. Positron annihilation spectroscopy (using two variants of this technique, coincidence Doppler broadening and lifetime spectroscopy) was adopted for determining the local chemistry in the proximity of open volume defects. The geometrical parameters of the distribution (size, volume fraction, numerical density of the solute aggregates) were obtained by small-angle X-ray scattering. The results of the investigation provide new information regarding: two families of vacancy-rich clusters formed during or immediately after quenching; Guinier-Preston zones formed at 95 degrees C after room-temperature pre-ageing; growth of eta' and eta phases at 150 degrees C; solute clusters formed at room- temperature in conditions of secondary ageing after preliminary heating at 150 degrees C.