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°C after room-temperature pre-ageing; growth of η′ and η phases at 150°C; solute clusters formed at room-temperature in conditions of secondary ageing after preliminary heating at 150°C.

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Si $p$-type (100) samples were coimplanted at room temperature with ${\mathrm{He}}^{+}$ ions at $30\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$ and successively with ${\mathrm{H}}^{+}$ ions at $24\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$. A series of samples was thermally treated for $2\mathrm{h}$ from $100\text{to}900°\mathrm{C}$ at $100°\mathrm{C}$ steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to $300°\mathrm{C}$ annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to $400–450\mathrm{nm}$. Starting from $300°\mathrm{C}$ annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around $280\mathrm{nm}$, where extended defects are expected; this indicates complete filling of the defects by H and He. From $300\text{to}600°\mathrm{C}$ decorated vacancy clusters of different sizes appear progressively in the region below $280\mathrm{nm}$, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities.