A comparative study of precipitate composition and volume fraction in an Al–Zn–Mg alloy using tomographic atom probe and small-angle X-ray scattering

The composition and volume fraction of precipitates in an Al–Zn–Mg alloy are investigated by tomographic atom probe (TAP), small-angle X-ray scattering (SAXS) and transmission electron microscopy. Three states of ageing are characterized in detail: slightly underaged, containing mostly the metastable η′ phase; slightly overaged, containing η′ and the stable η phase; overaged, containing only η. The different methods are in full agreement about the last state, with a composition of η close to the bulk composition MgZn2, and a volume fraction of 4.5%. The TAP and SAXS results obtained for microstructures with smaller precipitates show some discrepancy. Notably, the TAP predicts a higher aluminium concentration in the precipitates as well as a higher volume fraction. The artefacts of the two techniques are reviewed in detail and a critical analysis of the results is carried out.