Characterisation of the composition and volume fraction of η′ and η precipitates in an Al–Zn–Mg alloy by a combination of atom probe, small-angle X-ray scattering and transmission electron microscopy

The composition and volume fraction of nanometre size precipitates has been characterised in an Al–Zn–Mg alloy (AA7108.50) in two states of ageing, namely the T6 state, containing mostly metastable η′ precipitates, 2.5 nm in radius, and the T7 state, containing mostly stable η precipitates, 4 nm in radius. Three complementary experimental techniques have been used in order to provide for the first time two independent measurements of the precipitate composition and volume fraction: tomographic atom probe, small-angle X-ray scattering, and transmission electron microscopy. The composition of precipitates in the T7 state are shown to be very close to the equilibrium MgZn2 composition, with an aluminium content smaller than 10%. In the case of the T6 material, the most probable composition is proposed as 55% Zn, 25% Mg and 20% Al.