Abstract: The nanosized precipitates in Al-3.65Zn-2.27Mg-0.08Zr (wt%) alloy aged at 120 °C for 48 h (T6 temper) have been characterized quantitatively in detail by atom probe tomography (APT) and transmission electron microscopy (TEM). The TEM results show that most spherical particles are T' phase, and the rod-shaped particles are η' phase, with some exceptions, in which some spherical particles are not T' phase but η' phase instead, having the hcp structure. The APT results show that the atomic ratio of Zn/Mg of the particle has a bimodal distribution, exhibiting two distinct peaks at the Zn/Mg atomic ratio values of 1.1 and 1.7 for the T' and η' phases. Accordingly, the volume fraction and number density of the two-phase precipitates in the alloy are 1.38 % and 1.34 × 10²³/m³, respectively. For the two-phase precipitates, 75 % of the volume is T' phase and 25 % is η' phase. The mean composition of the T' phase is: 48.3 % Al, 28.5 % Zn, 23.2 % Mg; while the mean composition of the η' phase is: 51.5 % Al, 30.7 % Zn, 17.8 % Mg. The volume fractions for the two phases obtained from TEM and APT are different by about 30 %. The reason for this discrepancy has been discussed. These quantitative results of the precipitate composition and volume fraction are essential for designing new alloys.

Key words: Precipitate quantification, T'/η' phase distinction, Atom probe tomography, Al-Zn-Mg-Zr alloy