Metastable precipitates play important roles in a super high strength Al–Zn–Mg based alloy (Mesoalite®). The present study was undertaken to examine the composition of the metastable precipitates in Mesoalite® and the effect of Mn addition on the composition. The (Zn+Mg) concentration of η′ metastable precipitates detected by 3DAP (three dimensional atom probe) was ~25% in the alloy Al–4.1Zn–3.1Mg–0.57Cu–0.01Ag (at%). By comparing with the results of G.P. zones in an Al–Zn alloy, it was suggested that the intrusion of Al atoms from the matrix lowered the (Zn+Mg) concentration of metastable precipitates detected by 3DAP. A model to quantify the intrusion of Al atoms was developed and applied to the metastable precipitates. The (Zn+Mg) concentration was quantified to be ~65 at% at 383 K and ~75 at% at 413 K. When the alloy contained Mn, furthermore, it was suggested that Mn affected the Zn/Mg ratio of metastable precipitates by lowering the virtual Zn/Mg ratio of the specimen composition.
