Atom probe tomography of nanoscale microstructures within precipitate free zones in Al-Zn-Mg(-Ag) alloys

Solute distributions in the vicinity of grain boundaries in Al-Zn-Mg(-Ag) alloys were studied using a three-dimensional atom probe, in order to elucidate the mechanism of formation of precipitate free zones (PFZs) and the fundamental role of Ag in controlling PFZ width. It is shown that nanoscale clusters are formed within the PFZ in Al-Zn-Mg, despite the solute concentration remaining at the levels in the as-quenched state. Such observations have not previously been possible, and show unambiguously that vacancy depletion is the dominant mechanism of formation of PFZs in this alloy. In the Ag-containing alloy, a narrower PFZ is observed, with a reduced solute level, showing that here the dominant mechanism of PFZ formation is solute depletion. The role of Ag in this change of mechanism appears to be due to its favorable interactions not only with Mg and Zn atoms but also with vacancies.