A Positron Lifetime Study of Cooling and Natural Aging of Solutionized Aluminum Alloys

Fast cooling of aluminum alloys from the solutionizing temperature to “room temperature,” during which solute atoms and vacancies evolve rapidly, is the step defining all ensuing age‐hardening treatments. Six Al–Mg and Al–Mg–Si alloys are solutionized and air‐cooled at a moderate rate, after which cooling is interrupted in various stages by fast quenching and positron lifetime measured at 20 °C. In the ternary alloys, two main processes during cooling are found, continuous vacancy loss followed by solute cluster formation, whereas in the binary alloy, the latter process is absent. The extent of clustering is higher for high solute content, especially Si, while Sn addition to the alloy delays it. Compared to analogous calorimetry and resistivity measurements, positron lifetime measurement shows a unique sensitivity to vacancy evolution and an enhanced sensitivity to solute clustering.