Secondary precipitation in an Al–Mg–Si–Cu alloy

Interruption of a conventional T6 heat treatment at 177 °C for the Al–Mg–Si–Cu alloy 6061 after a short period of time (20 min), by inserting a dwell period at a lower temperature (e.g. 65 °C), promotes secondary precipitation of Guinier–Preston (GP) zones. As a consequence, a much greater number of precursors to the β″ precipitates are produced so that a finer and denser dispersion of this phase is formed when T6 ageing is resumed. This change in microstructure causes significant and simultaneous improvements in tensile properties and fracture toughness. Secondary precipitation of GP zones occurs through a gradual evolution of a large number of Mg–Si(–Cu)-vacancy co-clusters formed during the initial ageing at 177 °C. The precise mechanism of secondary precipitation has been revealed by three-dimensional atom probe microscopy supplemented by transmission electron microscopy and differential scanning calorimetry.