A study of ageing in an AlCuMgZn foundry alloy

A coupled differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) study of the ageing mechanism at 190 °C after solution treatment, quenching and room temperature annealing was carried out for an AlCu4.5Mg0.35Zn3 foundry alloy of standard 249.0. It was found that three different precipitation sequences are present during the ageing process: ( i ) α sss → GPZ → θ " → θ ʹ → θ – CuA l 2 ( ii ) α sss → GPBZ → S ʹ → S – CuMgA l 2 ( iii ) α sss → GPZI → GPZII → η ʹ − η - MgZ n 2 There are no indications of possible interactions between the different sequences which seem to develop independently. The sequence based on the θ-CuAl2 phase is clearly accelerated in comparison with AlCuMg alloys having the same Cu:Mg ratio already investigated and reported in literature. In particular GPZ, θ″ and θ′ precipitates are already detected after room temperature annealing, and a number of equilibrium θ-phase particles is present after 15 min ageing at 190 °C, which grow during the annealing. The acceleration of the precipitation kinetics is explained in terms of a decrease in the stacking fault energy induced by the presence of Zn atoms.