Abstract
Abstract The precipitation sequence at 190°C in a complex A1-4Cu-0·35Mg-0·7Ag-1·2Zn foundry alloy belonging to the commercial AVIOR composition has been investigated by means of differential scanning calorimetry and transmission electron microscopy, and a comparison with a Zn-free alloy has been made in order to evaluate the influence of Zn on the precipitation sequence. It is found that Zn modifies the sequence generally found in AlCuMgAg alloys having a high Cu: Mg ratio. The Ω′ precursor of the Ω-CuAl2 phase found in the Zn-free alloy is in fact not detectable and a different mechanism for Ω nucleation must be invoked. An active role of early θ″ and θ′ precipitates is proposed as precursors of Ω. It is also found that the presence of Zn strongly influences the kinetics of Ω precipitation, which is promoted at the expense of the θ′-CuAl2 and S'-CuMgAl2 phases. Well-assessed Ω particles are present after 15 min of ageing at 190°C, and in relevant amounts after 600 min, while only few θ′ precipitates and rare S′ particles heterogeneously nucleated on defects and homogeneously precipitated inside the grains can be found. The differences detected in the Zn-additioned alloy are interpreted in terms of reduction of the stacking fault energy obtained by the Zn alloying.
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