PRECIPITATION SEQUENCE IN AN AL-CU-MG-AG-ZN ALLOY

The precipitation sequence at 190 degrees C in a complex Al-4Cu-0.35Mg -0.7Ag-1.2Zn foundry alloy belonging to the commercial AVIOR compositi on 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 p recipitation sequence. It is found that Zn modifies the sequence gener ally found in AlCuMgAg alloys having a high Cu:Mg ratio. The Omega' pr ecursor of the Omega-CuAl2 phase found in the Zn-free alloy is in fact not detectable and a different mechanism for Omega nucleation must be invoked. An active role of early theta '' and theta' precipitates is proposed as precursors of Omega. It is also found that the presence of Zn strongly influences the kinetics of Omega precipitation, which is promoted at the expense of the theta'-CuAl2 and S'-CuMgAl2 phases. Wel l-assessed Omega particles are present after 15 min of ageing at 190 d egrees C, and in relevant amounts after 600 min, while only few theta' precipitates and rare S' particles heterogeneously nucleated on defec ts and homogeneously precipitated inside the grains can be found. The differences detected in the Zn-additioned alloy are interpreted in ter ms of reduction of the stacking fault energy obtained by the Zn alloyi ng.