A STUDY OF AGING IN AN ALCUMGZN FOUNDRY ALLOY

A coupled differential scanning calorimetry (DSC) and transmission ele ctron microscopy (TEM) study of the ageing mechanism at 190 degrees C after solution treatment, quenching and room temperature annealing was carried out for an Al-Cu4.5-Mg0.35-Zn3 foundry alloy of standard 249. 0. It was found that three different precipitation sequences are prese nt during the ageing process: (i) lpha(SSS)-->GPZ-->theta('')-->theta' -->theta-CuAl2 (ii) alpha(SSS)-->GPBZ-->S'-->S-CuMgAl2 (iii) alpha(SSS )-->GPZI-->GPZII-->eta'-->eta-MgZn2 There are no indications of possib le interactions between the different sequences which seem to develop independently. The sequence based on the theta-CuAl2 phase is clearly accelerated in comparison with AlCuMg alloys having the same Cu:Mg rat io already investigated and reported in literature-In particular GPZ, theta('') and theta' precipitates are already detected after room temp erature annealing, and a number of equilibrium theta-phase particles i s present after 15 min ageing at 190 degrees 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 pr esence of Zn atoms.