INFLUENCE OF SI ADDITIONS ON THE AGING PROCESS OF AN AL-CU-MG-AG ALLOY

The influence of minor additions of silicon in the range 0.15-0.50 wt% on the precipitation sequence of an Al-Cu-Mg-Ag alloy 201 at 190-degrees-C after solution treatment and quenching has been investigated by means of joint differential scanning calorimetry and transmission electron microscopy techniques. Silicon additions inhibit the nucleation of OMEGA' and formation of the OMEGA-CuAl2 precipitate. As a result of the lack of OMEGA' and OMEGA both GPS --> S' and GP --> theta'' --> theta' --> theta sequences are influenced. In fact an uniform and homogeneous distribution of S' particles is obtained after 600 min at 190-degrees-C (T7 temper) instead of the heterogeneous precipitation on dislocations found in the Si-free alloy as well as a more dense precipitation of theta' particles. Furthermore the presence of small theta-CuAl2 equilibrium precipitates is found in the T7 temper. The strong affinity of Si atoms with GPB and vacancies is invoked to explain the observed modifications in the precipitation sequence of the Al-Cu-Mg-Ag alloy, namely: (a) Si atoms stabilize the GPB zones so eliminating the availability of Mg atoms for the alpha(SSS)--> OMEGA' --> OMEGA sequence; (b) more stable GPBZs drive the uniform and homogeneous precipitation of S' particles. In this way Cu atoms not involved in the formation of the OMEGA phase are available for the GPZ --> theta'' --> theta' --> theta precipitation sequence so increasing the amount of theta' precipitates detected after 600 min of ageing.