EFFECTS OF SILICON ADDITION ON THE 1ST STAGE OF PRECIPITATION IN THE AL-1.52 PCT CU-0.75 PCT MG ALLOY

The influence of silicon on the precipitation kinetics of Al-Cu-Mg alloys is not yet understood. although its effect on the improvement of mechanical properties is well established. In this study, the kinetics of the first stage of precipitation in Al-1.52 pct Cu-0.75 pct Mg alloy containing 0.49, 0.76, and 1.03 pct Si were investigated by differential scanning calorimetry. From the calorimetric data, the extent of reaction Y and the reaction rate (dY/dt) were evaluated as functions of the reaction temperature. The rates are expressible by the relation (dY/dt) = (1 -Y)k0 exp (-Q*/RT), where k0 is the frequency factor and Q* is the activation energy. For the alloys containing 0.49, 0.76, and 1.03 pct Si, the activation energies are 76.7, 70. 1, and 64.6 kJ/mole, respectively, and k0 changes systematically with silicon content. Critical analysis of these results and those available in the literature on the silicon-free and 0. 23 pct Si containing alloys shows that GP zones precipitate in the 0.49, 0.76, and 1.03 pct Si alloys, while only GPB zones precipitate in the silicon-free and 0.23 pct Si containing alloys. The sudden change in the precipitation behavior occurs due to the preferential removal of magnesium from the matrix by the insoluble particles which are present in 0.49, 0.76, and 1.03 pct Si alloys. The alteration of the matrix composition is also responsible for systematic decrease in the activation energy.