INTERACTION BETWEEN PRECIPITATING PHASES IN QUENCHED AL-CU-SI ALLOYS

The precipitation in two solution-treated Al-Cu-Si alloys, one with an d one without free silicon particles, was studied by differential scan ning calorimetry, X-ray diffraction and hardness measurements. In both alloys, theta'-phase and Si-phase precipitation processes proceed sim ultaneously. As in binary Al-Si and Al-Cu alloys aged at the same temp erature, precipitation occurs at clearly separate time intervals; this finding suggests an interaction between the two precipitation process es. It is shown that the interaction of stress fields around newly for med precipitates can result in a significant decrease of the nucleatio n barrier and thus can explain the synchronous precipitation. The acti vation energy of the combined precipitation reaction is 0.95 +/- 0.06 eV.