STUDY ON THE KINETICS OF PHASE TRANSFORMATION β → α IN THE HOMOGENEOUS HEAT TREATMENT OF Al-Mg-Si SERIES ALLOYS

According to the present experiment results about the transformation from beta-AlFeSi to alpha-Al(FeMn)Si phase in Al-Mg-Si series alloys, the model and kinetics of phase transformation, and microstructure evolution at 555 degrees C were systematically investigated. The results show that, the homogeneous temperature T can give a significant effect on the transformation kinetics, and with increasing T, the transformation rate increases fast. The nucleus radius of alpha phase only affects the breakthrough time along the thickness direction of beta phase. The distance l between two a phases mainly has an influence on the dissolving of rim for beta phase. The thickness of beta phase normally has a greatest influence on the phase transformation beta -> alpha, especially for the breakthrough stage of beta phase, with increasing the thickness of beta phase, the main transformation with longer time in the whole phase transformation process can changes from rim dissolving of beta phase to breakthrough along the thickness direction of beta phase. The breakthrough time of beta phases with the thickness of 0.1, 0.2 and 0.3 mu m is above 5, 10 and 15 h, respectively, which corresponds with the real experiment results. In addition, if the thickness of beta phase is smaller than 0.3 mu m, the transformation rate f(alpha) easily reaches above 0.9 after homogeneous heat treatment at 555 degrees C for 16-24 h. Therefore, the importance of optimizing casting process to make sure the thickness of beta phase below 0.3 mu m, is the same as the homogeneous heat treatment for the better controlling the phase transformation beta -> alpha.