SI PHASE NUCLEATION ON SIC PARTICULATE REINFORCEMENT IN HYPEREUTECTIC AL-SI ALLOY MATRIX

The solidification process of a hypereutectic Al-17wt.%Si-0.31wt.%Mg alloy with 15 wt.%SiC particles was investigated by quenching samples through the solidification region and by conventional casting of samples from the mechanically stirred composite melts. The nucleation and growth processes of the primary Si phase under both stirred and unstirred conditions were examined, and the Si/SiC(p) interface was analysed by SEM and EDX with X-ray image analysis. The Si phase of Al-Si alloys has a more important role than the alpha phase in the bonding of the SiC reinforcement with the matrix, because the Si phase nucleates preferentially on the SiC particles even when the spinel interfacial layer exists. The crystal structures of the phases in the composites were identified by X-ray diffraction, and the factors which influence the heterogeneous nucleation of Si crystal on the substrates are discussed.