Microstructural evolution of SiC_p/ZA27 composite modified by Zr during partial remelting

The microstructural evolution of SiC_p/ZA27 composite modified by Zr during partial remelting was investigated using the back-scattered electron imaging technique in scanning electron microscopy (SEM), and also compared with that of the unmodified SiC_p/ZA27 composite as well as that of the monolithic ZA27 alloy. The results indicated that the matrix microstructure of the composite modified by 0.2 wt% Zr experienced in succession the rapid merging of the secondary dendritic arms and the resulting growth of the primary dendrites, the melting of the interdendritic regions and the structure separation, and finally formed the slurry needed by thixoforming with the uniform, small and globular solid particles through the spheroidization during heating at 460°C (0 min - 30 min). No obvious coalescence of the solid particles was found during the subsequent isothermal holding (30 min-80 min). The primary dendrites of the unmodified composite changed into interconnected and irregular grains during the heating stage and did not spheroidize completely even when holding for 80 min. The separation of the microstructure of the monolithic alloy was faster than that of the modified composite during the initial stage, but the particle coalescence was likely more obvious during the subsequent holding period. The rapid coalescence of the primary dendrites during the initial stage of the heating resulted from the diffusion of the interdendritic eutectics towards the dendrites. In addition, the microstructural evolution was also discussed through analyzing the phase transformations which occurred during this process.