Consolidation of cryomilled Al–Si using spark plasma sintering

Abstract

Cryomilled eutectic aluminum–12% silicon powder was sintered using spark plasma sintering (SPS) to create bulk compacts. The cryomilling serves to break up and disperse the eutectic phase in the powder to create a well-distributed Si phase throughout the matrix and to modify the morphology of the Si phase from plate-like to spherical, whilst refining the aluminium grain size to the nanometric level. The effects of different sintering times and temperatures using SPS on the densification of the powder, the aluminium grain size evolution, the growth of the Si phase and the morphology change of the Si phase were investigated. The compacts were analysed using X-ray diffraction, scanning electron microscopy and optical microscopy. The initial stages of densification appear to be highly dependent on the yield strength of the powder. An estimate of the temperature gradient seen in the powder bed was made and calculated to be near 200 °C at the highest point. The Al and Si phase growth was investigated and it was observed that the Si coarsening rate is increased due to the increased volume of grain boundaries. As the Si coarsens, any pinning effect on the Al grains is lost, resulting in a highly unstable microstructure that coarsens rapidly.