Aluminium precipitates in the primary silicon of as-spray-formed hypereutectic aluminium-silicon alloys

Precipitates have been observed in the primary silicon of as-spray-formed hypereutectic aluminium-silicon alloys with and without various alloy additions. The precipitates have been investigated using transmission electron microscopy and high-resolution transmission electron microscopy. They are aluminium based and generally adopt a spheroidal morphology, with diameters in the range of approximately 5-10 nm, unless located on a stacking fault or grain boundary. Spray forming is a rapid solidification process and precipitation occurs because of the low solid solubility of aluminium in silicon (maximum of 0.016 plus or minus 0.003 at.% at 1190 degree C). The morphology of the precipitates suggests that they form from aluminium or aluminium-rich liquid. The orientation relationships of several precipitates in one of the primary silicon grains have been determined.