HETEROGENEOUS NUCLEATION OF SOLIDIFICATION OF SI IN AL-SI

The heterogeneous nucleation of the solidification of Si by Al has bee n studied in melt-spun Al-3wt.%Si alloys, using a combination of diffe rential scanning calorimetry (DSC) and transmission electron microscop y (TEM). Melt-spun alloy samples have been heated in the DSC apparatus to above the Al-Si eutectic temperature of 577 degrees C, followed by cooling, to monitor the solidification exotherms of Si-rich liquid dr oplets embedded in a solid Al matrix. The kinetics of the heterogeneou s nucleation of the solidification of Si by the surrounding matrix are difficult to analyse, because of compositional variations during cool ing. Alloys made with 99.9999% pure Al have a microstructure of eutect ic droplets which contain a fine cluster of Si particles embedded in a n Al matrix. The eutectic droplets nucleate at about 517 degrees C, ie . 60 K below the eutectic temperature. The effect of the cooling rate on the shape of the solidification exotherms has been analysed to give a contact angle of 43 degrees at the Al-Si-liquid triple point.