Heterogeneous nucleation of solidification of Si in AlSi

Abstract

The heterogeneous nucleation of the solidification of Si by Al has been studied in melt-spun Al-3wt.%Si alloys, using a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Melt-spun alloy samples have been heated in the DSC apparatus to above the AlSi eutectic temperature of 577 °C, followed by cooling, to monitor the solidification exotherms of Si-rich liquid droplets embedded in a solid Al matrix. The kinetics of the heterogeneous nucleation of the solidification of Si by the surrounding matrix are difficult to analyse, because of compositional variations during cooling. Alloys made with 99.9999% pure Al have a microstructure of eutectic droplets which contain a fine cluster of Si particles embedded in an Al matrix. The eutectic droplets nucleate at about 517 °C, i.e. 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° at the AlSi-liquid triple point.