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Abstract
The present investigation was carried out on hypoeutectic Al-Si alloys containing two levels of Fe, 0.5 and 1.1 wt pct, and Sr in the range of 30 to 500 ppm. The addition of Sr in excess of 100 ppm significantly reduced the number of eutectic grains and also resulted in the formation of polygonal-shaped Al^sub 2^Si^sub 2^Sr intermetallics. Transmission electron microscopy studies revealed that the Al^sub 2^Si^sub 2^Sr phase surrounded the P-rich particles. This may suggest that the otherwise potent nuclei for the Al-Si eutectic, aluminum phosphide (AlP), become poisoned or deactivated by the formation of the Al^sub 2^Si^sub 2^Sr phase around the particles. At the high-Fe level (1.1 wt pct Fe), pre-eutectic formation of β-Al^sub 5^FeSi platelets further reduced the number of eutectic Al-Si nucleation events. It is proposed that both eutectic silicon and β-Al^sub 5^FeSi are preferentially nucleated on AlP particles. Nucleation of eutectic silicon, therefore, becomes more difficult when it is preceded by the formation of Al^sub 2^Si^sub 2^Sr or β-Al^sub 5^FeSi, because fewer nuclei are available to nucleate silicon. Addition of up to 60 ppm P to the alloys increased the formation temperature of the β-Al^sub 5^FeSi platelets but did not significantly alter the size, whereas the addition of Sr decreased the β-Al^sub 5^FeSi nucleation temperature by reducing the potency of the AlP particles. [PUBLICATION ABSTRACT]