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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.
DOI: 10.1007/s11661-008-9580-8
© The Minerals, Metals & Materials Society and ASM International 2008
I. INTRODUCTION
THE Al-Si alloys are the most widely used aluminum foundry alloys today, and the control of their microstructure is one of the most important methods to improve the mechanical properties and the casting quality. Commercial Al-Si foundry alloys usually contain more than 50 vol pet of Al-Si eutectic, and extensive research to control their microstructure by eutectic modification has been carried out. The addition of alkali or alkaline earth elements changes the morphology of eutectic silicon from flakelike to branched fibres. The mechanism of eutectic modification is still not yet fully understood. For a long time, alterations in the growth of eutectic silicon by a large increase in twin density were used to explain eutectic modification,[1] but more recent studies have shown that modification changes the nucleation frequency and dynamics of eutectic grains with associated effects on the growth rate.[2,3] In...