Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy
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I.
INTRODUCTION
THE commercial importance o f A1-Si casting alloys has attracted the interest o f many researchers.t1.2.31 In the early 1920s, it was found that the microstructure o f A1-Si casting alloys can be modified either by chill casting o r by adding impurities, such as Na or Sr. The Si phase undergoes a morphological transition from coarse flakes to fine fibers, with a dramatic improvement in the mechanical properties o f the cast alloys. Two mechanisms o f microstructural modification have been proposed by previous authors: a change in the Si growth mode [2-m or a change in the Si nucleation temperature. [12,'3~ Hellawell a n d c o - w o r k e r s 12'4-71 and H o g a n and co-workers 13,8-111 investigated the Si crystallography and showed that the addition o f impurities, such as Na and Sr, increases the number o f twins in the Si phase. They proposed that this high twin density changes the Si growth mode and, therefore, modifies the cast A1-Si alloy microstructure. In contrast, not much work has been done to understand the effect o f impurity additions on the Si nucleation temperature, though it has consistently been reported that addition o f Na increases the undercooling o f AI-Si solidification. [14,151 Liquid eutectic droplets embedded in a primary solid matrix were f i r s t used by Wang and Smitht~61 to study heterogeneous nucleation and have since been used in a wide variety o f different alloy systems. [14,17,'81 When the alloys are prepared by rapid solidification, the small scale o f the microstructure enables detailed study o f droplet D.L. ZHANG, formerly with the Oxford Centre ~'for Advanced Materials and Composites, Department of Materials, Oxford University, is Research Associate, Department of Materials S c i e n c e and Engineering, Carnegie Mellon University, Pittsburgh, P A 15213-3890. B. CANTOR, Reader and Director, is with the Oxford Center for Advanced Materials and Composites, Department of Materials, Oxford University, Oxford OX1 3PH, United Kingdom. Manuscript submitted September 2 4 , 1990. METALLURGICAL TRANSACTIONS A
nucleation kinetics by differential scanning calorimetry and droplet solidification structure by transmission e l e c tron microscopy. The present article describes the results o f a combined differential scanning calorimeter and transmission electron microscope investigation into the heterogeneous nucleation o f Si by A1, using liquid A1-Si eutectic droplets embedded in a primary A1 matrix in a series o f melt-spun A1-Si alloys, concentrating on the effects o f alloy purity and ternary additions o f Na and Sr.
H.
EXPERIMENTALTECHNIQUE
Binary hypoeutectic A1-3 wt pct Si ingots were m a n ufactured by induction-melting Si o f 99.999 pct n o m i n a l purity and A1 o f three different nominal purities, 99.995 pct (LP), 99.999 pct (HP), and 99.9999 pct (UHP), in recrystallized alumina crucibles under a dynamic A r atmosphere. Ternary hypoeutectic A1-3 wt pct Si ingots containing n o m i n a l 0.1 wt pct Na and 0.15 wt pct and 0.3 wt pct Sr additions were manufa
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