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Effect of sodium on the solidification process of high purity Al-12 wt% Si alloys and the epitaxial growth of Si phase from (111) Si or (100) Si substrate has been studied by means of thermal analysis, microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Results show that normal modification with Na has little influence on the eutectic arrest temperature, melting point, latent heat of freeze and heat of fusion of the Al-12 wt% Si alloy, and that Na adsorbs on (111) Si and affects both nucleation and growth of the Si. Kinetic features and mechanism of the modification are discussed and it is suggested that modification of AlSi eutectic with Na is a kinetic problem rather than a thermodynamic one and the transition of growth morphology of eutectic Si from platelike to fibrous results from the effects of Na on the nucleation and growth of both Al and Si phase.

By inserting single crystal silicon seeds with (111)Si or (100)Si faces into the melt of high purity A1-22 wt% Si alloy, epitaxial growth of silicon from the seeds and the effect of modification with sodium were studied. Results show that primary silicon grows platelike epitaxially from certain sites of the surfaces of the seeds, with lateral faces of {111}Si and preferred directions of Si. It appeared that an addition of 0.6 wt% Na into the melt decreases markedly the number of epitaxial primary silicon plates from the seeds and causes primary silicon to grow more packed and thicker in lateral size, while the lateral faces and preferred directions of epitaxial primary silicon plates remain the same as in the pure alloy. After a discussion on the experimental phenomena, it is suggested that modification with sodium affects both nucleation and growth of silicon phase, whereas the TRPE mechanism still plays an important role in the growth of silicon phase when Na is present.

Analyse par spectrometrie Auger et par spectrometrie XPS de l'adsorption de Na sur Si. Influence de cette adsorption sur la formation de fibres de silicium eutectique dans l'alliage Al-22% Si. Analyse du mecanisme de modification de l'alliage par le sodium