SOLIDIFICATION OF BINARY AND SR-MODIFIED AL-SI EUTECTIC ALLOYS - THEORETICAL-ANALYSIS OF SOLUTE FIELDS

This paper analyses experimental data in available literature concerning the eutectic spacing-lambda and growth undercooling DELTA-T in the Al-Si binary eutectic and in the strontium-modified eutectic. The expression for DELTA-T obtained in Jackson and Hunt's model is used, before application of the minimum undercooling growth criterion. It is shown that, for the binary eutectic, the results can be interpreted quantitatively by means of this equation. On the other hand, it is shown that the results for the modified alloy cannot be interpreted on this basis and that a contribution of the kinetic undercooling for growth of Si has to be taken into account. A high level of kinetic undercooling is found (ranging from 5 to 15 K depending on the solidification rate). This analysis also predicts a significant silicon-enrichment of the interfacial liquid, especially in the modified eutectic (up to 2 wt %).