GROWTH OF INTERDENDRITIC EUTECTIC IN DIRECTIONALLY SOLIDIFIED Al-Si ALLOYS.

Interdendritic eutectic microstructures in Al-Si (6 to 12. 6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology as well as the behavior of the eutectic spacing and undercooling suggest that the resultant microstructure is governed by two different growth processes. At low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, lambda **2V equals constant, where lambda is the eutectic spacing and V is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The findings are interpreted in the light of models for the columnar to epitaxial transition and or irregular eutectic growth.