Influence of gravity on the microstructure of directionally solidified Ag-Si eutectic alloy

In order to study the influence of gravity on the eutectic growth of Ag-Si and Al-Si alloys, both the downward (direction of gravity) and upward (opposite to direction of gravity) unidirectional solidifications were carried out. In the case of the downward U.D.S., the molten metal is confined between the crucible and solid. This inevitably produces the shrinkage cavity at the solid/liquid interface during solidification. In this study, the zone-melting technique was used to avoid this cavity formation. The results of this study were as follows: (1) In the case of the downward U.D.S. of the Ag-Si alloy, the eutectic silicon phase grew discontinuously and dispersed in the α-Ag phase with a diameter of several micrometers. This is caused by the large difference in specific density between silver and the silicon phase. On the other hand, in the case of the upward U.D.S., the silicon phase grew continuously and its morphology showed the typical faceted/non-faceted eutectic. (2) The eutectic silicon phase of the Al-Si alloy grew continuously regardless of growth direction. There was no morphological difference between the upward and downward U.D.S. specimens.