Gravitational influence on the microstructure of directionally solidified monotectic and eutectic alloys

In order to study the influence of gravity on the microstructure of directionally solidified Al-In monotectic alloy, Ag-Si and Al-Si eutectic 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. by the current Bridgman method, the molten alloy must be confined between the crucible and a solid. This causes a shrinkage cavity at the solid/liquid interface during solidification. In this study, to avoid this `cavity formation', a zone-melting technique was used. The results are summarized as follows. (1) Regarding the unidirectional solidifications of Al-In alloy, the monotectic In(L<sub>2</sub>) phase grows continuously in both growth directions. However, the morphology in the downward U.D.S. shows a tendency to form a so-called `banded structure.' (2) In the case of the upward U.D.S. of the Ag-Si alloy, the eutectic silicon phase grows continuously and its morphology shows the typical faceted/non-faceted eutectic structure. On the other hand, in the case of the downward U.D.S., the eutectic silicon phase grows discontinuously and disperses in the α-Ag phase with a diameter of several micrometers. This is caused by the large difference in specific density between the silver and silicon phases. (3) The eutectic silicon phase of the Al-Si alloy grows continuously regardless of the growth direction. There is no morphological difference between the upward and the downward U.D.S. specimens.