Influence of Gravity on Microstructure of Directionally Solidified Al-Ni Eutectic Alloy

The influence of the growth direction on the structure of the Al-Ni eutectic alloy was studied. In order to investigate the solidification phenomena in a gravitational environment, downward unidirectional solidification (in the direction of gravity) and upward unidirectional solidification (in the opposite direction of gravity) were carried out. A zone-melting technique was used for the evaluation of the unidirectional solidification under constant experimental conditions. The upward unidirectional solidification leads to the solid/liquid interface with the morphology of the typical faceted/non-faceted eutectic. On the other hand, in the case of the downward unidirectional solidification, the leading phase of the eutectic solidification transforms from Al3Ni into alpha-Al with the increasing solidification rate. This must be caused by the concentration distribution of nickel due to the density difference between aluminum and nickel. At the front of the solid/liquid interface upward solidified, the nickel concentration decreases with the increasing distance from the interface even though the liquid phase during the downward solidification has a constant concentration value. Therefore, the result indicates that the interface morphology must be influenced by the amount of nickel supplied to the Al3Ni phase. The supply depends on the nickel concentration corresponding to the solidification direction.