Microgravity Studies in the Liquid-Phase Immiscible System: Aluminum-Indium
As part of a study to understand the influence of gravity, cooling rate, and composition on the structure of liquid-phase immiscible materials, two alloys, AI-40 wt% In and Ai-70 wt% In, were treated thermally on the SPAR II rocket flight. The treatments involved homogenization at a temperature above the miscibility gap and then rapid cooling through the miscibility gap, followed by complete solidification. The last two steps were performed while acceleration levels were <4 x.10 ~5 #v Ground-base reference samples similarly treated showed the expected separation into indium-rich and aluminum-rich layers. The flight alloys, however, produced unexpected results. Instead of fine uniform structures, the alloys consisted of a macroscopically sized aluminumrich central region surrounded by indium-rich metal. It was reasoned from past results and analyses on configurations formed by gas-liquid mixtures at low g that the structures observed on the flight alloys correspond to those having minimum surface and interfacial energies. Since fluid motion in the microgravity environment is necessary to achieve the final structural configuration, a number of different flow mechanisms were analyzed. Of those analyzed thus far, convection due to density differences, thermocapillary convection, and capillary effects all appear to have a possible role.