Influence of gravity on the microstructure of the MnBi/Bi eutectic

Directional solidification of MnBiBi eutectic in space produced MnBi fibers that were significantly finer and closer together than when solidification was carried out on earth under otherwise identical conditions. Use of a strong magnetic field during solidification on earth gave about the same results as solidification in space, indicating that convection is the cause of the difference in microstructure. However, 15 years of theoretical and experimental research have failed to reveal the mechanism for this phenomenon. It has been found that temperature gradient has no effect; the concentration field in front of the freezing interface is not altered sufficiently by buoyancy-driven convection to explain it, even if the MnBi fibers project out in front of the Bi matrix; and the Soret effect is not sufficiently large. On the other hand, vigorous forced convection caused a change in microstructure in agreement with theory.