INFLUENCE ON GRAVITY ON THE MICROSTRUCTURE OF THE MNBI-BI EUTECTIC

Larson and Pirich showed that the MnBi fiber spacing in the Mn-Bi syst em is finer when the alloy is solidified in space or in the presence o f a magnetic field. In completion of a program aimed at explaining the se results, the influence of electric current pulses on the MnBi fiber spacing is determined. Small, frequent electric current pulses cause fiber spacing to increase. The fiber spacing is roughly proportional t o the length of the current pulse and its amplitude. An additional pie ce of information is the recent results by Smith, showing no influence of low gravity on the MnBi fiber spacing. Consequently now the Larson -Pirich results are believed to be caused by a fluctuating freezing ra te on earth, resulting from time-dependent buoyancy-driven convection. The fiber spacing adjusts more readily to a sudden decrease in freezi ng rate than it does to a sudden increase in freezing rate. That is, t he MnBi fibers are terminated more readily than they branch. If the re verse is true, then a fluctuating freezing rate would cause a decrease in fiber spacing rather than an increase.