THE INFLUENCE OF MICROGRAVITY ON THE DENDRITIC GROWTH-RATES OF NH4CL-H2O - AN INTERNATIONAL MICROGRAVITY LABORATORY EXPERIMENT

Ten NH4Cl-H2O Bridgman directional solidification experiments were run on an International Microgravity Laboratory Space Shuttle flight in J anuary 1992 using a matrix of temperature gradients and cooling rates. Ground-based experiments using otherwise duplicate conditions were co nducted on earth in the flight science modules. Dendritic front growth rates were obtained using reconstruction holograms taken periodically during the runs. The ground-based growth rates of the dendritic front s were approximately one-half those of the flight growth rates. This i s attributed primarily to changes in the concentration of the bulk flu id supplying NH4Cl to the tips as a result of the onset of convective flow and the concomitant dendritic coarsening. Reduced transparency of the overall mushy zone on earth is attributed to the increased ammoni um chloride concentration within the interdendritic fluid. These resul ts indicate that microgravity is a valid experimental platform for min imizing flow effects and therefore ennabling the optimum production of directional materials.