EUTECTIC SPACING SELECTION IN LEAD-BASED ALLOY SYSTEMS

Directional solidification studies have been carried out in Pb-Au, Pb-Pd, Pd-Cd, and Pb-Sn systems to characterize the variation in eutectic spacing with velocity. In the Pb-Cd, Pb-Sn, and Pb-Pd systems, statistical distributions of spacings at each velocity were determined and a significant spread was observed. The smallest and the largest spacings, along with the average spacing, have been characterized. A broad distribution curve has been observed for low velocities which becomes sharper as the velocities increase. A comparison with the theoretical model shows that the average spacings are consistently larger than the spacings predicted by the minimum undercooling criterion, whereas the smallest observed spacing corresponds to the theoretical extremum value. Dynamical studies have been carried out to examine the spacing selection process at a given velocity by initially starting with a higher or a lower average spacing. The final spacing distribution at a given velocity was found to be the same irrespective of the initial starting condition which establishes that a definite distribution of spacings exists in eutectic systems without any hysteresis effect and that dynamical effects are not responsible for the existence of a band of stable spacing.