Dendritic Growth of Undercooled Nickel-Tin: Part III

Studies were made of structure and solute distribution in undercooled droplets of nickel-25 wt pct tin alloy and the eutectic nickel-32.5 wt pct tin alloy. Structures of levitation melted droplets of the Ni-25 wt pct Sn alloy showed a gradual and continuous transition from dendritic to fine-grained spherical with increasing initial undercooling up to about 180 K. Results suggest that all samples solidified dendritically and that the final structures obtained were largely the result of ripening. Experimental data on minimum solute composition in the samples produced are bounded by two calculated curves, both of which assume equilibrium at all liquid-solid interfaces during recalescence and subsequent cooling. One assumes complete diffusion in the solid during recalescence; the other assumes limited diffusion, but partial remelting to avoid superheating of the solid. Several observations support the view that the eutectic alloy solidifies dendritically, much as the hypoeutectic alloy does. Surface dendrites were seen in regions of surface shrinkage cavities and a coarse “dendritic” structure can be discerned on polished sections, which seems to correspond to the large surface “dendrites” seen by high-speed photographs of the hypoeutectic alloy. The structure of highly undercooled eutectic samples is composed fully of an anomalous eutectic. Samples solidified with intermediate amounts of undercooling possess some lamellar eutectic which, it is believed, solidified after recalescence was complete.