CONTAINERLESS LIQUID TO SOLID NUCLEATION PATHWAYS IN 2 REPRESENTATIVEGRADES OF COMMERCIALLY AVAILABLE ZIRCONIUM

Experimental measurements were conducted to determine the solid metal nucleation pathways of radiatively cooling, molten zirconium spheres o f two different commercially available purity grades in a high-vacuum, high-temperature electrostatic levitator. The ensemble distribution o f maximum undercooling temperatures was interpreted using Poisson stat istics to determine the temperature dependence of the solid metal nucl eation rate. For a sample of nominally 99.95% pure zirconium, the resu lts are consistent with heterogeneous solid metal nucleation either on static catalyst particles at least similar to 30 nm diameter or on a surface coating. For a sample of nominally 99% pure zirconium, however , it appears that heterogeneous solid metal nucleation occurred either on a polydispersion of similar to 10 nm (mean diameter) static cataly st particles or on dynamic catalyst particles that precipitated from a solution that became supersaturated as the melt cooled. Copyright (C) 1997 Acta Metallurgica Inc.