HETEROGENEOUS NUCLEATION OF SOLIDIFICATION IN ATOMIZED LIQUID-METAL DROPLETS

A model is developed where the heterogeneous nucleation of solidification in atomized liquid metal droplets is controlled by one of two types of catalyst particles. One type, designated "static", is pre-existing in the bulk melt and changes little throughout subsequent processing. Static catalyst particles are modeled as small spheres whose potency is prescribed by wetting angle and size. Large particles are more potent catalysts than small particles of the same species. The distribution of static catalyst particles amongst the atomized droplets is quantified by an expression based upon Poisson statistics which considers both the number density and potency of catalysts in the melt. The model can account for the major features displayed by the nucleation of droplets as measured by droplet emulsification experiments. Consideration of nucleation in droplets not containing a pre-existing catalyst particle leads to the hypothesis that a second type of catalyst particle, termed "dynamic", can form in the individual droplets after the atomization event and at large supercoolings. The importance of careful processing on the control of the two types of catalyst particles and hence on the generation of consistently large supercoolings in atomized droplets is emphasized.