Continuous cooling transformation diagrams for the heterogeneous nucleation of Sn-5 mass%Pb droplets catalyzed by surface oxidation

A model was developed that permits computation of CCT diagrams for surface oxidation catalyzed heterogeneous nucleation of alloy droplets from a few experimental data. The model addresses the progress of droplet surface oxidation during continuous cooling in terms of oxide nucleation and growth kinetics. Avrami-corrected surface oxidation kinetics are adopted to account for the impingement of oxide islands growing on the droplet surface during continuous cooling. The model predicts droplet nucleation kinetics in relation to the oxidation potential in the atmosphere, droplet diameter, cooling schedule and four temperature-independent constants. CCT diagrams were computed for Sn-5 mass%Pb droplets cooling in nitrogen atmospheres with various oxidation potentials using nucleation data and cooling simulation of traveling mono-size droplets. Calculated CCT diagrams show that droplet surface oxidation sharply increases the droplet nucleation temperature particularly at high cooling rates. Computation can be extended to predict CCT curves for droplet nucleation in industrial thermal spraying processes.