Thermodynamics of Nucleation and Supersaturation for the Aluminum-Deoxidation Reaction in Liquid Iron

The nucleation process of alumina in aluminum-deoxidized liquid iron was investigated by computer simulation, in which the Gibbs free-energy change of the parent liquid iron, the dependence of interfacial free energy between liquid iron and (data)-alumina on oxygen content, and the dependence of the interfacial free energy on the curvature of a nucleus were considered. The calculated curve of the Gibbs free-energy change of the systems ((lambdal)G), with respect to nuclear radius, has a maximum and a minimum. Nucleation occurs rapidly when the initial oxygen content is higher than the critical point of nucleation (Co^cr), but the growth of nuclei stops just after ((lambdal)G) reaches its minimum. At the minimum, the small alumina nuclei are suspended in liquid iron for an extended period of time. This suspension is one reason for the presence of excess oxygen in liquid iron above the a-alumina equilibrium level, which is characteristic in this system. The residual dissolved aluminum and oxygen at the((lambdal)G) minimum remain supersaturated in the liquid iron. At an initial oxygen content below Co^cr), no nucleation can occur, and the components in the liquid iron remain in the supersaturated state. This supersaturation is another reason for the phenomenon of excess oxygen in liquid iron.