Thermodynamics of nucleation and supersaturation for the aluminum-deoxidation reaction in liquid iron

Abstract

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 ␣-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 (⌬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 (C cr O ), but the growth of nuclei stops just after ⌬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 ␣-alumina equilibrium level, which is characteristic in this system. The residual dissolved aluminum and oxygen at the ⌬G minimum remain supersaturated in the liquid iron. At an initial oxygen content below C cr 0 , 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.

Thermodynamic Analysis on Metastable Alumina Formation in Aluminum Deoxidized Iron Based on Ostwald's Step Rule and Classical Homogeneous Nucleation Theories.

“…2). 13 where subscript i means each alumina. The s Fe-i calculated from the molar enthalpies of aluminas, H i°, 14) using Eqs.…”

“…Thus the model does not consider the free oxygen in the usual concentration ranges of oxygen and aluminum in liquid iron. In a previous report, 13) the present authors applied this model to liquid iron containing excess oxygen. The results showed that the model could be applied to liquid iron with excess oxygen and the excess oxygen should originate, partly, from the suspension of fine alumina deoxidation products and partly, from the supersaturation of alumina-associated compounds.…”

“…The results showed that the model could be applied to liquid iron with excess oxygen and the excess oxygen should originate, partly, from the suspension of fine alumina deoxidation products and partly, from the supersaturation of alumina-associated compounds. 13) The suspended aluminas could coalesce during cooling and the solidification process to form secondary inclusions. Thus, the structure of the coalesced secondary alumina depends on the original suspended alumina, which is the primary deoxidation product.…”

“…In the present study, the formation of unstable alumina will be discussed on the basis of the Ostwald's Step Rule, 10,11) and two homogeneous nucleation theories (namely, one in common use and a modified theory which follows the original equation of the homogeneous nucleation thoery 13) ). In both homogeneous nucleation theories, the dependence of the interfacial free energy between liquid iron and alumina on oxygen concentration has been taken into account in different ways.…”

“…In both homogeneous nucleation theories, the dependence of the interfacial free energy between liquid iron and alumina on oxygen concentration has been taken into account in different ways. In the latter theory, which was discussed in detail in a previous report, 13) the Gibbs free energy change of parent iron phase has been considered.…”