A mechanism for the formation of unstable alumina (δ, γ, and κ-alumina) in Al-deoxidized iron has been investigated on the basis of Ostwald's Step Rule and two homogeneous nucleation theories. The two homogeneous nucleation theories include a simplified theory in widespred use and the other is a modified theory in which the Gibbs free energy change of the parent iron phase is taken into consideration.
Comparison of the chemical potential of various aluminas from the standpoint of the Ostwald's Step Rule showed that the unstable alumina could form from the liquid iron alloy supersaturated with oxygen. Moreover, the rule revealed that there is a possibility of liquid alumina formation. The two homogeneous nucleation theories also proved the possibility of the formation of unstable and liquid alumina. The latter nucle-ation theory indicated the easier nucleation of liquid and unstable aluminas than that for α-alumina from the liquid iron containing oxygen contents in excess of the critical value of nucleation at 1 873 K. In contrast, nucleation was rare for oxygen contents less than the critical point at 1 873 K. However, unstable alumina can be formed during cooling and solidification since the oxygen contents of the critical point decreases with temperature decrease. When iron solidifies whilst in the supercooled condition, the nucleation rates of unstable and liquid alumina can be accelerated as a result of the oxygen enrichment at the solidifying front. The authors propose a mechanism of the network-like or coral like inclusion formation by taking into consideration of the liquid alumina formation.