Aluminum-deoxidized iron at 1 873 K was solidified at 3 different cooling speed; (1) the ultra-rapid cooling of iron using twin rollers, (2) the quenching of iron into copper mold, and (3) the quenching of the iron-bearing crucible in a water bath; the most rapid cooling rate achieved with (1), which was probably about 10 5 K/s, followed (2) and (3). Dendritic, maple-like, polygonal, network-like, coral-like and spherical inclusions were observed in the samples. The dendritic, maple-like and polygonal inclusions varied in size from several tens to a few mm and were classified as primary inclusions since their sizes were independent of cooling speed. However, the network-like and coral-like inclusions (in the sample cooled ultra-rapidly and quenched into copper mold), and spherical inclusions were classified as secondary inclusions since they decreased in size with increased cooling speed. A few large spherical inclusions, which would be primary inclusions, were also present.The analysis of the electronic diffraction of the inclusions established that the a, g, d-alumina were present as secondary inclusions. An amorphous silica spherical inclusion was also observed.

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.

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