Atomic Cluster Aggregates in Nucleation of Solid Alumina Inclusion in the Aluminum Deoxidation for Liquid Iron

Nucleation plays a decisive role in determining the structure and size distribution of a solid inclusion particle in liquid iron. The literature does not yet provide a clear picture about the pathway leading to solid alumina inclusion from liquid iron. The main confusion surrounds the little-known structures and thermodynamic properties of metastable alumina phase (MAP) that appear in the nucleation of alumina inclusion. In this work, it is suggested that MAP could appear in the Al-deoxidation reaction, based on the summaries and analysis in terms of various equilibrium experimental results and thermodynamic modeling of the Al deoxidation in liquid iron since the 1950s. The structures and thermodynamic properties of a species of MAP, alumina atomic cluster aggregates (AACAs), are calculated by density functional theory (DFT) methods. The thermodynamics on formation and transformation of AACAs shows that AACAs are in equilibrium with Al and O with various contents measured in the Al-deoxidation equilibrium experiments for liquid iron. It is suggested that the residual Al and O, which cannot nucleate, appear in the form of AACAs in Al-deoxidized liquid iron. It can be concluded that AACAs are the metastable intermediates and structural units for alumina inclusion nucleating from liquid iron. Thus, the behaviors of AACAs are important to the control of the nucleation rate of alumina inclusions in the Al deoxidation for liquid iron.