Formation Mechanism of Dendritic and Spherical Alumina Formed When Iron is Deoxidized by Aluminum

Abstract

This work was carried out to obtain the knowledge of the formation mechanisms of dendritic and spherical deoxidation-products. Aluminum was added as liquid state on the surface of the quiescent molten iron. After the specimen was quenched, its vertical section was analysed by XMA. The shapes of the deoxidation-products in the specimen were observed by SEM after deep etching.
The results obtained are summarised as follows.
(1) When the molten iron contains more surface active elements, the alumina/molten iron interfacial energy becomes lower. The lower this energy is, the smaller the critical super-saturation ratio for nucleation of alumina becomes. This critical super-saturation ratio is obtained as 5.55×10⁸.
(2) The dendritic deoxidation-products grown to aluminum-rich direction are α-alumina, and another grown to oxygen-rich is hercynite. These different shapes depend on the crystal structures. The smaller the super-saturation ratio is, the thinner the dendrites become along their principal axes, that is, they become acicular.
(3) The spherical deoxidation-products are formed by remelting of α-alumina by increasing soluble aluminum, or from their liquid state as complex oxides with low melting points.
(4) The clusters are grown by gathering of various deoxidation-products which have different floating velosities.