Examples of faceted Al2O3 inclusion in steel

Examples of faceted Al2O3 inclusion in steel

Figure 1: Examples of faceted Al2O3 inclusion: (a) octahedral, (b) non-octahedral; (c) hopper morphologies (indicated by the arrow. SEM, scale bars: 1, 2, 10 µm.

Inclusion name: Alumina
Record No.: 1282
Inclusion formula: Al2O3
Inclusion type (Macro/Micro/Nano): Micro
Inclusion type (Exogenous/Indigenous): Indigenous
Inclusion classification: Oxide
Inclusion composition in weight %: No data
Sample: Steel
Steel composition in weight %: No data
Note: Inclusion with well developed facet planes and a three-dimensional character are classified as faceted inclusions (Fig. 1). They are also numerously found in industrial samples. Their size ranges from a few ìm to few tens µm. In the work of Dekkers, faceted inclusions are further classified as octahedral (Fig. 1 (a)) or non-octahedral (Fig. 1 (b)). The reason is the belief that corundum, which cannot develop an octahedral habit, is not the only Al2O3 phase present in liquid steel. Other Al2O3 polymorphs could explain the presence of octahedral inclusions.
The facets appear in the crystal habit as a result of slower growth rate, and the subsequent ability of the crystal to arrange the incoming growth units in the configuration that minimises the surface energy. Such growth conditions are obtained at lower supersaturation degree. From that point of view, faceted inclusions cannot be formed during cooling. Also, the size of the polygonal inclusions was found to be independent on the cooling rate of the Fe. As a result of different supersaturation degrees during the growth of the polyhedral crystal, flat, concave or hopper faces (Fig. 1 (c)) on the octahedral inclusions were observed.
Large faceted inclusions can also result from the sintering and densification processes of aggregates. Overgrowth of plate-like inclusions or dendrites was occasionally observed on small octahedral inclusions.
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