Plate-like overgrowth onto existing particles in steels

Plate-like overgrowth onto existing particles in steels

Figure 1: Plate-like overgrowth onto existing particles. SEM, scale bars: 1, 2 µm.

Table 1: Compositional ranges in ppm of the investigated steel grades.

Inclusion name: Alumina
Record No.: 1000
Inclusion formula: Al2O3
Inclusion type (Macro/Micro/Nano): Micro
Inclusion type (Exogenous/Indigenous): Indigenous
Inclusion classification: Oxide
Inclusion composition in weight %: No data
Sample: Different steels. Medium carbon aluminium killed (MCAK), low carbon silicon-aluminium killed (LCSAK), low carbon aluminium killed (LCAK) steels.
Steel composition in weight %: See the table
Note: Morphology of non-metallic inclusions depends on their crystallographic structure, the growth conditions and the presence of impurities. Inclusions were extracted from industrial aluminium killed steel samples and investigated under high-resolution scanning electron microscopy. In this chapter the morphology of these aluminium oxide inclusions, including their surface features, is approached from the viewpoint of crystal growth. Commonly aluminium oxide inclusions are considered to be corundum, but some inclusion shapes prove that other aluminium oxide polymorphs are present as well.
Some inclusion morphologies may be understood by considering overgrowth of corundum onto octahedral faces, e.g. plates that are overlying octahedral faces and extend into the steel (Fig. 1). The octahedral faces probably act as preferred sites for epitaxial overgrowth of corundum, because only a slight lattice misfit exists between (111) face of the hexagonal close-packed structure and the {111} face of the cubic closed-packed structure (e.g. spinel) [I. Sunagawa, in Morphology of Crystals, Part B, Chp. 7, Ed. I. Sunagawa, Terra scientific publishing: Tokio, D. Riedel publishing: Dordrecht (1987) 509-587.]. Similar to plate-like inclusions we assume that the plate-like habit of the overgrown phase is caused by the influence of impurities. Based on our observations we are not able to tell, whether the overgrowth occurs at a certain moment of primary growth or whether it is due to reoxidation (secondary growth).
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