Nucleation of proeutectoid ferrite at ceramic/steel interfaces

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Intragranular ferrite nucleation at non-metallic inclusions in steel has drawn a considerable amount of attention because the phenomena may be used as a novel technique of the control of steel microstructure and properties. In order to elucidate the mechanism and identify the most important parameters controling the intragranular nucleation of ferrite, a number of ceramics (sintered polycrystals available commercially) were bonded to an Fe-C-Mn alloy at high temperatures and isothermally reacted for varying times below Ae_3 temperature. Among the ceramics studied either the bonding appaeared to be incomplete with TiO_2 and MnS (either a peculiar reaction occurred or the interface region was melted). The number of ferrite at VN/austenite interfaces was considerably larger with that at austenite grain boundaries, which may indicate that VN served as an innoculant for ferrite nucleation. A similar tendency was observed with Al_2O_3, but to a lesser extent. A SEM-EDX analysis has shown that an interfacial reaction or solute diffusion across the interface did not occur significantly with these ceramics. The thermodynamic driving force, the strain energy possibly caused by the difference in thermal expansion between iron and ceramics, the interfacial energy between ceramics and iron were calculated using a simple theory and the relative nucleation potency of each ceramic was evaluated on the basis of classical nuleation theory. As a result, the most important parameter governing the ferrite nucleation appears to be the difference in the interfacial energy between ceramics/austenite and ceramics/ferrite. However, further detailed studies may be necessary of atomic structure and energies of ceramics/steel interfaces, solute distribution near and within inclusions, etc.to utilize this interesting phenomena for the microstructure control of steels.