Local structure analysis of carbon in ferritic steel by C K-edge X-ray absorption fine structure and multiple scattering calculations

The ferritic steel with supersaturated solid solution carbon remarkable increase in hardness by heat treatment (so-called aging) at low temperature. This has been considered to cause by the formation of the carbon cluster in ferritic steel.¹ In this study, we aimed to understand the local structure changes of solid solution carbon in ferritic steel with aging by soft X-ray absorption spectroscopy. In the C K-edge X-ray absorption near edge structure (XANES) spectrum, extended X-ray absorption fine structure (EXAFS) analysis by peak fitting is difficult due to parameter restrictions based on the sampling theorem. Therefore, we proposed the analysis method of C-Fe bond length ratio based on Natoli's law.² Because solid solution carbon in bcc-Fe is a D_4h point group symmetry, it is expected that the peak position of C-Fe σ* state splits in the xy and z directions. We found that the C-Fe bond length ratio, l_xy/l_z, began to decrease with the formation of carbon clusters. Therefore, it suggests that the C-Fez bond was expanded by the increase of local carbon concentration due to the formation of carbon clusters, and structure transition of bcc to bct occurred.

¹H. Abe, Scand. J. Metall., 13, 226 (1984).
² J. Stöhr: NEXAFS Spectroscopy, Springer, Berlin (1992).