X-ray absorption fine structure analysis of interstitial (C, N)-substitutional (Cr) complexes in austenitic stainless steels.

Abstract

Local atomic environments around chromium atoms in 15Ni-15Cr austenitic steels with separate or combined addition of carbon, nitrogen and silicon are investigated by means of XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) measurements. A peak is observed on the rising slope of the chromium K-edge absorption spectrum. The peak is most distinct for the steels without alloying of carbon, nitrogen, or silicon. The peak is weaker in the steels with combined addition of silicon with carbon or nitrogen, and single addition of silicon than with separate addition of carbon and nitrogen. Height of the first peak in the Fourier transforms of EXAFS spectra of the solution-treated state lowers with addition of interstitial elements: carbon or nitrogen. These results suggest that interstitial atoms gather around chromium atoms to form a kind of Interstitial-Substitutional (I-S) complexes such as a shortrange ordered structure. Silicon probably promotes formation of the I-S complexes around chromium atoms. In the steels alloyed with carbon or nitrogen, the height of the first peak is elevated by cyclic or uniaxial tensile deformation probably because of the reduction in density of the I-S complexes. Plastic deformation produces no significant modification in the XANES spectra. The I-S complexes, which may form in Cr-Ni and Cr-Mn austenitic steels containing both or one of carbon and nitrogen, are considered to play an important role in solid solution hardening in the solution-treated state and also in work softening in the low cycle fatigue test owing to their breakaway during plastic deformation.