ANOMALOUS-X-RAY-SCATTERING STUDY OF LOCAL ORDER IN BCC FE0.53CR0.47

A single crystal of Fe0.53Cr0.47, quenched to 300 K after an anneal above the sigma-phase boundary, was studied at three x-ray energies: 5.969, 7.092, and 7.6 keV (where the x-ray contrast was approximately 0). With our analyzer detector we were able to remove the Compton and resonant Raman scattering experimentally to leave solely the diffuse scattering from concentration fluctuations and atomic displacements. Using the 7.6-keV data to determine and remove the combined pure static and thermal diffuse scattering, the other two data sets were least-squares fitted for short-range order and local-atomic-displacement parameters. These data sets both indicate a clustering tendency (Fe-Fe and Cr-Cr local environments). Inverse Monte Carlo and Krivoglaz-Clapp-Moss determinations of pair-interaction energies agree well and compare favorably with recent theoretical calculations. The average individual Fe-Fe and Cr-Cr distances were also determined and revealed a contraction of both Fe-Fe and Cr-Cr nearest-neighbor distances (Cr is the larger atom). This result disagrees with a conventional treatment of atom-size effects in solid solutions and must be understood using species-dependent solute-lattice couplings. Finally, no indication of premonitory sigma-phase fluctuations was found although diffuse scattering attributable to a related omega-phase softness was observed.