Neutron diffraction results on the ordering of an iron-chromium-carbon austenite

Abstract

July iS, 1994) Introduction Order on Fe-Cr-C austenites was first demonstrated by Arbuzov, Golub and Karpets (I), who observed extra reflections in single crystal X-ray diffraction experiments. The reflections were attributed to an Fe7Cr, face centered cubic superlattice (Space Group: Fm3m) with lattice parameter a~: 0.7200 nm. These results were obtained in alloys within a broad range of compositions (6-10wt%Cr and 0.8-1.2wt%C) and confirmed in the martensite produced after liquid nitrogen quenching of the samples (2). The maximum relative superlattice reflection intensities were found in an Fe-Swt%Cr- i.2wt%C alloy, solubilized at 1473K (I). The of carbon could not be assured from the X-ray diffraction data due to experimental limitations of the technique [1,2), but the authors argue that some kind of short-range ordering of carbon in the neighborhood of chromium should be expected due to thermodynamic considerations (2). In a previous paper by some of the authors (3), the results of a conversion electron M~ssbauer spectroscopy (CEMS) experiment in a Fe-Swt%Cr-l.2wt%C austenite was interpreted as evidence of interstitial ordering, with most carbon atoms having at least one chromium atom as near neighbor. The M~ssbauer measurements, however, were not sufficient to evaluate the detailed distribution of both carbon and chromium atoms in the lattice of Fe-Cr-C austenites and a complementary experimental technique was required. The aim of this work is to further investigate the order structure of the austenite of an iron-chromium-carbon alloy using neutron diffraction and, with the help of the M6ssbauer results of (3), to determine whether or not the carbon is also ordered in Fe7Cr superlattice. Experimental A Fe-8wt%Cr-l.2wt%C alloy (nominal composition) was prepared and solubilized at 1473K/200h using the procedure described in (3). The as-solubiiized sample was submitted to meta]iographical and X-ray diffraction analysis in order to assure the homogeneity of the microstructure. A cylindrical sample with ~lSmm diameter and ~30mm height was taken from the sclubilized material and used in the neutron diffraction experiments. 1639