PART VI - The Location of Carbon in the Lattice of an Austenitic Manganese Steel

Neutron-diffraction pattens were obtained at room temperature from two austenitic manganese steels, oxc wth n carbon content of 1.23 zct PC/ and the olher 0.63 wt pct. Analysis of the data showed that the curOan atols (ions) occupy tile octaheadral site. any of the cavbon is in the tetahedral site, tile concentration is too strzall lo be detecled by tzeulr-on d$facton. AS part of a long-range investigation of eutectoid decomposition, a study of the structure of austenite has been undertaken in order to characterize as best as possible the parent phase in the Fe-C system. Austenite is a solid solution of carbon atoms in fcc iron, the carbon atoms being located in interstitial positions. Of the two kinds of interstitial sites, octahedral (0, +, +) and tetrahedral (4, a, $), in this lattice, the larger octahedral sites are usually considered as the probable position of carbon. The strain energy involved in pytting a neutral carbon atom (covalent di; ameter 1.54)' into a tetrahedral site (diameter 0.56A) is considerably greater than that associated with p2tting carbon into the octahedral site (diameter 1.08A). Dayal and arkeen' observed the migration of carbon in iron under the influence of an electric field indicating tha tye carbon is ionized. pe ionic radius of C'4 is 0.15A and that of C" is 0.29. Consequently, it is conceivable that carbon ions could occupy the tetrahedral sites if one considers only the strain energy. Size considerations alone do not provide a complete explanation for the formation of the interstitial solid solutions, however. because metals such as copper, zinc, and cadmium do not appear to form interstitial solid solutions in spite of their large interstitial sites. Evidence which suggests partial ioni-zation of the carbon in austenite has also been obtained by Speiser, Spretnak, and alor who derived formu- lae relating the observed change in lattice parameter to the effective diameter of the solute atom. Assuming the carbon occupies the octahedral site. they computed the effective diameter of carbon to be 1.33A whereas the covalent diameter is 1.54A. petch5 concluded from powder X-ray diffraction experiments that carbon atoms occupy the octahedral sites. His conclusion was based on the general trend observed in the intensity of the diffraction lines as the carbon content of the austenite was increased. However, the major difficulty associated with application of X-ray diffraction to this particular problem is the relatively poor scattering power of carbon. Williamson and smallman' utilized X-ray diffraction to study the line broadening caused by the strain field about carbon atoms inferrite: these investigators concluded that at least 85 pct of the carbon in ferrite occupies the octahedral (0: 3, B) site rather than the larger