ATOMIC DISPLACEMENTS CAUSED BY CARBON INTERSTITIALS IN AUSTENITE

Full reciprocal volume diffuse X-ray scattering measurements were performed on a single crystal austenite specimen of composition Fe-25 wt% Ni-0.88 wt% C employing a synchrotron radiation source. An X-ray energy of 8800 eV was used to minimize the scattering contrast between Fe and Ni which allowed the data to be treated as a binary ''Fe''-C system, greatly simplifying the analysis. The intensity data was used to determine directly the Fe-C pair displacements in this alloy. These were compared to both lattice statics computations and a recent molecular dynamics simulation of C in austenite. The measured first neighbor Fe-C distance of 1.886(+/-0.006)angstrom indicates that Johnson's Fe-C potential underestimates the pair interaction. A pair potential which will produce this measured Fe-C separation is presented. The diffuse components associated with possible C-C ordering were not of sufficient intensity for conclusions to be drawn about the state of order on the interstitial sublattice in this alloy.