Clustering in Fe-3.9at.%Mo

The diffuse X-ray scattering from a single crystal of Fe-3.9 at. % Mo has been measured at room temperature. A volume in reciprocal space was explored, which was chosen so that the intensity due to local order could be separated from the effects of different atomic sizes of the species and first-order thermal diffuse scattering making use of the symmetry of these contributions according to a theory by Boric & Sparks. The Warren local order parameters have been derived and used in a computer program to generate the corresponding atomic distributions. The Mo atoms formed clusters on the average 7 to 8 Mo atoms in size, the clustered regions having about twice the Mo concentration as the average alloy composition. The largest clusters contained about 40 Mo atoms. The shapes of all these regions were irregular. An analysis of the scattering due solely to the differences in atomic size of Fe and Mo indicates that the Fe atoms are displaced from lattice points by about 0.001 Å.