ON THE STRUCTURE AND HOMOGENEITY OF SOLID-SOLUTIONS - THE LIMITS OF CONVENTIONAL X-RAY-DIFFRACTION

The characterization of solid solutions by conventional X-ray diffract ion (XRD) is critically assessed using the example of Cu-Co alloys. In particular, the limits of this technique for investigating compositio nal inhomogeneities are discussed. Coherent Cu-Co multilayer films are taken as a model for a decomposed solid solution, with the layer thic knesses being taken as representative for the sizes of heterogeneous d omains. It is shown that the XRD patterns of these multilayer films co ntinuously change with decreasing layer thickness from that of two bul k phases to a single pattern identical to that of a homogeneous solid solution, the transition occurring at a layer thickness of about 10 nm . Below this layer thickness, the lattice spacing derived from the sin gle diffraction pattern exhibits an average value determined by the ov erall composition. By definition, this average lattice spacing varies continuously between the pure elements with a Vegard law dependence wh en the overall composition is changed. However, since the multilayer i s heterogeneous, the average lattice spacing is nowhere present in the specimen. The results indicate that coherent inhomogeneities in Cu-Co solid solutions must be larger than several nanometres before they ca n be detected using conventional XRD. With these conclusions XRD measu rements of Cu-Co alloys after preparation as well as during subsequent annealing are considered. The results demonstrate the important role of structural coherence on diffraction measurements. It is concluded t hat it is not possible to determine the supersaturation of a solid sol ution using conventional XRD. Complementary measurements have to be pe rformed in order to prove the homogeneity of a supersaturated alloy.