Determination of displacements from nanoprecipitates in a Cu-Ni-Fe single crystal using anomalous small-angle and large-angle X-ray scattering

Single crystals of Cu-41.5 at.% Ni-16 at.% Fe have been studied by small-an gle and large-angle X-ray scattering in order to determine the displacement s induced by the disc-like precipitates formed during decomposition. The sm all-angle X-ray scattering (SAXS) patterns gave information on the size and the organization of the precipitates, while the scattering near Bragg peak s allows a determination of the distortions of the lattice created by these precipitates. The variations of the atomic scattering factors of Fe, Ni an d Cu (for the large-angle measurements) were used to determine both the par tial structure functions ('chemical' data) and the partial displacements. T he precipitates were found to be enriched in Fe and Ni, which induced a con traction of the lattice, while the depleted matrix (mainly Cu) was dilated. This succession of compressed and dilated regions is very stable and preve nts a single precipitate from growing much in this direction during coarsen ing. This can explain why a single precipitate tends to grow mainly in the orthogonal directions, where the extension of the displacements is smaller than the precipitated region.