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Single crystals of different Cu–Ni–Co and Cu–Ni–Fe alloys, forming spheroid or plate-like precipitates during decomposition, have been studied by small-angle (SAXS) and wide-angle X-ray scattering (WAXS). The SAXS patterns gave information on the sizes and the organization of the precipitates, while the scattering near Bragg peaks allowed a determination of the distortions of the lattice created by these precipitates. The variations of the SAXS spectra with the atomic scattering factors of Co (or Fe) and Ni were used to determine the composition variation between matrix and precipitates (i.e. the `chemical' term), while those of the WAXS spectra enabled the determination of the displacements in the matrix and in the precipitates. The precipitates were found to be enriched in respectively Co and Ni, or Fe and Ni, inducing a local contraction of the lattice, while the matrix (mainly Cu) was of course depleted in the same elements, and its lattice was dilated. These precipitates are piled up along one of the three 〈100〉 `soft' directions. The variations of the deformation field with the precipitate sizes (along both directions parallel and orthogonal to the pile-up orientation), and with the distance between precipitates have been determined. From the knowledge of such variations, it was possible to modify the respective deformation fields in such a way that the respective precipitates have the same sizes, but with a different geometry, and then to isolate what was induced solely by the geometry of a precipitate. Moreover, these experiments yielded the determination of the secondary components of the displacement field (displacements orthogonal to the pile-up direction), which were found to be weak compared with the primary one (parallel to the same direction). Finally, from the variation of the aspect ratio of the precipitates with the aging duration, it was possible to estimate the surface energy of the precipitates, and a comparison between SAXS and WAXS results shows the applicability of this new approach to the decomposition process.

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