Displacements from Cu₄Ti in a Cu-Ti single crystal using small-angle and diffuse X-ray scattering: a synchrotron radiation study

Single crystals of Cu-2.5 at.% Ti have been studied by small-angle and large-angle X-ray scattering to determine the displacements induced by the formation and the coarsening of ordered Cu₄Ti precipitates. The variation of the atomic scattering factor of Cu near its absorption edge was used to confirm the nature of the precipitates and to show that the scattering appearing near Bragg peaks was due to the displacements of the atoms from their ideal positions, induced by the formation of the precipitates, and not to the segregation of the Ti atoms. The displacement field was found to be dependent on the precipitate mean size, with a single component aligned along a soft <100> direction: it is composed of a region of dilatation, where the Cu-atom positions follow the increase of the atomic distance induced by the Cu₄Ti precipitate formation, followed by a region of contraction. This effect is responsible for the pile-up of the precipitates along the soft directions: the interaction between precipitates is repulsive at long distances and attractive at shorter distances.