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 larg e-angle X-ray scattering to determine the displacements induced by the form ation and the coarsening of ordered Cu4Ti precipitates. The variation of th e atomic scattering factor of Cu near its absorption edge was used to confi rm 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 idea l positions, induced by the formation of the precipitates, and not to the s egregation of the Ti atoms. The displacement field was found to be dependen t on the precipitate mean size, with a single component aligned along a sof t [100] direction: it is composed of a region of dilatation, where the Cu-a tom positions follow the increase of the atomic distance induced by the Cu4 Ti precipitate formation, followed by a region of contraction. This effect is responsible for the pile-up of the precipitates along the soft direction s: the interaction between precipitates is repulsive at long distances and attractive at shorter distances.