X-ray study on single crystal superalloy SRR99: Mismatch γ/γ′, mosaicity and internal stress

The aim of the work is to set up an X-ray diffraction technique capable of detailed study of the γ-γ′ lattice mismatch and the anisotropy of both phases as a function of location in thermo-mechanically fatigued specimens. A four-axis diffractometer with a copper source is used. X-ray profiles are recorded on SRR99 single crystals coated with a NiAl layer. Two measurements (into the bulk and close to the coating) are performed on two different samples before and after thermo-mechanical fatigue testing. Rocking curve acquisitions are achieved by integration of the individual (θ–2θ) scans at different rocking angles (ω), while the evolution of intensity as a function of tilting angle (χc) is recorded in order to obtain information about the range of orientation of crystallites. A numerical separation of the two strongly overlapping γ and γ′ peaks, which is carried out by means of the PROFIL software developed for this purpose, discloses information related to each of both phases. Assuming a zero shear stress in the material, the stress and the lattice parameter of the strained cell are calculated, in three dimensions, for each of the phases. This specific data acquisition strategy and processing allows the γ-γ′ mismatch to be evaluated successfully in the three main directions of the lattice and in all the samples studied. The remarkable local evolution of mosaicity as a function of the testing demonstrates the usefulness of this parameter for the microstructural characterization of the material in the vicinity of the coating. Internal stresses which are quantified in both γ and γ′ phases are also pointed out as sensitive parameters characterizing the material microstructure. L'objectif de l'étude est de mettre au point une technique de mesure par diffraction des rayons x capable de caractériser en détail les paramètres de maille, la mosaïcitéet l'anisotropie de déformation des 2 phases γ-γ′ en fonction de la position dans unéchantillon fatiguéthermo-mécaniquement. Un diffractomètre 4 axes equipéd'une source de cuivre est utilisé. Des profils de raies de diffraction x sont enregistrés sur un monocristal de SRR99 revètu d'un dépot de NiAl. Deux mesures (au coeur de l'échantillon et sous le revètement) sont effectuées sur deuxéchantillons différents, avant et après fatigue thermo-mécanique. Des acquisitions du type “rocking curve” sont obtenues par intégration de diagrammes (θ, 2θ) enregistrésàdifférents angles ω. L'évolution de l'intensitéen fonction de χc (an