Local strain redistribution in a coarse-grained nickel-based superalloy subjected to shot-peening, fatigue or thermal exposure investigated using synchrotron X-ray Laue microdiffraction
Etude par microdiffraction Laue de la redistribution des déformations dans un superallliage gros grain à base de nickel après grenaillage, maintien isotherme et fatigue.
Résumé
The Laue microdiffraction technique was used to investigate the strain field caused by the shot-peening operation and its redistribution after thermal hold or fatigue in a model nickel-based superalloy with an average grain size of 40$\mu$m. Micrometer and millimeter size mappings showed that the plastic deformation introduced by shot-peening in the whole sample partially relaxes after a thermal exposure at 450∘C and was fully redistributed by the fatigue of the material, except in the hardened layer close to the sample edge. Diffraction patterns permitted to measure separately the strains related to the average alloy ($\gamma$+$\gamma$′) and to the γ′ phase. No difference was observed between the two deviatoric strain fields. Even if there were small stresses in the inner part of the samples, the sensitivity of the Laue microdiffraction method was large enough to quantitatively characterize the crystal misorientations and the deviatoric strain redistributions. Useful data were provided not only at the grain scale but also at the mesoscopic scale, thus bridging the gap between the sin$^2$$\psi$ and Ortner’s methods used to determine residual stresses, respectively, in fine and single-grain microstructures. The obtained results are also of first interest for a quantitative comparison with HR-EBSD measurements in the scanning electron microscope. Energy coupled measurements with an energy-dispersive point detector were also performed to determine the full elastic strain tensors associated with the $\gamma$ and $\gamma$′ phases. We demonstrated that, for Ni-based superalloys, the accuracy on strains and stresses was, respectively, of the order of 1×10$^{−3}$ and $250$ MPa for the diagonal components of tensors. The measurements suffered from the 150 eV resolution of the detector which made it difficult to the separate the energies of the $\gamma$ and $\gamma$′ phases. Owing to large crystal misorientations, the microdiffraction technique was not able to determine elastic strains and hardening in the highly deformed layer, where a large amount of plastic strain and a number of defects were accumulated. Some improvements are proposed to overcome these difficulties.
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