# 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

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## Abstract

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 \(40\,{\upmu }\hbox {m}\). 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 \(450^{\circ }\hbox {C}\) 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 (\(\gamma +\gamma ^{\prime }\)) and to the \(\gamma ^{\prime }\) 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 \(\hbox {sin}^{2}\psi \) 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 \(\gamma ^{\prime }\) phases. We demonstrated that, for Ni-based superalloys, the accuracy on strains and stresses was, respectively, of the order of \(1\times 10^{-3}\) 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 \(\gamma ^{\prime }\) 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.

## Notes

### Acknowledgements

This work benefited from the support of the REMEDDIES project (ANR-13-BS09-016) of the French National Research Agency (ANR) and the French Aeronautical and Space Research Foundation (FRAE). The synchrotron data used in this article were collected during ESRF experiments MA3096, 32-02 778 and IN995. Authors gratefully acknowledge the BM32 staff for helpful discussions and technical support, as well as D. Locq, C. Liard and P. Kanouté from ONERA for the alloy development and mechanical testing.

### Compliance with ethical standards

### Conflict of interest

The authors declare that they have no conflict of interest.

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