Residual stress relaxation in shot peened Inconel 718 under thermal loading: in-situ study by high-energy X-ray diffraction

Surface enhancement methods, such as shot peening, are widely used in the aerospace industry to improve the fatigue life of critical components. These surface processes are efficient only if the thermo-mechanical relaxation of the generated residual stress field at operating temperatures is moderate. Recent studies carried out on Inconel 718 showed that almost 50% of the stress relaxation could take place during the first two hours at 550°C. To assess the relaxation kinetics during this very short time, in-situ High energy X-ray diffraction (XRD) measurement under thermal loading were carried out at the PETRA P07 Engineering Materials Science (HEMS) beamline. Samples with different work-hardening generated by surface mechanical attrition treatment and shot peening were tested at different temperatures. Residual stress determination from XRD measurements revealed that residual stress relaxation was more important for the samples with higher work-hardening. However, it is difficult to quantitatively evaluate work-hardening and its evolution. Full width at half maximum (FWHM) of XRD line profiles, microstrain and dislocation density were chosen as indicators to inform about the work-hardening and plastic strain levels before, during and at the end of the thermal loading. These indicators will be used in future work to build relaxation models and predict stress relaxation phenomenon.