Effect of Rejuvenation Treatment on SX Ni-Based Superalloys Subjected to Low Cycle Fatigue

Kurzfassung

Rejuvenation treatments with integrated hot isostatic pressing have been proven to re-establish the gamma/gamma′ microstructure of single crystalline nickel-based superalloys after creep deformation, close porosity, and recover creep strength to a significant extent. Therefore, rejuvenation treatment is expected to be effective in extending the service life of components such as turbine blades for gas turbines resulting in reduced costs and improved sustainability compared to replacing the component. Since such components are subjected to combined creepfatigue loading, this investigation is aiming to answer the question if a rejuvenation procedure, which has proved to recover single crystalline superalloys after creep, is effective in rejuvenating these materials after low cycle fatigue. For this purpose, test samples have been subjected after high temperature low cycle fatigue experiments to a rejuvenation procedure. The materials microstructure has been studied by non-destructive X-ray computer tomography before and after rejuvenation. Subsequently, metallographic sections were prepared from rejuvenated samples and investigated by scanning electron microscopy. Pores and precipitates detected in high resolution section images were identified in the reconstructed volume and used to fit the metallographic section images into this volume. Pores and cracks emanating at pores during fatigue testing were closed by rejuvenation as long as they were not connected to the surface. Electron back scatter diffraction performed on the sections revealed that, in contrast to creep samples, the fatigue samples were recrystallized during rejuvenation. It is concluded that fatigue damage cannot be reversed by the same rejuvenation procedure that is effective for creep damage.