Fatigue failure behavior and strength prediction of nickel-based superalloy for turbine blade at elevated temperature

The fatigue failure behavior and fatigue strength prediction were performed on a Ni-based superalloy for the turbine blade in 750 °C elevated temperature environment. The asymmetric load tests with the stress ratios R = -1 and 0.1 were tested, followed by microstructure characterization and fracture mode analysis via two & three-dimensional microscopic observation and electron-backscattering diffraction, etc. Results show that as stress level decreases, fatigue failure is less likely to be induced by pore, while the possibility of grain cracking induced failure increases. The larger roughness of the fracture surface is attributed to the geometric incompatibility of grains and the plastic deformation at the crack tip. For the grain related failure, crack nucleation is mainly in Goss grain along the direction of the maximum Schmid factor. Moreover, the threshold values of small & long cracks, the transition crack size from small to long, are all lower for interior failure due to the effect of vacuum environment. Finally, based on El-Haddad model, a new fatigue strength prediction model is proposed, and the predicted results are in good agreement with the experimental ones.