Thermomechanical fatigue experimental study on a notched directionally solidified Ni-base superalloy

Ni-base superalloys are important to the gas turbine airfoils due to their excellent high temperature properties. Though sufficient works have done for their application, the attentions on performance of complex geometries under thermal mechanical fatigue loadings still do not match its importance. In the present paper, thermomechanical fatigue (TMF) of a notched directionally solidified (DS) nickel based superalloy DZ125 was experimentally investigated. 500 °C to 1000 °C Out-of-phase (OP) TMF cycles were performed in longitudinal grain orientation of the cylindrically-notched specimens. The test results show a significant fatigue life reduction effect is found with the increased notch-severity. Through Scanning Electron Microscope (SEM) observations, the cracks initiate around the notches with heavy oxidation. However, the cracks located at the maximum Mises stress or Principle stress tend to form as the main cracks and cause the final failure. Hence, the combination of surface oxidation and stress concentration (K_t) dominates the failure procedure.