Temperature dependence of high-cycle fatigue behavior of a Ni-base single crystal superalloy

Smooth specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 degrees C, 760 degrees C, 850 degrees C and 900 degrees C in air atmosphere. The results demonstrated that conditional fatigue strength reached the maximum at 760 degrees C and decreases with increasing temperature. Analysis on fracture surface showed a trend for cleavage rupture at 850 degrees C and 900 degrees C and ductile rupture at 700 degrees C and 760 degrees C. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through SEM observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal gamma' precipitates tended to agglomerate and spheroidized, meanwhile a larger number of secondary gamma' particles were formed in the gamma matrix in specimens fatigue tested at 700 degrees C, which would have a significant effect on the high temperature properties.