High cycle fatigue behavior of a single crystal superalloy at elevated temperatures

Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [0 0 1] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C, 760 °C, 850 °C and 900 °C in ambient atmosphere. The results demonstrate that the fatigue strength of smooth specimens reached the maximum at 760 °C and decreased with increasing temperature. The alloy became more notching sensitive with increase of temperature while the notch sensitivity declined at 900 °C. Analysis on fracture surfaces of both smooth and notched specimen shows that a transition from ductile fracture at lower temperatures to cleavage mode at higher temperatures were observed. Evolution of the microstructure was investigated by SEM and TEM observation. With the process of cyclic plastic deformation at elevated temperatures, the primary cuboidal γ′ precipitates tended to dissolve into the matrix channels, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix. In addition, different types of dislocation structures were developed during the cyclic deformation, which would have a significant impact on the fatigue life of the material.