Elevated temperature, high cycle fatigue behavior of carbon-containing single crystal Ni-Based superalloys

Abstract

Although first removed from single crystal alloys, alloy developers have added carbon back into single crystal alloys for improved castability, cleanliness and defect tolerance. However, in this study, the fatigue properties of model third generation (i.e., 6 wt% Re) single crystal Ni-base superalloys containing carbon additions were found to be greatly influenced by the carbon additions. All of the samples were tested at 850 °C in load controlled high cycle fatigue with a stress range of 690 MPa. The addition of a small amount of carbon (~0.01 wt% carbon) resulted in an increase in the volume fraction of carbides and porosity which caused a greater amount of scatter and, on average, a slight decrease in fatigue properties. However, further additions of carbon (>0.05 wt% carbon) resulted in significant reductions in both the amount of scatter and a reduction in the average fatigue life. Porosity, which were much larger in size than carbides, were observed to be fatigue crack initiation sites and as the volume fraction of porosity and carbides increased the fatigue lives were observed to decrease. Interestingly, as the carbon content increased, and the volume fraction of carbides and porosity increased the average size of the carbides and porosity slowly decreased preventing the application of Kitagawa-Takahashi diagrams.