抄録
Nickel-base high temperature superalloys having three different micro-structures have been tested by a rotating fatigue machine at the rate of 3500r.p.m. at 815°C. The test was carried out on the specimens both without and with a 60° notch. The fatigue life of the cast one with a notch was considerably shorter than those of other specimens, but the notch sensitivity was eliminated by the solution heat treatment followed by aging.
Fractographic observations were conducted on fracture surfaces. Large MC type carbides have a marked influence in addition to the notch effect on fatigue life. High temperature fatigue fracture surface showed four distinctive features. The planar region including the origin of fracture was the first pattern appearing along the main fatigue crack propagation direction and was very similar to the Stage 1 fracture surface at room temperature in fatigued single crystal of nickel-base superalloy. The second region was the shear mode pattern. The third was the characteristic region containing both transgranular and intergranular fractures. Well defined striations were observed in the transgranular fracture region except the cast one with a notch which showed dull striations. The flat plane, which may be attributed to crystallographic fracture mode, was also observed. The final was the overloading fracture region.
