Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 degreesC and 900 degreesC was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating loads were studied. The results show that, there is a defined threshold for K40S alloy at elevated temperatures. The fatigue threshold is 23. 9 MPa . m(1/2) at 700 degreesC and 12 MPa . m(1/2) at 900 V. The significant decrease of the threshold with increasing temperature is associated with the oxidation induced embrittlement at crack tip. Observation on the fatigue fracture surfaces indicates a ductile fracture mechanism related to the fatigue crack growth.
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