High-cycle fatigue of ULTIMET® alloy

ULTIMET(R) alloy, a relatively new commercial Co-26Cr-9Ni superalloy, was studied to examine its fatigue behavior at low and high frequencies and in different environments. The microstructure of the alloy in the solution-annealed condition consisted of face-centered-cubic, single phase, relatively fine, and uniform grains with annealing twins. Stress-controlled fatigue tests were performed at room temperature with an R-ratio (the ratio of the applied minimum stress to the maximum stress levels) of 0.05. The high-frequency (1,000 Hz) high-cycle fatigue tests gave results comparable to the low-frequency (20 Hz) tests. Interestingly, there was a plateau region in the S-N (maximum applied stress level versus number of cycles to failure or run-out) curves of this alloy regardless of the testing frequencies and environments. At 20 Hz, the maximum stress level of the plateau was around 600 MPa, which is approximately equal to the yield strength of 586 MPa, and at 1,000 Hz the plateau region occurred at 540 MPa. The microstructural characterization of tested specimens revealed that the evolution of stacking faults played an important role in the fatigue process. The results of fractographic studies indicated that cracking was initiated either on the specimen surface or subsurface depending on test conditions, and the crack-initiation sites were cleavage-like in nature, typical of stage I crack initiation. The presence of oxygen and/or moisture in the test environments was found to have significant effects on the fatigue-crack initiation behavior and fatigue life of ULITMET alloy. The fatigue-fracture surfaces were observed to have a crystallographic appearance for all of the conditions investigated.