Phenomenological aspects of the high-cycle fatigue of ULTIMET® alloy

ULTIMET® alloy is a commercial Co–26Cr–9Ni wt.% superalloy, which possesses good resistance to both wear and corrosion. The microstructure of ULTIMET® alloy in the as-received condition exhibited a single face-centered-cubic phase with relatively fine, uniform grains, and annealing twins. Stress-controlled fatigue tests were performed at room temperature with different R ratios, in air and vacuum. The experimental method, uniform design, was employed to plan fatigue tests in order to study systematically the effects of the testing variables. A statistical model was formulated to estimate the effects of maximum stresses, R ratios, and environmental conditions on the S–N curves. The statistical analysis showed that these three factors had significant effects on fatigue life, but there was no interaction effect within the ranges of parameters investigated. Interestingly, there were plateau regions in the S–N curves of this alloy regardless of the environment. The plateaus were around a maximum stress level of 600 MPa, which was approximately equal to the material yield strength of 586 MPa. Fractographic studies showed that fatigue cracks were generally initiated either on the specimen surface or subsurface, and the crack-initiation sites were cleavage-like in nature, typical of stage I crack initiation. Fatigue-fracture surfaces had a crystallographic appearance. The stress-induced phase transformation of ULTIMET® alloy during fatigue was characterized by X-ray diffraction. The plateaus of S–N curves were associated with the stress-induced phase transformation and the change of the crack-initiation site from the surface to subsurface.