Phenomenological aspects of the high-cycle fatigue of ULTIMET (R) alloy

ULTIMET (R) alloy is a commercial Co-26Cr-9Ni wt.% superalloy, which posses ses good resistance to both wear and corrosion. The microstructure of ULTIM ET (R) alloy in the as-received condition exhibited a single face-centered- cubic phase with relatively fine, uniform grains, and annealing twins. Stre ss-controlled fatigue tests were performed at room temperature with differe nt R ratios, in air and vacuum. The experimental method, uniform design, wa s employed to plan fatigue tests in order to study systematically the effec ts 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 h ad significant effects on fatigue life, but there was no interaction effect within the ranges of parameters investigated. Interestingly, there were pl ateau 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 app roximately equal to the material yield strength of 586 MPa. Fractographic s tudies showed that fatigue cracks were generally initiated either on the sp ecimen surface or subsurface, and the crack-initiation sites were cleavage- like in nature, typical of stage I crack initiation. Fatigue-fracture surfa ces had a crystallographic appearance. The stress-induced phase transformat ion of ULTIMET (R) alloy during fatigue was characterized by X-ray diffract ion. The plateaus of S-N curves were associated with the stress-induced pha se transformation and the change of the crack-initiation site from the surf ace to subsurface. (C) 2001 Elsevier Science B.V. All rights reserved.