High-frequency metal fatigue: the high-cycle fatigue behavior of ULTIMET® alloy

ULTIMET® alloy is a relatively new commercial Co–26Cr–9Ni (wt.%) alloy, which exhibits good resistance to both wear and corrosion. A state-of-the-art high-frequency, 1000-Hz, material test system was used to study the high-cycle fatigue behavior of ULTIMET alloy up to 109 cycles. Fatigue experiments were conducted at high (1000 Hz) and conventional (20 Hz) frequencies in air at room temperature. The effects of the test frequency, the temperature increase during fatigue, and the change of crack initiation sites from the surface to subsurface on fatigue life are discussed. Although the fatigue life was comparable at test frequencies of 1000 and 20 Hz, the equilibrium temperature at 1000 Hz was considerably higher than that at 20 Hz. The fractographic study showed different morphologies of fracture surfaces at various frequencies. The high-cycle fatigue behavior of ULTIMET alloy at both high- and low-frequencies exhibited a typical two-stage fatigue-crack-growth process, i.e., (a) stage I fatigue-crack initiation in which the cracks formed on those planes most closely aligned with the maximum shear–stress direction in the grains of the fatigue specimen; and (b) stage II fatigue-crack growth in which the maximum principal tensile stress controlled crack propagation in the region of the crack tip.