High-frequency metal fatigue: the high-cycle fatigue behavior of ULTIMET (R) alloy

ULTIMET (R) alloy is a relatively new commercial Co-26Cr-9Ni (wt.%) alloy, which exhibits good resistance to both wear and corrosion. A state-of-the-a rt high-frequency, 1000-Hz, material test system was used to study the high -cycle fatigue behavior of ULTIMET alloy up to 10 degrees cycles. Fatigue e xperiments were conducted at high (1000 Hz) and conventional (20 Hz) freque ncies in air at room temperature. The effects of the test frequency. the te mperature 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 equ ilibrium 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 a t both high- and low-frequencies exhibited a typical two-stage fatigue-crac k-growth process, i.e., (a) stage I fatigue-crack initiation in which the c racks formed on those planes most closely aligned with the maximum sheer-st ress direction in the grains of the fatigue specimen; and (b) stage II fati gue-crack growth in which the maximum principal tensile stress controlled c rack propagation in the region of the crack tip. (C) 2001 Elsevier Science B.V. All rights reserved.