Low-cycle fatigue behavior of ULTIMET® alloy

The low-cycle fatigue behavior of ULTIMET(R), a wrought cobalt-based alloy, was studied at the temperatures of 294, 873, and 1,173 K under isothermal conditions. A constant strain rate of 3.0 x 10(-3) S-1 was used with fully reversed strain ranges between 0.4 and 2.5 pct. Observations on the strain vs fatigue-life curves, cyclic stress-strain responses, and fatigue fracture modes were obtained. The microstructure evolution of the ULTIMET alloy was characterized using X-ray diffraction, scanning-electron microscopy (SEM), and transmission-electron microscopy (TEM). For fatigue tests performed at 294 and 873 K, plastic-strain-induced, fcc-to-hcp phase transformations were observed. Twinning and carbide precipitation were found to contribute to the significant cyclic hardening observed at 1,173 K.