The effect of environment on the mechanism of Stage I fatigue fracture

Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes. Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance. Both of these factors indicate that the environment may play an important role in this mode of propagation. To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200. The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced. Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance. In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines. At high magnifications, a dimpled structure is observed on these fracture surfaces. The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip. A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue.