Low cycle fatigue of René 77 at elevated temperatures

Abstract The low cycle fatigue behavior of Rene 77, a nickel-base superalloy, was studied at 927 °C. It was found that, even though the precipitate structure is stable with respect to thermal exposure, reversed plastic deformation had the effect of producing rapid coarsening which was associated with cyclic softening. The substructure consisted of networks of near-edge dislocations stored on the large γ′ precipitates and results in part from the misfit between matrix and precipitate. There were relatively few dislocations in the interprecipitate channels. It was thus concluded that the damage associated with dislocation debris was minimal. However, oxidation occurred on the surface, preferentially at grain and twin boundaries. The degree of oxidation appeared to be enhanced by cyclic deformation. It was shown experimentally that prior oxidation could greatly reduce the fatigue life and on the basis of these observations the point of failure appears to be determined by a balance between the rate of structural coarsening (increased ductility, reduced stress) and environmental damage. Since a reduction in the fatigue life was not observed when the test frequency was reduced, the conventional ideas of a negative creep-fatigue interaction do not apply to this system.