Effects of environment and structural stability on the low cycle fatigue behaviour of nickel-base superalloys

The effects of microstructural stability and environmental degradation on the low cycle fatigue life of nickel-base superalloys at elevated temperature are considered. Alloys such as René 80 and René 77, in which the μ/μ´ mismatch is large, exhibit rapid μ´ coarsening during testing, and the life is determined by a trade-off betwee beneficial structural coarsening and environmental degradation. For Rene 80, and probably for Rene 77, the life increases with decreasing strain rate and increased hold time. Other alloys such as Waspaloy and Nimonic 90 have low misfit parameters and the μ´ is relatively stable. However, they exhibit deformation-induced carbide precipitation on grain boundaries, twin boundaries and slip bands which appears to adversely affect the fatigue life. In addition, twinning is observed in Nimonic 90 at higher temperatures.A model, based on the premise that the fatigue life is determined by a critical combination of environmental damage and the maximum stress in a cycle, is reviewed and it is shown that many of the fatigue data cited in this report are in agreement with the model. Some suggestions are made for improving the model and developing life prediction methodologies based on observed damage mechanisms.