Evaluation of time dependent thermomechanical fatigue crack growth in a nickel alloy for power plant applications

In the present study fatigue crack growth rates were measured under isothermal and nonisothermal conditions up to 700 °C for the nickel base material alloy 617. The microstructure and mechanical properties of alloy 617 are influenced by the thermal heat treatment and the following thermal exposure in service. Hence, a solution annealed and a long-time service exposed material were studied. The service exposed material was extracted from the component test plant, which was operated for a time of about 20.000 hours within the EUproject COMTES700 whereof 13.000 h were at a temperature above 680°C. The measured fatigue crack growth rates results in a function of material condition, temperature and load waveform. Furthermore, the results of the non-isothermal tests depend on the phase between thermal and mechanical load (in-phase, out-of-phase). A fracture mechanic based, time dependent model is upgraded by an approach to consider environmental effects, where almost all model parameters represent directly measureable values. A consistent description of all results and a good correlation with the experimental data is achieved.