Crack Growth under Isothermal and Thermomechanical Fatigue and Dominant Failure Mechanisms in Heat-Resistant Alloy

The paper presents an analysis of crack growth rates and an assessment of fracture mechanisms based on the experimental data on isothermal and nonisothermal fatigue. Test results were obtained in the temperature range of 400–650°C under conditions of harmonic fatigue, creep–fatigue interaction, in-phase and out-of-phase thermomechanical cyclic loading. The object of study was heat-resistant nickel-based alloy EI698. After testing, the specimens underwent detailed fractographic analysis using scanning electron microscopy. It was found that the fatigue fracture diagrams were in the following order in terms of crack growth acceleration: isothermal creep–fatigue interaction, nonisothermal in-phase thermomechanical fatigue, isothermal pure fatigue, and nonisothermal out-of-phase thermomechanical fatigue. The crack growth rate curves were arranged according to the dominant intergranular and transgranular fracture mechanisms, and features of transition from one to another were identified.