Crack paths in single-crystal Ni-base superalloys under isothermal and thermomechanical fatigue

Fatigue crack paths in single-crystal Ni-base superalloys do not follow the classical Stage I/Stage II behavior. In fact, the opposite often occurs with cracks growing initially in Mode I opening and then later transitioning to crystallographic shearing. The crack paths under isothermal and thermomechanical fatigue (TMF) are influenced by several factors including temperature, strain range, dwells during the cycle, the microstructure and crystal orientation, the environment, the strain-temperature phasing in TMF, and frequency. The relative influences of the several crack growth parameters on the transition between Mode I opening and crystallographic shearing crack growth are graphically summarized.