Competing Mechanism of Creep Damage and Stress Relaxation in Creep-Fatigue Crack Propagation in Ni-Base Superalloys

Effect of creep deformation at crack tip on fatigue crack propagation behavior in a single crystal and a directionally solidified superalloys at 900 °C was investigated. Creep-fatigue crack propagation tests with single tension hold introduced into cyclic fatigue loading were conducted. In specimen extracted from the single-crystal superalloy, ICMSX-4, when the cyclic fatigue loading was restarted after the tension hold, nascent crack was immediately initiated followed by significant crack retardation. This crack propagation behavior was ascribed to mechanisms based on two different concepts of residual compressive stress and crack closure. From the viewpoint of mechanism based on the residual stress concept, material degradation at crack tip induced by the tension hold was investigated using scanning electron microscope, while stress relaxation and the resultant residual compressive stress at crack tip were quantified by elastic-plastic-creep finite element analysis coupled with digital image correlation technique. Finally, crack propagation behavior in polycrystalline specimen extracted from the directionally solidified superalloy, MGA1400, was investigated focusing on effect of grain boundary. An insight into criteria of a transition from crack retardation to accelerated intergranular cracking was suggested based on a relative grain size to “creep affected zone.”