Crystal Plasticity Assessment to Crystallographic Stage I Cracking in a Ni-Based Single Crystal Superalloy

Stage I fatigue crack propagation along crystallographic slip planes was experimentally and analytically investigated in a single crystal Ni-base superalloy, NKH-304. Fatigue crack propagation tests at room temperature were conducted using four types C(T) specimens with different combinations of primary and secondary orientations. It was revealed in the experiments that the fatigue cracks propagated along crystallographic slip plane in mixed mode with Mode I, II and III components. Mixture ratio and fatigue crack propagation rate was strongly influenced by the primary and secondary crystal orientations. In order to interpret the effect of crystal orientations on the Stage I cracking, a crystal plasticity finite element analysis was conducted considering the actual geometry of the crystallographic crack planes. Analytical results for slip activities on the individual octahedral slip systems and a damage parameter based on the critical plane approach provided reasonable explanations to the effect of crystal orientations on the cracking path and propagation rate of the crystallographic Stage I cracking.