Crystal plasticity assessment of crystallographic Stage I crack propagation 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 compact specimens with different combinations of primary and secondary orientations. It was revealed in the experiment that the fatigue crack propagated along crystallographic slip planes in a mixed mode with Mode I, II and III components. The mixture ratio and fatigue crack propagation rate were strongly influenced by the primary and secondary crystal orientations. To interpret the effect of crystal orientations on the Stage I cracking behavior, a crystal plasticity finite element analysis was conducted considering the actual geometry of the crystallographic crack planes. The slip activities of the individual octahedral slip systems were evaluated, and then a damage parameter was proposed based on the critical plane approach. As a result, a reasonable explanation was obtained for the effect of crystal orientations on the cracking path and propagation rate of the crystallographic Stage I cracking.