Transition behavior from Mode I cracking to crystallographic cracking in a Ni-base single crystal superalloy

The fatigue crack propagation of Ni-base single crystal superalloy strongly depends on the crystallographic orientation and usually shows transition behavior from Mode I cracking to crystallographic cracking at low temperature. To clarify these behaviors, fatigue crack propagation test at 450 degrees C and crystal plasticity analysis are conducted for a Ni-base single crystal superalloy. The Mode I cracking, the transition, and the crystallographic cracking are observed to be dependent on the level of stress intensity factor range, and to be affected by the crystallographic orientations in a compact specimen. Crystal plasticity finite element analysis for the Mode I cracking, the transition, and the crystallographic cracking are all conducted to investigate the slip activity of an octahedral slip system in front of the crack tip by carefully considering the actual 3D geometry of the crack tip. By taking account of the fatigue damage on the individual slip plane, a damage parameter provides reasonable explanations for the fatigue crack propagation rates during Mode I cracking and crystallographic cracking regardless of crystallographic orientation. This damage parameter has also quantified the critical condition that induces the transition from Mode I cracking to crystallographic cracking in a single crystal superalloy.