STAGE I FATIGUE CRACK PROPAGATION IN A SINGLE CRYSTAL AND A DIRECTIONAL SOLIDIFIED Ni-BASE SUPERALLOY

Stage I fatigue crack propagation along crystallographic slip planes in Ni-base superalloys were experimentally investigated at room temperature employing miniature CT specimens. At first, effects of crystallographic orientation on the Stage I fatigue crack propagation were investigated, using a single crystal (SC) Ni-base superalloy, NKH-304, with different combinations of primary and secondary orientations. Based on the test result of the NKH-304, influences of grain orientations and grain boundary on Stage I crack propagation was investigated employing a directional solidified (DS) superalloy, CM-247LC, changing the grain orientation in front of the initial mechanical notch of CT specimen. A series of experiments revealed the Stage I cracking in the SC superalloy was significantly influenced by both primary and secondary crystal orientation, and the grain boundary in the DS superalloy caused the retardation of crack propagation depending on angular difference between crack planes across the grain boundary. In order to numerically investigate the effect of elastic anisotropy and the geometry of the Stage I crack, a 3-D finite element model for the CT specimen was developed. It was found from fracture mechanics analysis that geometries of 3-D inclined crack, containing mode I, II and II components, played an essential role to determine the driving force of the Stage I crack propagation in the superalloys.