Electron-microscopic analyses on high-temperature fatigue crack growth mechanism in a Ni-based single crystal superalloy

The microscopic mechanism of high temperature (900 degrees C) fatigue crack growth in a single crystal Ni-based superalloy is investigated. Particular attention is focused to its relation to the cyclic misorientation markings (CMMs), formerly termed EBSD (electron backscattered diffraction) striation, left along the crack wake. Observation of a cross-sectioned crack tip is conducted by various electron microscopy i.e. SEM, EBSD, highvoltage TEM and STEM-EDS. A {100} fatigue crack subjected to a relatively high Delta K (40 MPam(1/2)) shows CMMs with an interval corresponding to crack growth rate (2-4 mu m/cycle), consisting of heavily dislocated gamma (Ni) phase and less dislocated gamma' (Ni3Al) phase. It is also revealed that local oxidation along the gamma/gamma' interface at the crack tip existed at the current temperature/Delta K level. On the bases of these findings, a rational yet qualitative crack growth model that incorporated both brittle/ductile processes is proposed.