Abstract
The microscopic mechanism of high temperature (900 °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, high-voltage TEM and STEM-EDS. A {100} fatigue crack subjected to a relatively high ΔK (40 MPam1/2) shows CMMs with an interval corresponding to crack growth rate (2–4 μm/cycle), consisting of heavily dislocated γ (Ni) phase and less dislocated γ’ (Ni3Al) phase. It is also revealed that local oxidation along the γ/γ’ interface at the crack tip existed at the current temperature/ΔK level. On the bases of these findings, a rational yet qualitative crack growth model that incorporated both brittle/ductile processes is proposed.
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