Measurement of fatigue crack plastic zones in fine grained materials usingelectron backscattered diffraction

Electron backscattered diffraction (EBSD) techniques have been used to meas ure the plastic zones of fatigue cracks in a powder, nickel based superallo y with an average grain size of 5 pm. Two main approaches were taken: misor ientation measurements and pattern quality analysis. Calculation of misorie ntations from EBSD orientation mapping was found to identify only a heavily deformed plastic region bordering the crack and not the entire plastic zon e. This disappointing result was explained by reference to dislocation theo ry and which suggests that more sensitive methods of measuring the misorien tations are required. By comparison the dimensions of the plastic zone meas ured by direct observations of the EBSD pattern quality were very close to the values predicted by elastic-plastic fracture mechanics.