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

Electron backscattered diffraction (EBSD) techniques have been used to measure the plastic zones of fatigue cracks in a powder, nickel based superalloy with an average grain size of 5 μm. Two main approaches were taken: misorientation measurements and pattern quality analysis. Calculation of misorientations from EBSD orientation mapping was found to identify only a heavily deformed plastic region bordering the crack and not the entire plastic zone. This disappointing result was explained by reference to dislocation theory and which suggests that more sensitive methods of measuring the misorientations are required. By comparison the dimensions of the plastic zone measured by direct observations of the EBSD pattern quality were very close to the values predicted by elastic–plastic fracture mechanics.