The anomalous behavior of microstructurally “short” cracks that can control fatigue life at very high cycles can be attributed to the local conditions around these cracks, since the length scale involved requires the consideration of anisotropic material behavior and the effect of changes in grain orientation as the crack grows. The effect of local crystallography was studied in multicrystalline Compact-Tension (CT) specimens of pure nickel and a cast Ni-based superalloy. Orientation Imaging Microscopy (OIM) was used to map the crystallography of the grains ahead of the notch. A standard fatigue crack growth test was then carried out to characterize the crack path in relation to the grain orientations. Two extreme cases were identified: at one end cracks grew with small deviations through all the grains ahead of it, whereas at the other end large deflections from a path perpendicular to the applied load were observed. Intergranular cracks were found to prefer high angle boundaries, whereas transgranular cracks had a tendency to nucleate and display stage I growth along slip traces of systems with high Schmid factors, as determined by the uniaxial conditions expected at the notch tips. In addition, crack path tortuosity was more pronounced in grains with loading axes close to 〈111〉. Finally, the influence of changes on slip geometry as cracks moved across grain boundaries is also discussed.
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