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Fatigue crack propagation in a polycrystalline Ni-base superalloy was experimentally investigated at room temperature. Fatigue crack propagation tests were conducted under a constant ΔK condition to clarify the effect of grains crystal orientation, grain size and grain boundary, employing CT specimens with different grain size and grains orientations. A series of experiments revealed that crack propagation modes within the grain were roughly categorized into the shearing and opening mode, depending on crystal orientations and grain size. Crack propagation rate in the shearing mode was affected by the primary and secondary crystal orientation of grains and its fundamental mechanism could be explained by the findings for the single crystal material in our previous study. The crack propagation rate in the opening mode was affected by both Schmid factor and Young's modulus, which increased with reduction of the crack length relative to the grain size. It was also found that the grain boundary caused the crack retardation depending on the crack propagation modes in both grains across the boundary. In case the crack propagated in the shearing mode in two adjacent grains, the crack retardation became more pronounced due to the discontinuity and transition of the crack planes.

Influences of crystal orientations and grain boundary on crack propagation behaviour at room temperature in a polycrystalline Ni-base superalloy