Effect of a dislocation pile-up at the neutral axis on trans-crystalline crack growth for micro-bending fatigue

Micro-bending beams of the f.c.c. nickel based superalloy CMSX-4, machined by the focused ion beam technique (FIB), are used to study the effect of a stress gradient associated with the bending configuration on the fatigue crack growth behavior in small dimensions. To avoid an impact of dislocation reactions on the crack growth behavior, a trans-crystalline stage-I fatigue crack has been initiated at a notch placed on a slip plane in a single-slip geometry with the highest Schmid factor. da/dN-curve data was gathered by visual crack extension observation during in-situ fatigue in the SEM combined with experimental compliance measurements and FEM-simulations. A dislocation pile-up at the neutral axis results in a significant Bauschinger effect and a back stress acting on the crack tip causing a crack deceleration and an early change from stage-I to stage-II fatigue crack growth analyzed by FIB tomography.