Abstract
The effect of crystallographic orientation and microstructure on fatigue crack propagation behavior in ultrafine-grained specimens with the average grain size of less than 2μm was investigated by EBSP (Electron backscatter patterns). The results show that fatigue crack bifurcation, which has been proven to be the dominant factor of the excellent fatigue property introducing crack closure, was introduced either at grain boundaries or within grains with the ratio of 1 : 1. The mechanism of the bifurcation was explained by introducing the slip factor. ζ. Fatigue crack bifurcates at a grain boundary when it propagates from a grain containing a single slip system with the outstandingly largest ζ to the other grain holding many slip systems with large ζ. The intragranular bifurcation was observed in the grain with dozens of slip systems with the same largest ζ.
