The smooth specimens of ultrafine-grained steel with the average grain size of less than 2 μm were fatigued under cyclic axial tension compression at room temperature and the crack initiation and early propagation behaviors were investigated by crystallographic orientation analysis based on the electron backscatter patterns (EBSP). Two types of slip lines were observed. One is cross slip that initiates a fatigue crack as previously reported. The other is simple slip which does not initiate a fatigue crack and observed on the specimens with coarser grains as well. The crystallographic orientation analysis revealed that only one slip system was activated at the early stage of fatigue cycles, while the cross slip introduces several active slip systems resulting in the crack initiation at the grain boundary.
