Steel plates produced by an advanced thermo-mechanical control process have two regions: the ultrafinegrained surface layer with the average grain size of less than 2μm and the intermediate region with medium-sized grains. Smooth specimens made of ultrafine-grained and medium-grained regions were fatigued under cyclic axial tension compression at room temperature. The effect of the microstructure on the initiation and the early propagation of small fatigue cracks were investigated by observing the images of specimen surfaces replicated on plastic sheets by atomic force microscopy. In the medium-grained specimens, the fatigue cracks initiated from the simple slip lines. In the ultrafine-grained specimens, complex slip deformation in the shape of irregularities of the specimen surface was observed prior to the initiation of fatigue cracks. The fatigue cracks were nucleated at the grain boundary between the grains containing the complex slip deformation. The crack propagation rate in the ultrafine-grained specimens decreases because of the grain-boundary blocking and crack deflection in an early stage of the propagation.
