BY ATOMIC FORCE MICROSCOPY

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