The microstructure of the fatigue crack tip in Fe–Al–Mn–0.4% C alloy near the stress intensity threshold

This work is an investigation of the dislocation structures close to the crack tip near the stress intensity threshold, and the dislocation morphology of surface crack initiation. In these experiments, a material (Fe–Al–Mn–0.4% C alloy) with the planar dislocation structure at early stages in fatigue and wavy type at the later step in fatigue was used. The crack propagation was produced by the Instron hydraulic testing machine to obtain the near threshold of the crack propagation. The microstructure was observed by the JEOL 200CX transmission electron microscope. The results are (1) the dislocation structure in front of the crack tip at the stress intensity threshold is the cell with an average diameter about 0.5 μm and with a range of about 2–3 μm. Out of this range, the dislocation is similar to the dislocation of a planar type. (2) The surface crack initiates when the multiple slip system is created and the accomplishment of dislocation is triggering multiple ladder-like wall in persistent slipband (wavy dislocation) embedded in a matrix–planar dislocation (persistent slip Lüder band).