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 cr ack 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 fatigu e and wavy type at the later step in fatigue was used. The crack propagatio n was produced by the Instron hydraulic testing machine to obtain the near threshold of the crack propagation. The microstructure was observed by the JE:OL 200CX transmission electron microscope. The results are (1) the dislo cation structure in front of the crack tip at the stress intensity threshol d is the cell with an average diameter about 0.5 mu m and with a range of a bout 2-3 mu m. Out of this range, the dis:location is similar to the disloc ation of a planar type. (2) The surface crack initiates when the multiple s lip system is created and the accomplishment of dislocation is triggering m ultiple ladder-like wall in persistent slipband (wavy dislocation) embedded in a matrix-planar dislocation (persistent slip Luder band). (C) 2000 Else vier Science S.A. All rights reserved.