Wide-Area Observation of Dislocation Structures around Fatigue Crack using ECCI Method

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Dislocation structures formed near fatigue cracks in copper single crystals and ferritic stainless steels were observed with electron channelling contrast imaging (ECCI) method. The ECCI method is based on the electron channelling phenomenon that intensity of backscattered electrons depends on the angle between incident electron beam and lattice plane. We can observe dislocations lying under surface in a scanning electron microscope (SEM). In the present research, it was confirmed that the dislocation structures such as the persistent slip band (PSB) and the labyrinth structure -- which are generated due to fatigue loading -- were imaged with the ECCI method over a wide area including the crack tip vicinity, at which the TEM observation had been difficult. At the fatigue experiments on the copper single crystals, the dislocation structures formed around fatigue cracks were sensitive to loading axes. At the [001] loading axes, the ladder-like PSB structure and the labyrinth structure we … More re observed around the fatigue crack. The formation of these dislocation structures depended on the angle from crack plane. The distribution of the dislocation structures could be explained by the stress distribution calculated from the linear fracture mechanics. In the crack growth experiments on Fe-30%Cr single crystals, the dislocation structures depended on the mode of fatigue cracks. The cell structure surrounded the fatigue crack having Mode-III component which appeared at <112> and <221> loading axes. On the other hand, no significant dislocation structures were found near the Mode-II fatigue crack propagating at the specimen with <110> loading axis. Only a band-like dislocation structure parallel to the crack plane was emitted from the crack tip. In the polycrystalline Fe-20%Cr alloy, the relationship between crack growth rate and the surrounding dislocation structure was investigated. At a single-slip-oriented grain, the dislocation structure mostly observed in the vicinity of the crack was the vein and the crack growth rate was relatively fast. On the other hand, the cell structure was well developed near the crack in double-slip oriented grains. The fatigue crack with the cell structure propagated at slow crack growth rate. Less