Dislocation Structure Analyses by Surface Observation using ECCI Method and Its Application to Fatigue Damage Estimation

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Electron channeling contrast imaging (ECCI) method has some advantages for dislocation structure observation in comparison with conventional transmission electron microscopy : we can observe the dislocations lying under free surface nondestructively at considerably wide area. With a help of such characteristics of the ECCI method, the dislocation structures formed at all the grains contained in gage part of fatigued strip specimens of stainless steel and copper We could obtained the vein structure and ladder-like PSB structure, which have been observed using TEM. It was found that the formations of the PSBs were preferred to the grains having large diameters, whereas Schmid factor seemed to have no relation to the PSB formation. It was also recognized that annealing twins played an important role on the PSB formation : the number of the PSBs generated both along and across the annealing twin boundaries were larger than that predicted statistically.Dislocation structures near fatigue cracks of polycrystalline copper specimens were also observed using the ECCI technique. Prior to the ECCI observations, optical microscope observations were conducted to classify the fatigue crack morphologies into several kinds. It was found that the dislocation structures were correlated with the slip morphologies observed using the optical microscope. The cell structure almost corresponded to the severely deformed plastic zone where the individual slip bands could not be identified. The labyrinth dislocation structure was detected at the double-slip region. Ladder-like dislocation structure was detected ahead of the Stage I type fatigue crack. Hence, it can be said that the persistent slip band (PSB) was a favorable crack path. However, the microscopic route of the crack growth was not along the PSB but along the cell structure which was developed locally in the vicinity of the crack tip.