Nondestructive Observation of Dislocation Structure using ECCI Method and Embryology of Fatigue Cracks.

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Intragranular fatigue cracks are initiated frequently along the persistent slip bands (PSBs) at which cyclic plastic strains are localized. Dislocation structure of the PSB is characterized by a ladder-like structure which consists of dislocation wall and channel. Hence, by detecting this ladder-like dislocation structure, we can predict the fatigue crack initiation. However, a material must be shaped to a thin foil in order to observe the dislocation structure using a conventional TEM. In the present research project, we make attempt to perform nondestructive dislocation observation using Electron Channelling Contrast Imaging (ECCI) method, with a view to predicting the fatigue cracking.Copper single crystals having defined stress axes were prepared by the Bridgman method. The copper single crystals were cyclically deformed at a constant plastic strain amplitude γ_<p1>=2x10^<-3> until various numbers of cycles. Surfaces of the fatigued single crystals were observed by the ECCI method. … More During the cyclic deformation tests, we recorded shapes of hysteresis loops and calculated the Bauschinger energy parameter β_E from the hysteresis loop shape. It has been proposed that the β_E value became a local maximum when the PSBs begin to develop. In the single crystal having [123] tensile axis, the β_E value became a local maximum at 1200 cycle. The ECCI observations after 1500 cycles revealed band-like contrasts along a primary slip plane. On the other hand, no characteristic contrast could be detected at the ECCI observation at 500 cycles. When we observed the band-like contrast at a high magnification, the ladder-like dislocation structure where the wall spacing was about 1.4 μm. Being very similar to the PSB observed using the TEM, the observed band-like contrast must be the PSB structure in the vicinity of specimen surface. At the single crystal having [001] tensile axis, grid-like contrast was detected in the ECCI observation. It is likely that this contrast is attributed to the labyrinth dislocation structure which has been reported in TEM observation at the [001] tensile axis.From the abovementioned observations, we can suggest that the prediction of fatigue cracking is possible by detecting the PSB structure using the nondestructive ECCI observation. Less