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

沿持续滑移带（PSBs）频繁地萌生晶内疲劳裂纹，而循环塑性应变则局限于此。PSB的位错结构是由位错壁和位错通道组成的阶梯状结构。因此，通过检测这种阶梯状位错结构，我们可以预测疲劳裂纹的起裂。然而，为了使用传统的透射电镜观察位错结构，材料必须被塑造成薄片。在本研究项目中，我们尝试使用电子通道对比成像（ECCI）方法进行无损位错观察，以期预测疲劳裂纹。采用Bridgman法制备了具有明确应力轴的铜单晶。铜单晶在恒定的塑性应变幅值γ_<p1>=2x10^<-3>下循环变形，直至不同的循环次数。用ECCI法对疲劳单晶表面进行了观察。在循环变形试验中，我们记录了滞回线的形状，并根据滞回线的形状计算了包辛格能量参数β_E。有人提出，β_E值在psb开始发展时成为局部最大值。在具有[123]拉伸轴的单晶中，β_E值在1200循环时达到局部最大值。1500个周期后的ECCI观测显示沿初级滑移面有带状对比。另一方面，在500个周期的ECCI观测中，没有检测到特征对比。在高倍镜下观察到带状对比，发现壁面间距约为1.4 μm的阶梯状位错结构。由于与透射电镜观察到的PSB非常相似，因此观察到的带状对比一定是试样表面附近的PSB结构。在具有[001]拉伸轴的单晶处，在ECCI观察中检测到网格状对比。这种对比很可能是由于在TEM观察中发现的[001]拉伸轴的迷宫位错结构。从上述观察结果来看，我们可以建议使用非破坏性ECCI观察检测PSB结构来预测疲劳裂纹是可能的。少

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