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

与传统的透射电镜相比，电子通道对比成像（ECCI）方法具有观察位错结构的优点：可以在相当大的范围内对自由表面下的位错进行无损观察。利用ECCI方法的这一特性，在不锈钢和铜的疲劳条形试样中，各晶粒均形成位错组织，得到了TEM观察到的脉状组织和阶梯状PSB组织。结果表明，粒径较大的颗粒对PSB的形成更有利，而施密德因子似乎与PSB的形成无关。人们还认识到退火孪晶对PSB的形成起着重要作用：沿退火孪晶边界和跨退火孪晶边界产生的PSB数量比统计预测的要大。利用ECCI技术还观察到了多晶铜试样疲劳裂纹附近的位错结构。在进行ECCI观察之前，进行了光学显微镜观察，将疲劳裂纹形态分为几种类型。在光学显微镜下观察到的滑移形貌与位错结构相关。细胞结构几乎与严重变形的塑性区相对应，其中单个滑移带无法识别。在双滑移区发现了迷宫位错结构。在I级疲劳裂纹前检测到阶梯状位错组织。因此，可以说持续滑移带(PSB)是一个有利的裂纹路径。然而，裂纹扩展的微观路径不是沿着PSB扩展，而是沿着裂纹尖端附近局部发育的细胞结构扩展。

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.