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

用电子通道对比成像（ECCI）方法观察了铜单晶和铁素体不锈钢在疲劳裂纹附近形成的位错结构。ECCI方法是基于电子通道现象，即背散射电子的强度取决于入射电子束与晶格面的夹角。我们可以用扫描电子显微镜（SEM）观察到表面下的位错。在本研究中，证实了ECCI方法在包括裂纹尖端附近的大范围内成像了由于疲劳载荷而产生的位错结构，如持久滑移带（PSB）和迷宫结构，这是TEM观察困难的。在铜单晶的疲劳实验中，疲劳裂纹周围形成的位错结构对加载轴敏感。在[001]加载轴上，疲劳裂纹周围多观察到阶梯状PSB结构和迷宫式结构。这些位错结构的形成与裂纹面夹角有关。位错结构的分布可以用线性断裂力学计算得到的应力分布来解释。在Fe-30%Cr单晶的裂纹扩展实验中，位错结构与疲劳裂纹的模式有关。在<112>和<221>加载轴处，胞体结构围绕着具有iii型分量的疲劳裂纹。另一方面，在<110>加载轴处扩展的ii型疲劳裂纹附近没有发现明显的位错组织。裂纹尖端只发出与裂纹面平行的带状位错结构。在多晶Fe-20%Cr合金中，研究了裂纹扩展速率与周围位错组织的关系。在单滑移取向晶粒中，裂纹附近的位错组织多为脉状，裂纹扩展速度较快；另一方面，在双滑移取向晶粒中，裂纹附近的晶胞结构发育良好。具有胞状结构的疲劳裂纹以慢裂纹扩展速率扩展。少

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