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

用电子沟道衬度成像（ECCI）方法观察了铜单晶和铁素体不锈钢疲劳裂纹附近的位错结构。ECCI方法基于电子沟道现象，即背散射电子的强度取决于入射电子束与晶格平面之间的角度。在扫描电子显微镜（SEM）中，我们可以观察到位于表面下的位错.在本研究中，确认了诸如持久滑移带（PSB）和迷宫结构之类的位错结构-由于疲劳载荷而产生-在包括裂纹尖端附近的广泛区域上用ECCI方法成像，其中TEM观察是困难的。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， ...更多信息 在疲劳裂纹周围观察到裂纹扩展。这些位错结构的形成取决于与裂纹面的夹角。位错结构的分布可以用线性断裂力学计算的应力分布来解释。在Fe-30%Cr单晶的疲劳裂纹扩展实验中，位错结构取决于疲劳裂纹的模式。疲劳裂纹周围的胞状组织具有III型成分，出现在<112>和<221>加载轴上。另一方面，没有发现明显的位错结构附近的模式-II疲劳裂纹扩展的试样与<110>加载轴。裂纹尖端只发射出平行于裂纹面的带状位错结构。研究了多晶Fe-20%Cr合金中裂纹扩展速率与周围位错结构的关系。在单一滑移取向的晶粒中，裂纹附近的位错结构多为脉状，裂纹扩展速率相对较快。而双滑移取向的晶粒在裂纹附近胞状结构较发达。疲劳裂纹以缓慢的裂纹扩展速率扩展，具有胞状结构。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