Elucidation and modeling of nano-scale mechanisms of early fatigue damage based on analysis of crystal orientation

在这项研究中，疲劳裂纹的萌生和扩展行为进行了研究的基础上的纳米尺度观察的结果与晶体取向的滑移变形。宏观上在轧制平面上有100的纤维织构。对于裂纹起始位置的情况，观察到具有110纤维织构的殖民地。在这一地区，有32个[111]方向的滑移系被激活。{321}和{132}的最大Schmid因子为0.412<111><111>。这些滑移可以在试样表面上观察到为与加载轴成70.5度的线。滑移角与实验结果非常吻合。对于超细晶钢，这32个滑移系引起的横向滑移对裂纹的萌生起着重要的作用。其次，从裂纹尖端滑移变形和扩展方向两个方面研究了疲劳裂纹扩展行为的机理。考虑到裂纹尖端的应力奇异性，提出了与光滑试件的Schmid因子相对应的滑移因子。对于超细晶钢，裂纹分叉导致的大的裂纹扩展阻力是非常重要的。研究了分叉的机理。在滑移系数最大值的方向上形成分叉。对于晶粒内部的分叉，当多个滑移系存在于同一滑移线上时，分叉发生在滑移方向位于试件表面的晶粒内部。

In this study, initiation and propagation behaviors of fatigue cracks were investigated on the basis of the results of the nano-scale observations of slip deformation concern with the crystal orientation. There is a fiber texture of 100 on the rolling plane macroscopically. For the case of the location of crack initiation, the colony which has a 110 fiber texture was observed. In this region, 32 slip systems for [111] directions were activated. The largest Schmid factor is 0.412 for {321}<111> and {132}<111>. These slips can be observed as a line with 70.5 degree from the loading axis on the specimen surface. The slip angle agreed very well with the experimental result. For the ultrafine-grained steels, the cross slips due to those 32 slip systems acted as a very important role for crack initiation. Secondly, mechanism of the propagation behavior of fatigue cracks was investigated on the basis of the slip deformation near the crack-tip and the propagation direction. The slip factor, which was corresponding to the Schmid factor for smooth specimens was proposed by considering the stress singularity near the crack-tip. For the ultrafine-grained steels, large resistance of crack propagation due to crack bifurcation is very important. The mechanism of the bifurcation was investigated. The bifurcation was formed in the direction of the maximum value of the slip factor. For the bifurcation within the grain, when the many slip systems existed on the same slip line, the bifurcation was formed in the grain whose slip direction exists on the specimen surface.