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

点击翻译按钮获取中文摘要

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.