Dislocation Structures in 18-8 Austenitic Steel during Fatigue Deformation Process

Abstract

Since transmission electron microscopy began to be applied to observations of the structure of dislocation that takes place during fatigue deformation process, many investigations on this field have been reported. Consequently it has been made clear that the dislocation structure in fatigued metals is greatly affected by the stress or strain amplitude and also by the stacking fault energy in face centered cubic metals.
The authors have investigated the fatigue deformation process in 18-8 austenitic steel having very low stacking fault energy from the viewpoint of the morphology of microscopic slip line and the crystallographic micro-lattice strain, and have been able to clarify the features of fatigue deformation in the materials of low stacking fault energy to a certain extent. Hitherto only a few studies on dislocation structure in austenitic stainless steel have been reported by means of transmission electron microscopy. Therefore for the purpose of clarifying the behavior of dislocation in fatigue stressed 18-8 austenitic steel, electron microscopic observation was made of the specimens cycled variously at both low and high stress levels, and at the same time their dislocation structure in the course of their tension was also observed, and on the results of these observations the characteristic features of fatigue deformation in the materials of low stacking fault energy were examined.
Some differences in dislocation structure during the fatigue process between at low and high stress levels were observed. In the former, dislocation bundles are formed at the very early period, and then those structures gradually develop into band structures composed of dislocation dipoles and elongated dislocation loops, and finally cell structures are built up, while in the latter the highly developed band structures and cell structures are observed, especially near at the grain boundary, in the earlier stage of fatigue process, and with the more stress cycles, these configurations gradually change into clear cell structures consisting mainly of dislocation loops. The cell structure at the time of micro-crack initiation is more distinct than that at low stress level. However, the cell structure in 18-8 austenitic steel is imperfect in contrast to those in iron and aluminum, namely cell walls are broad and the misorientations between cells are very small. Those differences can be explained by the ease of cross slip mechanism.
The dislocation structure during the fatigue process is characterized by the band structures lying along {111} trace, while in the tensile deformation the uniform distribution of dislocations is a distinctive feature. As for the cell structures made in the fatigue and in the tensile deformation, the following differences are observed; in the former case the cell size is larger than in the latter, and the cell walls are mostly composed of the dislocation loops, rather than joggy tangled dislocations, and the cell structure in the former is more distinct than that in the latter.