Hideaki Kawabe

Microscopic Observations of Fatigue Crack Initiation Process in 18-8 Austenitic Steel

It is suggested that fatigue failure mechanisms in the face-centered cubic metals are greatly affected by stacking fault energy. It was from this viewpoint that the fatigue crack initiation and stage I type crack propagation in 18-8 austenitic steel which had the lowest stacking fault energy among the face-centered cubic metals on the market, were discussed by the X-ray microbeam diffraction technique and the replica electron microscopic method.The fatigue slip line at room temperature tests has fine and straight configurations which are characteristic of low stacking fault energy materials, while the wavy and coarse slip line is typical of fatigue tests at 200°C where screw dislocation is more easily prone to cross slip. In both of these cases very complicated deformation configurations are caused by double slip and cross slip near the grain boundaries, and frequent extrusion and intrusion are observed in these areas. Fatigue crack nucleation might be caused by stress concentration at the notch and peak topography. The stage I type fatigue crack propagates immediately after the initiation, mainly along and frequently across the intense slip lines. Its growth in the area neighbouring the grain boundary runs in the direction that depends on both the crystal orientations of the grain containing the crack and the grains adjacent to it.Total misorientation and micro lattice strain measured by X-ray micro-beam diffraction technique increase abruptly at early fatigue stressing, then equilibrium values are attained and finally increase rapidly. These three periods during the fatigue deformation might be corresponding to those of the fine slip line initiation, the intensification of each slip line without new slip line formation and of final initiation of micro crack, respectively. Furthermore it will be noticed that the equilibrium value of total misorientation of 18-8 austenitic steel during the fatigue process is about one-tenth of that of carbon steel.The area of substructural zone near the fatigue crack of 18-8 austenitic steel is smaller than that of aluminum of high stacking fault energy and body-centered cubic iron. Therefore, the formations of subgrain and martensite could not be detected by X-ray micro-beam diffraction technique when irradiated by Cr-Kα of 200μφ beam size. In order to clarify these points the detailed observation at the fatigue crack tip must be made by using transmission electron microscopy. These results will be reported elsewhere.

Electron Microscopic Studies of Pure Aluminum Fatigued at Elevated Temperatures

Fatigue deformation studies have been widely undertaken by many investigators, still the fatigue mechanism are not clarified yet. In the present investigation the fatigue deformation and also the fatigue mechanism of annealed polycrystals of pure aluminum (99.99%) have been studied at room temperature and at elevated temperatures of 80°C and 130°C. The deformation behavior of specimens during fatigue was examined by both the replica and the transmission electron microscopy techniques. At the same time the ease of cross slipping at elevated temperatures was taken into consideration.The results obtained were as follows:(1) At elevated temperature fatigue, the surface of the specimen is deformed more than at room temperature and the cracks propagate in such a way as to link many micro-cracks which are made of the pores.(2) Large numbers of dislocation loops were found in the specimen at elevated temperature fatigue. These loops play an important part in forming the cell and they are also closely related to the initiation and the propagation of a crack.(3) The cell walls are formed by dislocations which are clustered along low index planes, e. g. {100} and {110}. The cell size was larger at elevated temperature (5 to 9μ) than at room temperature (2 to 4μ).

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

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.

The Surface Structural Changes of Surface-Rolled Low Carbon Steels during Fatigue

It has been understood that the fatigue strength of machined materials increases by the hardening work in surface layers. In these hardened surface layers, microstructures, such as dislocation structure, might be variable with the extent of working. And there might be variation in surface structure during the fatigue.Therefore, in order to clarify the surface structural changes during the fatigue of the annealed low carbon steels and the surface-rolled ones under three different working conditions, in this paper, the observations on the notched part of the rotating bending fatigue specimen are performed by light microscope and electron microscope with replicas, and the properties of slip line and fatigue crack initiation have been made clear.The summarized results are as follows:(1) The morphology of slip lines are variable from the annealed specimens to the surface-rolled specimens. In the former the fine slip lines are formed near the grain boundaries, and in the latter the slip bands are formed locally in the ferrite grains. These slip bands increase in length and width as the number of cycles increases.(2) The higher the rolling pressure rises, the more preferential slip bands are formed in the ferrite grains. The microcracks in the highly rolled specimens tend to initiate within the slip bands before these slip bands interfere each other.(3) Under the same cyclic stress condition, the time of emergence of the slip lines is earlier in the surface-rolled specimens, but that of microcracks is slower than that in the annealed specimens.(4) The direction of the slip lines in the surface-rolled specimens is perpendicular to the maximum tensile stress axis.(5) The slip ratio during fatigue increase more slowly in the surface-rolled specimens than in the annealed specimens.