18-8 オーステナイト鋼の疲労き裂発生過程の微視的観察

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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 micro-beam 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℃ 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 200μ^φ 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.
社団法人日本材料学会の論文
1971-04-15

18-8 オーステナイト鋼の疲労き裂発生過程の微視的観察

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