Deformation structure and subsurface fatigue crack generation in austenitic steels at low temperature
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I.
INTRODUCTION
OUR research group has investigated the cryogenic high-cycle fatigue properties of some alloys, like titanium alloys and austenitic steels, which are the candidate materials for cryogenic structural use.[1,2] In general, fatigue crack initiation is understood to occur on the specimen surface owing to the irreversible process of extrusion and intrusion through slip deformation.[3] Most of the tested materials, however, clearly exhibited two kinds of fatigue crack initiation.[4] One was at the specimen surface, and the other was in the specimen interior. The subsurface crack initiation was dominant in the long-life range and at lower temperatures, while the surface crack initiation occurred in high-peak stress tests and in short-life tests. It is revealed that the initiation site shifts from the surface to the interior at lower stress and that the subsurface crack initiation behavior can be clearly detected at cryogenic temperatures. Generally, the tensile strength of alloys is increased as the temperature is decreased. It appears that low-temperature fatigue tests make clear the features of fatigue crack generation behavior in high-strength alloys such as the Ti-6Al4V alloy.[5] Nitrogen-strengthened 24Cr-15Ni[6] and 32Mn-7Cr[7] austenitic steels were designed to have high yield strength, improved toughness, and a high stability of austenite (fcc) phase at 4 K. In particular, 24Cr-15Ni steel has a good balance of strength and toughness at 4 K. Few studies have been done for the low-cycle fatigue behavior of those steels. The 32Mn-7Cr steel containing carbon and nitrogen showed low-cycle fatigue softening, which corresponded to the planar dislocation structure.[8] The 24Cr-15Ni steel also exhibited low-cycle fatigue softening at 4 K.[9] However, there have been almost no studies performed on the sub-
surface fatigue crack generation due to intergranular cracking in austenitic steel at low temperatures. At low-temperature, high-cycle fatigue, where the lattice dislocations blocked by a grain boundary are unable to climb, most of the dislocations will remain in place at the boundaries. The planar dislocation structures have been observed in austenitic steels and titanium alloys.[4] In the investigation of the subsurface crack initiation for a Ti-5Al-2.5Sn ELI (extra low interstitials) alloy, the present authors pointed out not only that the localized deformation and/or strain concentration by dislocation pileups in the vicinity of grain boundaries were a potential source of microcracking,[10] but also that a specific region with microstructural inhomogeneity provides a potential site against strain incompatibility and microcrack generation.[11] Hence, it is important to make clear the correspondence between the fatigue crack initiation site and microstructure from the viewpoint of heterogeneity in both microstructure and deformation structure. To evaluate fatigue crack generation, furthermore, it is necessary to consider the size of the subsurface crack initiation site. The size mostly reflects the st
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