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Recently, three-dimensional grain mapping techniques for polycrystalline materials called X-ray diffraction contrast tomography (DCT) have received considerable attention. DCT can simultaneously provide information on the three-dimensional grain arrangement of a sample, such as the shapes, locations, and crystallographic orientations of grains, together with the misorientation of grains. In the present study, the twinning and the detwinning in tension and compression loading in a magnesium alloy was observed by the DCT using ultra-bright synchrotron radiation X-ray, and the electron backscatter diffraction (EBSD) analysis. The shape of each grain in a polycrystalline magnesium alloy was successfully reconstructed and misorientation of each crystallographic plane was successfully measured by DCT. With increasing applied compression stress, the average value of the misorientation was decreased, while it is usually increased with plastic strain for materials without twinning. The value of misorientation increased with unloading process, indicating that the twinning was occurred under the compressive applied stress. The shape of diffraction spots of some grains also shows the twinning under the compressive stress and the detwining under the following tensile stress. The EBSD analysis of specimen surface also shows that the twinning was occurred under the compressive applied stress, and the following applied tension stress brought detwinning.

To clarify the effects of artificial defects and sulphide inclusions on rolling contact fatigue (RCF) in high strength steel, crack initiation and propagation behaviours from defects were evaluated by using synchrotron radiation computed laminography. Artificial defects and sulphide inclusions lead to RCF damage “flaking” through the same damage process, but considerably different crack initiation lives. Finite element analyses (FEA) for RCF simulated the different stress states between two kinds of defect. The FEA results suggest the reason for the different crack initiation lives in the RCF test.

A three-dimensional grain mapping technique for polycrystalline materials, called X-ray diffraction contrast tomography (DCT), was developed at SPring-8, which is the brightest synchrotron radiation facility in Japan. The developed technique was applied to an austenitic stainless steel. The shape and location of grains could be determined by DCT using the apparatus in a beam line of SPring-8. To evaluate the dislocation structure in fatigue, the total misorientation of individual grains was measured by DCT. The average value of the total misorientation over one sample was increased with the number of cycles. In a grain, the change of the total misorientation was largest for the primary slip plane. The maximum change of the total misorientation in fatigue was larger for planes with larger Schmid factor, and the first fatigue crack initiation was occurred in a grain, which had the greatest change of the total misorientation.

In the very high cycle regime, the duplex S-N property consisting of S-N curve for surface-initiated fracture and that for ineterior-initiated fracture was often reported for high strength steels. However, recent studies show us the fact that the surface-initiated fracture can be sometimes observed even at low stress levels in the very high cycle regime. In the case of interior fracture mode, a non-metallic inclusion is usually found at the center of the fish-eye. If we suppose that such inclusions are distributed at random inside the material space, a certain number of inclusions would be located within the thin surface layer of the specimen. In such cases, the fatigue crack take place within the surface layer giving the surface-initiated fracture even in the very high cycle regime. In the present work, the authors have attempted to construct a probabilistic model on the overall feature of these crack initiation modes in the very high cycle regime under the loading type of rotating bending. Thus, based on the distribution property of the inclusions inside the material, the apparing probability of the surface-initiated fracture and the distribution characteristics of the fatigue strengh at giga-cycles were well explained by the present model.

Rolling contact fatigue (RCF) property is a crucial one for machine elements subjected to rolling contact such as bearings. In particular, properties under pure RCF, that is, without macroscopic slip and under oil lubrication, are known to be influenced by non-metallic inclusions. The objective of the present paper is to clarify the RCF crack initiation and propagation mechanisms in high strength steels using synchrotron radiation laminography. The present authors succeeded in detecting small cracks below a flaking damage and around an artificial defect in thin wires with diameter of 0.6 mm by micro CT imaging using synchrotron radiation. This size limitation of sample is caused by the attenuation of X-ray in material. The crack initiation mechanism in the thin wire, however, was found to be different from that in bulk sample, then plates with width of 10 mm and thickness of 1.0 mm was employed for the present study. To conduct 3D imaging of plate sample, newly developed technique, called as laminography, was successfully applied, and the effect of size and orientation of MnS inclusions are discussed. The material of the sample and ball were high carbon-chromium bearing steel, modified JIS SUJ2. The material has intentionally contains a high concentration of sulfur to enable the observation of crack initiation from MnS inclusions. To conduct rolling contact fatigue test for thin plate nearby the experimental hatch of a beam-line of the synchrotron radiation facility, a special test machine was developed. Fatigue tests are interrupted to conduct laminography. The imaging was carried out at BL46XU beam line of SPring-8, which is the brightest synchrotron radiation facility in Japan. Slice images were reconstructed from the series of the projection images by a filtered-back projection algorithm. The laminography angle between beam line and normal of sample plate was 30deg. The fatigue life for flaking depended on the length, width, and orientation of inclusions. In either case, vertical cracks, whose faces were perpendicular to the rolling direction, were first appeared, then horizontal cracks, those faces were parallel to the sample surface were formed after the vertical cracks reached critical length. The initiation life of the vertical cracks and horizontal cracks depended on the length, width, and orientation of the MnS inclusions. The depth of the horizontal crack initiation, however, was almost independent of these parameters. These mechanism of flaking process, which was directly observed by laminography using synchrotron radiation, is completely different from that previously supposed from the surface observations by conventional microscopies.