Papers by Author: Hiroyuki Toda

Abstract: X-ray microtomography (XMT) has been utilized for the in-situ observation of various structural materials under external disturbance such as loading. In-situ XMT provides a unique possibility to access the three-dimensional (3D) character of internal microstructure and its time evolution behaviours non-destructively, thereby enabling advanced techniques for measuring local strain distribution. Local strain mapping is readily enabled by processing such high-resolution tomographic images either by the particle tracking technique or the digital image correlation technique. Procedures for tracking microstructural features which have been developed by the authors, have been applied to analyse localised deformation and damage evolution in a material. Typically several tens of thousands of microstructural features, such as particles and pores, are tracked in a tomographic specimen (0.2 - 0.3 mm³ in volume). When a sufficient number of microstructural features is dispersed in 3D space, the Delaunay tessellation algorithm is used to obtain local strain distribution. With these techniques, 3D strain fields can be measured with reasonable accuracy. Even local crack driving forces, such as local variations in the stress intensity factor, crack tip opening displacement and J integral along a crack front line, can be measured from discrete crack tip displacement fields.

Abstract: Synchrotron X-ray microtomography (SPring-8, Japan) has been used for the microstructure characterization in a closed cell Al-Zn-Mg alloy foam. Some sophisticated microstructure features, such as micropores and intermetallic particles inside the cell wall, were visualized and quantified three dimensionally(3D) by the high-resolution phase contrast imaging technique. By microtomographies tuned to energies above and below the Zn K-absorption edge, the 3D quantitation of Zn distribution was obtained using subtraction imaging technique. It has been clarified that the Zn distribution was inhomogeneous in the cell wall of the foam. And the agglomeration of Zn-bearing particles was confirmed to induce the brittle fracture of cell wall. The distributions of Ti and Ca in the foam were also visualized by subtraction method. The current tomographic techniques provide novel solutions for the 3D microstructure analysis in the highly inhomogeneous foam materials.

Abstract: Recently, three-dimensional (3D) observation and analysis have attracted considerable attention in materials science field. By using the synchrotron radiation, the tomography makes possible high-resolution 3D observation dynamically and the recent diffraction analysis is available for 3D orientation mapping. In this study, grain deformation behavior in polycrystalline aluminum alloy has been characterized by 3D observation method applying the synchrotron radiation. The method to measure inner strain distribution by means of microstructural features tracking provides strain distribution within the sample, which we could not access before. The effect of grain orientation and its interaction during tensile deformation was discussed with the obtained strain distribution.

Abstract: The new method to investigate and visualize a strain distribution at individual grains in three-dimensional has been developed based on synchrotron radiation X-ray tomography. The tensile specimen made of a heat-treated 2024 aluminum alloy to coarse precipitate particles has repeatedly scanned by the high-resolution X-ray tomography with applying a loading just before fracture would occur. Grain boundaries in the specimen were visualized by the tomographic scan after gallium application in order to detect grain regions in three-dimensional. Total 17 grains were found by 2D and 3D image processing and were provided with strain analysis by means of the microstructural features tracking. The actual strain development on a grain was visually demonstrated during tensile deformation. The strain map on a cross section of the grain shows inhomogeneous distribution. In the vicinity of grain boundary, the partly large strain is observed. The strain distribution within a grain seems to be affected by neighbor grains. The different evolution of strain distribution was also confirmed in the two grains while the grains were deformed almost equivalently on an average. The difference of deformation microstructure depending on grain orientation is suggested.

Abstract: Synchrotron X-ray microtomography was used to investigate the three-dimensional fatigue crack propagation behavior of an aluminum alloy. Local crack growth rates along specimen thickness were calculated. Crack observation revealed that crack propagation was retarded in three regions where there exist overlapping crack segments. Crack growth retardation was found to occur in these crack overlapping regions due to stress shielding. Observation of crack propagation process indicated that one of the overlapping crack segments initiated from the twisted crack. Crack tip opening displacement was measured and it was found that crack opening was larger in single planar crack region than that in crack overlapping region.

Abstract: 3D fatigue crack opening and propagation behaviors in an Al-Mg-Si alloy have been investigated in this study. A combined methodology of an in-situ 3D crack visualization via the synchrotron X-ray CT and image-based numerical simulation which takes into account real crack morphology has been applied for the study. The details of a crack with non-planar torturous crack morphology have been observed and many essential features of the crack and its opening/propagation inside the material have been identified. Typical example would be the complex deformation behaviors in the region where two crack segments lying on different horizontal planes overlap each other, and the influence of complicated crack front line on local crack driving forces. We recognized that two overlapping crack segments affected significantly the local crack driving force ΔkI each other, and large mode III displacement occurred near the edge of overlapping region. The region next to overlapping was influenced to a relatively large extent through shielding the near-tip stress/strain fields thereby reducing the local crack driving forces. We also found that the sharp extrusion on crack front line had no significant influence on neighboring cracks. The combined methodology of the in-situ 3D crack visualization and the image-based numerical simulation has been demonstrated to be powerful and feasible in revealing and understanding various fracture phenomena inside materials

Abstract: Traditional computational models always assume idealized crack geometry. However, actual crack geometry is very complex in real materials and thus, those simulations do not realistically represent the actual loading conditions of a real crack. In this paper, three-dimensional (3D) image-based simulation was performed to investigate the fracture behavior of an aluminum alloy, and the model takes into account the real crack geometry based on the 3D images of the crack. Accordingly, many essential features of fracture can be identified and interpreted, and some new insight into fracture behavior in real materials can be offered.

Abstract: Three-dimensional zinc mapping based on X-ray K-edge scanning has been performed. By microtomographies with energies above and below the K-absorption edges of the elements, the concentration distribution of the elements is evaluated during in-situ experiments, respectively. It is found that the Zn concentration distribution during the heat treatment was changed inside the cell wall of the aluminum foams and it has been homogenized. Also several precipitated phase transformation can be three-dimensionally visualized by the CT-method tuning X-ray energies.

Abstract: The surface of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical titanium alloy, Ti-6Al-4V ELI (Ti64). After gas nitriding, the microstructures near the surface of these alloys were observed by optical microscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. In both alloys, two titanium nitrides (TiN and Ti2N) are formed and the α phase precipitated by gas nitriding. Furthermore, oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO2 is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.

Abstract: The local elastic and plastic strain during deformation are very complicated and different form the macroscopic strain, because most materials have inhomogeneous microstructure. In this study, local strain distribution in three dimensions has been measured using the new developed method based on image analysis in high-resolution synchrotron radiation computed tomography (SR-CT). Model and practical specimens, which made of cupper alloy and aluminum alloy, respectively, were prepared for a development procedure and application of local strain measurements. Gauging intervals of microstructural features before and after deformation gave us information of inhomogeneous local strain distribution in three dimensions. High strain was observed in a necking region appeared after tensile deformation in the model sample. A combination of non-destructive measurements by SR-CT and three-dimensional analysis revealed inhomogeneous strain distributions in practical aluminum samples.