Papers by Keyword: Synchrotron Radiation

Abstract: The composites based on reactive metals (Zr, Ta, Nb, Ti) sheets explosively welded to stainless steel plates were investigated using X-ray synchrotron radiation, TEM and SEM to characterize phase transformations in near-the-interface layers. SEM and TEM investigations of the solidified melt regions unveiled amorphous and nanocrystalline non-equilibrium phases of variable chemical compositions, incorporating elements from the joined components. Phase analysis in layers near the interface carried out using high-resolution synchrotron radiation show predominantly reflections coming from the main elements of parent sheets/plates. Nevertheless, a closer look at the diffraction patterns shows the presence of reflections coming from the phases based on the two-component equilibrium phase diagrams. The measurements performed at the interface, but including only the steel plate, revealed significant amounts of α-Fe, γ-Fe and ε-Fe phases. Their appearance was attributed to the high pressure and fast cooling rates, which promoted a martensitic transformation in steel.

Abstract: To gain better understanding of rheological transitions from suspension flow to granular deformation and shear cracking, this research conducted shear-deformation on globular semi-solid Al-Cu alloys to study the rheological behavior of semi-solid as a function of solid fraction (38% - 85%) and shear rate (10^-4 – 10^-1 s^-1) under real-time synchrotron radiography observation. By analyzing 17 X-ray imaging datasets, we define three rheological transitions: (i) the critical solid fraction from a suspension to a loosely percolating assembly; (ii) from the net contraction of a loose assembly to the net dilation of a densely packed assembly, and (iii) to shear cracking at high solid fraction and shear rate. Inspired by in-situ observations of semi-solid deformation showing a disordered assembly of percolating crystals in partially-cohesive contact with liquid flow, we reproduced a two-phase sample using the coupled lattice Boltzmann method-discrete element method (LBM-DEM) simulation approach for granular micromechanical modeling. In DEM, each globular Al grain is represented by a discrete element, and the flow of interstitial liquid is solved by LBM. The LBM-DEM simulations show quantitative agreement of semi-solid strain localization with the experiments and are used to explore the components involved in the shear rate dependence of the transitions, and the role of liquid pressure on the initiation of shear cracking.

Abstract: We have investigated the surface effect of colloidally prepared CdSe nanocrystals (NCs) with the size range of 23-40 Å on their structural properties by changing the organic capping ligands. The TOPO/HDA-passivated NCs reveal a size-dependent behavior involving an elongated axial bond R⁽¹⁾ of an atomic tetrahedron and a shrunken equatorial bonds R⁽²⁾. After treatment of the NCs with pyridine, the bond length R⁽¹⁾ decreases significantly whereas R⁽²⁾ remains unchanged relative to the TOPO/HDA-passivated NCs, suggesting that a tensile stress along the [001] direction is contributed from the surface modification. In addition, we find that the expansion ratio of the pyridine-treated NCs along the c axis depends strongly on the density of stacking faults, which provides an evidence for the relaxation of atomic positions near the interface of stacking faults.

Abstract: We measured the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in a Mg₉₇Zn₁Y₂ alloy polycrystal. This was achieved by using a Gandolfi camera, which was attached on a high precision diffractometer at SPring-8 BL40XU beamline. By using this system, fine diffraction data were obtained from a Mg₉₇Zn₁Y₂ polycrystal at 6 different temperatures between 90 and 450 K. We succeeded to determine cell parameters of 18R-LPSO phase and α-Mg phase separately in the Mg₉₇Zn₁Y₂ alloy polycrystal. The thermal expansion coefficients were determined from the refined cell parameters. The differences of the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in the Mg₉₇Zn₁Y₂ alloy were much smaller than those of single-phase 18R-LPSO and α-Mg polycrystals.

Abstract: The rapid development of new materials and their application in an extremely wide variety of research and technological fields has lead to the request of increasingly sophisticated characterization methods. In particular residual stress measurements by neutron diffraction, small angle scattering of X-rays and neutrons, as well as 3D imaging techniques with spatial resolution at the micron or even sub-micron scale, like micro-and nano-computerized tomography, have gained a great relevance in recent years.Residual stresses are autobalancing stresses existing in a free body not submitted to any external surface force. Several manufacturing processes, as well as thermal and mechanical treatments, leave residual stresses within the components. Bragg diffraction of X-rays and neutrons can be used to determine residual elastic strains (and then residual stresses by knowing the material elastic constants) in a non-destructive way. Small Angle Scattering of neutrons or X-rays, complementary to Transmission Electron Microscopy, allows the determination of structural features such as volume fraction, specific surface and size distribution of inhomogeneities embedded in a matrix, in a huge variety of materials of industrial interest. X-ray microtomography is similar to conventional Computed Tomography employed in Medicine, allowing 3D imaging of the investigated samples, but with a much higher spatial resolution, down to the sub-micron scale. Some examples of applications of the experimental techniques mentioned above are described and discussed.

Abstract: The dislocation density of plastically deformed oxygen free copper (OFC) was evaluated by X-ray diffraction profile analysis with synchrotron radiation. The modified Williamson-Hall and modified Warren-Averbach methods were applied to the analysis. The dislocation densities of OFC samples with compressive plastic strains of 1 % and 4 % were 5.1×10¹⁴ m^-2 and 9.2×10¹⁴ m^-2, respectively.

Abstract: This work takes place in the general context of a better understanding of materials degradation mechanisms in extreme environments. In particular, the aim of the present study was to correlate microstructural elements to growth stress magnitude evolution and stress release mechanisms for thermally grown chromia thin films on NiCr alloys. Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Data have been acquired from oxides grown for different high temperatures evolutions on NiCr model alloys that form Cr₂O₃. Using synchrotron X-Ray at the ESRF (Beamline BM02) coupled with an induction furnace, Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded in air at temperature between 700-1000°C and during cooling. The distortion of the diffraction rings was analyzed to yield the in-plane strain. Thermal stresses imposed on Cr₂O₃/NiCr by abruptly reducing the sample temperature for a period of time, exploiting the thermal expansion difference between oxide and substrate, showed noticeable subsequent stress relaxation by creep. Such a mechanism was monitored using time-dependent in situ measurements of strain relaxation in the oxide. The main results obtained from these experiments are the kinetic of the growth stress from the isothermal measurements (isothermal plateau), and the study of the stress release mechanism after the low-temperature jumps. In complement, the oxide microstructure development during the course of oxidation is also investigated from both the peaks intensity and width evolution. In all cases, the steady stage growth strain was relatively low and compressive. Different degrees of relaxation were also found on cooling depending on scale microstructure through the variation of the initial oxidation conditions. Results are compared with other reports of residual stresses evolution in Cr₂O₃ scales.

Abstract: In the last decades, very significant advances have been made for what concerns bone and joint substitution and in the repair and regeneration of bone defects. Though some strong requirements are still to be met, biomaterials for these purposes have known an impressive evolution, for what concerns their mechanical behaviour, their bioresorbability and finally their capability to generate new bone tissue in a stable way in long periods. The validation of such materials necessarily depends on a suitable characterization of their properties. In this article a brief review of some works in this field, carried out by the authors’ research group, is presented. It was shown in particular how advanced experimental methods, such as synchrotron radiation µCT and synchrotron radiation diffraction can offer very important information, can be not only complementary methods to more standard techniques (electron microscopy, X-ray diffraction), but can also offer the possibility to measure parameters that cannot be obtained otherwise.

Abstract: Synchrotron X-ray radiography was used to in situ study the diffusion behavior and microstructural evolution of Al/Cu bimetal. The interface diffusion, dendritic/eutectic growth and the formation of intermetallic compounds around the Al/Cu bimetal interface were analyzed. During the isothermal diffusion process, a liquefied transition zone at the interface with a concentration gradient was formed when the Cu concentration exceeded eutectic composition of Al-Cu alloy. During the solidification of transition zone, the growth sequence of α-Al dendrites and eutectic structure were mainly dominated by the variation of Cu concentration and thermal field according to the temperature of the liquidus line of the equilibrium phase diagram. Finally, the transition zone around the interface were identified to be I (α-Al), II (Al+Al₂Cu), III (Al₂Cu) and IV (Al₂Cu, AlCu and Al₄Cu₉), respectively.

Abstract: Sapphire, an inorganic gem-material in a variety of corundum, mainly consists of alpha-alumina (α-Al₂O₃) structure. The geological origins of sapphire are related to either basaltic or metamorphic rocks. The causes of the color on sapphire are some trace elements such as Cr, Fe, and Ti. It could be mentioned that Ti atoms have cooperated with Fe atoms for creating the blue color. In this study, X-ray absorption spectroscopy (XAS) technique focused on the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS) is employed to identify the oxidation state of Ti atoms and Ti-O bond length on sapphire samples. The Ti K-edge XANES and EXAFS spectra of natural sapphires were carried out using the 13-channel array germanium detector in fluorescence mode. The XANES spectra showed that the oxidation state of Ti was Ti⁴⁺ regardless of Fe content. Moreover, the Ti-O bond length on a-Al₂O₃ was equal to the Ti-O bond length on rutile (TiO₂) analyzed from the EXAFS spectra. From these results, it could be concluded that the oxidation state of Ti atoms on natural sapphires was Ti⁴⁺ which substitutes Al³⁺ on the sapphire structure.