Materials Science Forum Vols. 638-642

Abstract: GKSS is currently investing heavily into new beamlines at DESY in Hamburg, Germany. After the completed installation of the wiggler beamline HARWI II at DORIS III GKSS is now building two new undulator beamlines at the new PETRA III storage ring. The High Energy Materials Science Beamline (HEMS) will allow high resolution diffraction experiments using samples and sample environments with masses up to 1 t, 3DXRD measurements, and high-energy micro-tomography experiments. The Imaging Beamline (IBL) will provide a nano-tomography as well as a micro-tomography station for X-ray energies up to 50 keV. Examples of typical experiments in the field of residual stress analysis, micro-tomography, and high-energy small-angle X-ray scattering will be given.

Abstract: A transmission imaging and a strain mapping in the vicinity of a crack tip in steel were investigated using a high energy white X-ray obtained from BL28B2 beam line at SPring-8 in Japan. Low-alloy and high-tensile steel was used as a specimen prepared in the G-type geometry with a rectangular sectional part of 5mm thickness for a four-point bending. A fatigue crack was introduced into the notch root on the tension side of the specimen by a pulsating bending load. The imaging of the crack in the specimen under the bending load was carried out by using the CCD camera that can detect indirectly the X-ray transmitted through the specimen. To measure the internal strain in the vicinity of the crack tip, the synchrotron white X-ray beam, which had a height of 80m and a width of 300m, was incident on the specimen with the Bragg angle  of 5 degrees using the energy dispersive X-ray diffraction technique. As the results, the transmitted image of the crack showed that the crack in the specimen was propagated deeper than that on the surface. The map of the internal strain near the crack tip could be obtained using the white X-ray with energy ranging from 50keV to 150keV. It became clear by the numerical simulation that the FWHM of diffracted X-ray profile measured near the crack tip was increased due to the steep change in the strain distribution. It was confirmed that the synchrotron white X-ray was useful for the imaging of the internal crack and the strain mapping near it.

Abstract: In this paper, the internal friction behaviors of unidirectionally solidified Al2O3/YAG eutectic that are examined by the authors in passed several years are reviewed. The internal friction of this eutectic increased with increasing temperatures above 1200oC. The apparent activation energy of the eutectic sample in the range from 1100 to 1400oC was same to that of Al2O3 single crystal. Above 1200oC, the internal friction of this material drastically increased with increasing temperatures. The magnitude of the internal friction strongly depended on the elevated temperatures and the cycle numbers of the torsional loading. For the 1400oC test, the internal friction gradually increased with the loading cycles and then saturated by 103 times of the loading cycles.

Abstract: Up to now the tendency in residual stress determination (using neutron diffraction) has been to assess the uncertainties in terms of the propagation of ‘fitting uncertainty’ of the Bragg peaks only. There are many other sources of uncertainty, some more obvious than others, that should be taken into account or at least be considered in terms of their impact when interpreting the data. These cover not just the instrument calibration and characteristics and the technique itself but also the properties of the sample. A discussion of how best to combine the uncertainty of all contributing factors will be made. These factors (on the sample side) will include variations in chemical composition, grain size related problems, surfaces/interfaces cutting through the sampling volume, texture variations within the sampling volume, presence of intergranular strains (plastic anisotropy) etc. The knowledge of appropriate elastic constants, for example, and their uncertainty is necessary for a more reliable stress determination. One should also be aware of the more subtle influences on the elastic constants such as texture or chemical variation. This should be a step in the right direction for a ‘unified uncertainty analysis’ covering all possible aspects of uncertainty in residual stress determination using neutron diffraction.

Abstract: The outstanding properties of synchrotron radiation (SR) allow manifold investigations of materials and processes which are not possible with conventional X-ray sources. Its high brightness allows extremely precise or time/spatial resolved measurements. In combination with its high collimation extremely high angular resolution is achievable. The white spectrum of SR allows tuning of the wavelength for spectroscopic applications or optimization according to the requirements of the experiment.

Abstract: Cracks induced by metal fatigue and structural aging effect can’t be fixed by traditional Gas Tungsten Arc Welding (GTAW), consequently cause the increasing of defect ratio. Although some cracks might be acceptable and qualified to field service standard subject to the military regulation, however, in certain areas, e.g. brazing and coating areas, the results are not ideal especially the HAZ and residual stress. In this study, Nd-YAG Laser welding and traditional GTAW processes were performed on Hastelloy X superalloy for comparison. Post-welding residual stress distribution was measured by X-ray diffraction method. Macro- and microstructure were observed by metallurgical OM and SEM in comparison to the hardness testing. Tensile test results show that traditional welding technology has better ultimate tensile strength and ductility. For the Nd:YAG laser welding, residual stress is limited to 3mm of the both sides of weld and drops drastically, while higher amplitude and widely spread in the GTAW welding. It is proposed that combining both methods, the repairing process can be optimized to reduce the defect ratio and save repairing time.

Abstract: This study highlites deformation behavior of austenitic and pearlitic steels by in-situ neutron diffraction and 3D/4D EBSD measurement with a particular attention to their hierarchy.In particular stress partitioning in these microstructures is examined from macroscopic as well as microscopic scale length levels, and they are correlated to each other.

Abstract: Sintering especially of loose particle packings is accompanied by dimensional changes of the specimen. The growth of inter particle contacts under the influence of the Laplace pressure is well understood and described by the two particle model. On the other hand the understanding of other fundamental sintering phenomena i.e. cooperative material transport processes is rather vague. To overcome this limitation for near net-shape production of components by powder metallurgy an improved model of particle rearrangement processes is required. First efforts to obtain necessary experimental data were performed at 1D and 2D models. But recent improvements of high resolution synchrotron computer tomography (SCT) setups allow the acquisition of in-situ data of particle rearrangements. In-situ studies of particle rotations during sintering were conducted at the ESRF in Grenoble. The rotations during free sintering of monocrystalline particles were investigated during continuous heating up to 1050 °C or with frequent interruptions of the heating by 1 hr dwell times every 100 °C. In contrast to 1D specimens measured by Wieters the 3D specimens showed negligible rotations. This must be attributed to the constraints in 3D samples. To obtain a more detailed insight in the rotations the particles of one sample were marked. It is possible to show that the particles perform intrinsic rotations. Therefore a new rotation model is developed. The intrinsic rotations are confirmed by complementary EBSD analyses as well.

Abstract: Electron tomography has developed into a powerful technique to image the 3D structure of complex materials with nanometer resolution. Both, TEM and HAADF-STEM tomography exhibit tremendous possibilities to visualize nanostructured materials for a wide range of applications. Electron tomography is not only a qualitative tool to visualize nano¬structures, but recently electron tomographic results are also exploited to obtain quantitative measurements in 3D. We evaluated the reconstruction and segmentation process for a heterogeneous catalyst and, in particular, tried to assess the reliability and accuracy of the quantification process. Furthermore, a quantitative analysis of electron tomographic results was compared to macroscopic measurements.

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.