Klaus-Dieter Liss

HIGH-ENERGY X-RAYS: A TOOL FOR ADVANCED BULK INVESTIGATIONS IN MATERIALS SCIENCE AND PHYSICS

The combination of these techniques is a strong issue for the construction and development of future instruments.

Evaluation of Residual Stresses in the Bulk of Materials by High Energy Synchrotron Diffraction

High energy synchrotron diffraction is introduced as a new method for residual stress analysis in the bulk of materials. It is shown that energy dispersive measurements are sufficiently precise so that strains as small 10−4 can be determined reliably. Due to the high intensity and the high parallelism of the high energy synchrotron radiation the sample gauge volume can be reduced to approximately 50 μm×1 mm×1 mm compared to gauge volume of one mm3 up to several mm3 achievable by neutron diffraction. The benefits of the high penetration depth and the small gauge volume are demonstrated by the results of stress studies performed on a fiber reinforced ceramic, a functional gradient material and a metal-ceramic compound. Furthermore, it is shown that in case of a cold extruded metal specimen the energy dispersive measurement technique yields simultaneous information about texture and residual stresses and thus allows a detailed investigation of elastic and plastic deformation gradients.

Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the ex...

On High-Resolution Reciprocal-Space Mapping with a Triple- Crystal Diffractometer for High-Energy X-rays

High-energy X-rav diffraction by means of triple-crystal techniques is a powerful tool for investigating dislocations and strain in bulk materials. Radiation with an energy typically higher than 80 keV combines the advantage of low attenuation with high resolution at large momentum transfers. The triple-crystal diffractometer at the High Energy Beamline of the European Synchrotron Radiation Facility is described. It is shown how the transverse and longitudinal resolution depend on the choice of the crystal reflection, and how the orientation of a reciprocal-lattice distortion in an investigated sample towards the resolution element of the instrument can play an important role. This effect is demonstrated on a single crystal of silicon where a layer of macro pores reveals satellites around the Bragg reflection. The resulting longitudinal distortion can be investigated using the high transverse resolution of the instrument when choosing an appropriate reflection.

Storage of X-ray photons in a crystal resonator

The temporal structure and high brilliance of the X-ray beams produced by third-generation synchrotrons open up new possibilities in time-dependent diffraction and spectroscopy, where timescales down to the sub-nanosecond regime can now be accessed. These beam properties are such that one can envisage the development of the X-ray equivalent of optical components, such as photon delay lines and resonators, that have proved indispensable in a wide range of experiments—for example, pump-probe and multiple-interaction experiments—and (through shaping the temporal structure and repetition rate of the beams) time-dependent measurements in crystallography, physics, biology and chemistry. Optical resonators, such as those used in lasers, are available at wavelengths from the visible to soft X-rays. Equivalent components for hard X-rays have been discussed for more than thirty years, but have yet to be realized. Here we report the storage of hard X-ray photons (energy 15.817 keV) in a crystal resonator formed by two plates of crystalline silicon. The photons are stored for as many as 14 back-and-forth cycles within the resonator, each cycle separated by one nanosecond.

The Non-Resonant Magnetic X-ray Scattering Cross Section of MnF2. 2. High-Energy X-ray Diffraction at 80keV

Results of high-energy non-resonant magnetic X-ray diffraction experiments performed on the model system MnF 2 at a photon energy of 80keV are presented. A surprisingly high peak intensity of the magnetic 300 reflection of 13 000 photonss -1 in the three-crystal mode and 19 000 photons s -1 in the two-crystal mode, with a peak-to-background ratio of 230 :1 and 10 :1, respectively, has been achieved. At 80keV, the penetration depth is 7mm. When the path length of the beam through the crystal is varied, the effect of volume enhancement of the intensity diffracted by magnetic reflections is demonstrated. The Q dependence of the magnetic and the charge Bragg reflections has been measured and agrees well with theory. The measurement of the temperature dependence of the sublattice magnetization allows a very accurate determination of the critical exponent β = 0.333 (3) and the Neel temperature T N = 67.713 (2) K. Finally, the multiple charge scattering is discussed, which is very pronounced for the magnetic reflections of MnF 2 .

Internal stress measurements by high-energy synchrotron X-ray diffraction at increased specimen-detector distance

High-energy X-ray diffraction has recently been shown to be a viable technique to measure volume-averaged lattice strains in the bulk of metallic polycrystals at increased speed compared to neutron diffraction. The established procedure is to irradiate the sample under investigation with monochromatic X-rays (∼100 keV) and to record complete diffraction rings with an area detector. The lattice strains are obtained by analyzing the minute distortions of these rings. In the present paper we present first results obtained using a setup in which two area detectors are positioned at a large distance (7 m) from the specimen. Although only segments of the rings can be recorded this way, this approach offers a number of advantages. In situ tensile tests were performed on a γ-TiAl-based alloy as an example to demonstrate the potential of the method. Both materials science aspects as well as consequences for further method development will be discussed.

Phase transition and ordering behavior of ternary Ti–Al–Mo alloys using in-situ neutron diffraction

Abstract Neutron diffraction has been used for in-situ investigations to elucidate the phase transformation behavior of two Mo-containing TiAl alloys with compositions of Ti-44Al-3Mo and Ti-44Al-7Mo (in at.%). Five different phases are present in these alloys. These include three ordered phases at room temperature, namely α2, β0 and γ and two disordered phases, and, which occur at higher temperatures. The sequence of the three phase transformations in each alloy has been determined. The phase transformation and disordering/ordering temperatures were determined on heating and cooling from the diffracted peak intensities. The neutron experiments are particularly sensitive to the order–disorder transitions in TiAl alloys, which are compared with the overall phase fractions obtained from previous high energy X-ray diffraction. Hysteresis and undercooling effects are observed for the various phase transformations and depend on the nature of atomic rearrangements.

Ultrasound-Induced Gradient Crystals Observed by High-Energy X-rays

High-energy X-ray scattering (90 keV) has been performed at the high-energy beamline ID15 at the European Synchrotron Radiation Facility, Grenoble, on perfect Si crystals excited by compressional ultrasound waves in the [111] direction. Two-dimensional intensity maps of the {\bar3}51 Bragg peak reveal a purely longitudinal distortion of the lattice with respect to the applied distortion field, which is, for the present scattering geometry, inclined by 72° to the diffraction vector. A maximum gain factor of 50 for the diffracted intensity is observed for the strongest sound excitation. A quantitative analysis of the line shape by a transfer-matrix method shows the transition from the diffraction behavior of an ideal crystal towards a crystal slightly deformed by internal stresses.