No abstract

High-energy X-rays between 30 keV and 1 MeV, such as provided by modern synchrotron radiation sources as the ESRF and HASYLAB, bear the advantage of high penetration into most materials. Even heavy element compositions can be accessed in their volume. The range of applications is huge and spreads from nuclear spectroscopy to the characterization of metal extrusion under industrial conditions. This article compiles an overview over the most common instrumental diffraction techniques.Modern two-dimensional detectors are used to obtain rapid overviews in reciprocal space. For example, diffuse scattering investigations benefit from the very flat Ewald sphere as compared to low energies, which allow mapping of several Brillouin zones within one single shot. Diffraction profiles from liquids or amorphous materials can be recorded easily. For materials science purposes, whole sets of Debye-Scherrer rings are registered onto the detector, their diameters and eccentricities or their intensity distribution along the rings relating to anisotropic strain or texture measurements, respectively. At this point we stress the resolution of this technique which has to be carefully taken into account when working on a second generation synchrotron source.Energy-dispersive studies of local residual strain can be studied by a dedicated three-circle diffractometer which allows accurately to adjust the scattering angle from a defined gauge volume.Triple axis diffractometry and reciprocal space mapping is introduced and can be employed for highest resolution purposes on single crystal characterization, even under heavy and dense sample environments. Thus, the perfection of single crystals can be mapped and strain fields and superstructures as introduced by the modulation from ultrasonic waves into crystals or epitaxially grown Si/Ge layers can be investigated in detail. Phase transitions as magnetic ordering can be studied directly or through its coupling to the crystal lattice. Time resolved studies are performed stroboscopically from a sub-nanosecond to a second time scale.The combination of these techniques is a strong issue for the construction and development of future instruments.

The distribution of Ge-0-Ge and Si-0-Si bond angles a in amorphous germania and silica is re-determined on the basis of diffraction experiments. The bond angle a joining adjacent tetrahedra is the central parameter of any continuous random network description (CRN) of these glasses. New high energy photon diffraction experiments on amorphous germania (at photon energies of 97 and 149 keV) are presented, covering the momentum transfer 0.6-33.5 A -'. In photon diffraction experiments on GeO, the contribution of the 00 pairs is very small. To obtain a similar information for amorphous SiO,, high energy photon diffraction experiments [l] have been combined with neutron diffraction data 12, 31 on amorphous silica in order to eliminate the 00-partial structure factor. With this technique it is shown that the Si-0-Si angle distribution is fairly narrow (a = 7.5") and in fact comparable in width to the Ge-0-Ge angle distribution (a = 8.3'), a result which differs from current opinion. The narrower distribution found in this study are in much better agreement to the determinations based on 29Si-MAS-NMR. Among the various models relating the chemical shift to the bond angle, best agreement is found with those models based on the secant model. Sharp components in the bong angle distribution can be excluded within the reached real space resolution of 0.09 A.

a b s t r a c tIn a b-stabilized Ti-43Al-4Nb-1Mo-0.1B alloy (composition in atomic percent) the correlation between the occurrence of b-phase and temperature was analyzed experimentally and compared to thermodynamic calculations. Results from in situ high-energy X-ray diffraction, texture measurements, heat treatments, scanning electron microscopy, and temperature-dependent flow stress measurements were used to study the evolution of the b-phase with temperature. Thermodynamic calculations based on the CALPHAD method were applied to correlate the phases developed in the b-solidifying TiAl based alloy under investigation. This alloy is characterized by an adjustable b-phase volume fraction at temperatures where hot-work processes such as forging and rolling are conducted. Due to a high volume fraction of bphase at elevated temperatures the hot-extruded alloy can be forged under near conventional conditions.

Studying the workplace: An industrial methanol synthesis catalyst operating at high pressure was studied by in situ neutron diffraction. The peculiar microstructure of Cu/ZnO/Al2O3 nanocatalysts was found to be stable under reaction conditions. Stacking fault annealing and brass formation was only observed at temperatures higher than used in the methanol synthesis process, providing support for active role of defects in this catalyst system.