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Organisationseinheit der BAM
Non-destructive evaluation of advanced materials by synchrotron radiation refraction topography
(2002)
The first hard X-ray beamline at BESSY II will be installed by BAM and PTB at a superconducting 7 T wavelength shifter. The main optical elements of the beamline are a Double-Multilayer-Monochromator and a Double-Crystal-Monochromator. The two devices can be used separately or in-line. Main applications of monochromatic radiation with photon energies up to 50 keV are X-ray fluorescence analysis, micro-computed tomography, X-ray topography, detector calibration and reflectometry. Calculable undispersed radiation up to 200 keV will be available for radiometric applications.
For the first time Metal Matrix Composites (MMC) have been investigated by 3D Computed Tomography combined with enhanced interface contrast due to X-ray refraction. X-ray refraction is a relatively new approach for the characterization of advanced materials. The related techniques of Refraction Topography and Refraction Computed Tomography have been developed and applied at our laboratory during the last decade to meet the actual demand for improved nondestructive characterization of high performance composites, ceramics and other low density materials and components. X-ray refraction occurs, when X-rays crosses interfaces of spherical or cylindrical shape (e.g. pores or fibres) in the same way as visible light is refracted by lenses. These X-ray optical effects can be observed at small scattering angles of few minutes of arc as the refractive index n of X-rays is nearly unity (n = 1 10-6). Due to the short X-ray wavelength of about 0.1 nm the technique determines the amount of inner surfaces and interfaces of nanometer dimensions. The technique is expected to solve many problems in understanding the meaning of micro and sub micro structures in materials science. With the results of the CT investigation, some questions could be clarified for a better understanding of fatigue failure mechanisms under cyclic loading conditions. The specimens for the test programme have been provided by MTU Aero Engines. They consist of a titanium matrix (Ti6242) reinforced by SiC fibres (SCS6). The investigations have been performed at the materials research station of BAM (BAMline) at the Synchrotron Facility BESSY in Berlin, Germany.
The employment of synchrotron radiation for refraction topography of materials has considerable advantages over standard x-ray sources. The much higher beam intensity and the parallel and monochromatic radiation provide faster measurements and better angular and spatial resolution. X-ray refraction techniques image the inner surface and interface concentration of micro-structured materials. This effect of x-ray optics is additional to small-angle scattering by diffraction, when the scattering objects reach micrometre dimensions. We have developed x-ray refraction techniques within the last decade in order to meet the growing demands for improved non-destructive characterization of high-performance composites, ceramics and other low-density materials. Sub-micron particle dimensions, the pore size of ceramics, the crack density distribution and single fibre debonding within damaged composites can be measured and visualized by computer-generated interface topographs. For this purpose investigations are now being performed at the new hard x-ray beamline of the Federal Institute for Materials Research and Testing (BAM) at BESSY, Berlin. This BAMline provides monochromatic radiation of photon energies from 5 to 60 keV from a double multilayer and/or a double-crystal monochromator. A separate instrument is dedicated to the further development and application of synchrotron radiation refraction (SRR) topography. Different from conventional small-angle scattering cameras with collimating slits and pinholes, scattering angles down to a few seconds of arc are selected by a single-crystal analyser, similar to a Bonse-Hart diffractometer. A 20 µm spatial resolution of the scattering micro-structures is achieved by a CCD camera with a fluorescent converter. First SRR topographs of aircraft composites [carbon fibre-reinforced plastics (CFRP), carbon fibre-reinforced ceramics (C/C), metal matrix ceramics (MMC)] will be reported.
A novel approach is suggested to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background signals 'backlighting' of 2D X-ray detectors. Depending on the detector geometry the mechanism may be different. Either based on the optical scattering of the fluorescent screen materials into the optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert Beer's law in single projections (radiographs) as a function of the detector area coverage and the magnitude of the attenuation. The absorption of multiple metal sheets is investigated by monochromatic synchrotron radiation, thus excluding beam hardening. The proposed correction procedure simply requires the individual subtraction of one and the same fraction of the primary and transmitted mean intensity, as a constant (non-local) scattering mechanism is assumed.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background Signals (backlight) of 2D X-ray detectors is suggested. Depending on the detector geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in single projections (radiographs).
When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation Phase and the samples aspect ratio (dynamics). The magnitude of the diffuse Background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shaded area and the stronger the total attenuation.
Moreover, the local intensity mismatch depends on the attenuation of the sample.
We present very basic reference data measured with multiple metal foils at a synchrotron radiation source. Beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure assumes a constant (non-local) scattering mechanism.
Beyond classical X-ray techniques are used for the purpose of preferably short-term applications, but some supplementary X-ray and synchrotron techniques for higher resolution microdiagnostics take advantage of scattering effects. In contrast to directly imaging methods their resolution is only limited by the diffraction limit of the X-ray wavelength, far below the atomic dimensions. These techniques of scanning topography and refraction synchrotron tomography may permit the systematic diagnostics for finding and exploiting structure/property relations like correlations among atomic, nano and microstructures with macroscopic properties. Their basic advantage over microscopic techniques is their potential for the non-destructive characterisation of materials, far from invasive sample treatments. They combine scattering and spatial resolution.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background signals ('backlighting') of 2D X-ray detectors is suggested. Depending on the detector geometry the mechanism may be different, either based on the optical scattering by the fluorescent screen materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert–Beer's law in single projections (radiographs). When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation phase and the samples aspect ratio (dynamics). The magnitude of the diffuse background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shadowed area and the stronger the total attenuation. This implies that the reconstruction suffers from additional anisotropic artifacts caused by elongated sample structures. This issue is studied simply by absorption of flat plates in a conventional laboratory radiography set-up and at a synchrotron radiation facility. In the latter case beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure requires simple integral intensity offsets as a constant (non-local) light scattering mechanism is assumed.