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A novel 3D-CT scanner was set up at BAM to extend the applicability of the X-ray transmission microtomography (µ-CT) to specimens up to 30 cm in diameter. The scanner consists of a bipolar 320 kV micro focus tube and a flat panel detector of amorphous silicon. Minimisation of scattered radiation, control of room and system temperature, and corrective preprocessing on the projection data are necessary to achieve expectable CT images. Statistical evaluations to show the limits and results of CT investigations are reported.
Summary
Beside several favourable properties, like high specific strength and damping capacity, most magnesium alloys also exhibit an excellent castability and are therefore usually fabricated by high pressure die casting. Due to the high casting speed, the melt flow is non-laminar and air can be entrapped during the casting process causing porosity when the melt solidifies. The volume fraction of porosity can be up to 4%. Porosity highly influences the mechanical properties, reducing significantly the values of elongation, tensile strength and especially fatigue strength [1-5]. Additionally, no heat treatment is possible to reach the optimum mechanical properties of the material. An alternative casting technology to reduce porosity is the Thixomolding process where the material is cast in the semi-solid state.
Different methods of non-destructive testing are possible to detect and remove affected components to ensure using components with a low content of porosity. One alternative of evaluating the porosity is the computerized tomography (CT). The CT enables a 3-dimensional view of the cast component.
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.