Non-destructive characterisation of polymers and Al-alloys by polychromatic cone-beam phase contrast tomography

Abstract

X-ray computed tomography (XCT) has become a very important tool for the non-destructive characterisation of materials. Continuous improvements in the quality and performance of X-ray tubes and detectors have led to cone-beam XCT systems that can now achieve spatial resolutions down to 1 {mu}m and even below. Since not only the amplitude but also the phase of an X-ray beam is altered while passing through an object, phase contrast effects can occur even for polychromatic sources when the spatial coherence due to a small focal spot size is high enough. This can lead to significant improvements over conventional attenuation-based X-ray computed tomography. Phase contrast can increase by edge enhancement the visibility of small structures and of features which are only slightly different in attenuation. We report on the possibilities of polychromatic cone-beam phase contrast tomography for non-destructive characterisation of materials. A carbon fibre-reinforced polymer and the Al-alloys AlMg5Si7 and AlSi18 were investigated with high resolution cone-beam X-ray computed tomography with a polychromatic tube source. Under certain conditions strong phase contrast resulting in an upward and downward overshooting of the grey values across edges was observed. The phase effects are much stronger for the polymer than for the Al-alloys. The influencemore » on the phase contrast of the parameters, including source-detector distance, focal spot size and tube acceleration voltage is presented. Maximum phase contrast was observed for a maximum distance between the source and the detector, for a low voltage and a minimum focal spot size at the X-ray source. The detectability of the different phases is improved by the edge enhancement and the resulting improvement of sharpness. Thus, a better segmentation of the carbon fibres in the fibre-reinforced polymer and of the Mg{sub 2}Si-phase in the AlMg5Si7-alloy is achieved. Primary and eutectic Si cannot be detected by attenuation-based X-ray computed tomography due to the small difference in X-ray attenuation between the Si and the Al-matrix in AlSi18. However, phase contrast effects lead to a significant detectability and the large primary Si particles become clearly visible in the tomograms of AlSi18. - Highlights: Black-Right-Pointing-Pointer Phase contrast increases with increasing distance object-detector and with decreasing U, X-ray spot. Black-Right-Pointing-Pointer The detectability of details is improved by the phase contrast. Black-Right-Pointing-Pointer For the carbon fibre-reinforced polymer the visibility of the carbon fibres is improved. Black-Right-Pointing-Pointer For AlMg5Si7 the detectability of the phases Fe-aluminides and Mg{sub 2}Si particles is improved. Black-Right-Pointing-Pointer For AlSi18 primary Si particles with a size > 50 {mu}m become detectable because of the phase contrast.« less