Multi-scale tomography for characterisation of metals

Determination of the 3-dimensional (3D) distribution of heterogeneities and structures is an issue of primary concern in the field of material characterisation and quality control. Heterogeneities may create exploitable property profiles, but can also degrade reliability and therefore are to be classified as defects. The quantitative description of heterogeneities is a prerequisite for evaluating their effects and their potential for strengthening or degradation. The knowledge of size, shape, location and arrangement of heterogeneities allows to evaluate the quality of materials and work pieces. To date, X-ray computed tomography (XCT) is the method to measure inner or hidden structures without destroying the specimen. The size and topology of different types of heterogeneities can vary over a wide range. The different size scales of the heterogeneities and of the affected material volume require appropriate tomographic methods and resolutions, such as industrial XCT (>10 m), microfocus X-ray tomography "-XCT" (>3 m), nanofocus XCT "nano-XCT" (>0.5 m), synchrotron tomography "s-XCT" (>0.1 m), or destructive focussed ion beam tomography FIBT (>10 nm). The indicated resolution limits scale with the maximum transmitted distance, which limits the specimen size to about 1000x resolution limit. Application related 3D characterisation of structural materials therefore requires multi-scale tomography. Goals: Main focus of this project is basic research on exploitability of multi-scale XCT-methods, ranging from industrial XCT down to s-XCT. In particular the following goals are targeted: Development of a methodology to use appropriate tomography methods on different length scales ranging from 0.5 mm resolution down to 100 nm. The examples chosen are: dendritic and interdendritic structures in aluminium-alloys; inclusions and pores of different number densities in cast carbon steel; pores and their connectivity in sintered iron-base alloys; 3D architecture of reinforcements in copper matrix composites and damage mechanisms. Verification of microstructural features detected by XCT and of contrast-simulations by correlation with the results of destructive sectioning methods (target metallography, FIBT). Generation of know-how and experience on the exploitable image resolution to assess the different tomographic methods appropriate to type and size of the heterogeneities in light alloys, steel and copper with positive and negative absorption contrast. Correlation of different methods for image processing and further development where needed to choose the appropriate image recording and evaluation parameters. Investigation of the complementarity of the results of -XCT, nano-XCT, s-XCT and FIBT exploiting the latest developments in tomographic methods and instruments. Results: A methodology to select appropriate tomographic methods for specific material inhomogeneities at different length scales verified by combination with destructive characterisation and simulation. Assessment of the applicability of CT methods regarding interpretation of CT contrasts, spatial and material resolution limits. XCT simulation methods to predict a correlation of CT measurement parameters with the detectability of heterogeneities. Enhanced expertise in application of CT methods, in order to quantify 3D structures of heterogeneities of different size scales in metallic materials.

X-ray computed tomography was introduced successfully for medical diagnostics in the middle of the 20th century. High energy X-ray sources with small focal spots enabled the application of X-ray tomography in non destructive testing of materials during the last decades. The increase in computing power provided the means for fast processing of 3 dimensional data sets. Engineering materials consist usually of different chemical and physical constituents, the spatial arrangement of which can be recorded by computed tomography. Synchrotron radiation sources are available for material research in Europe since the beginning of this century. The high brilliance of the synchrotron X-ray beam allows to produce high resolution tomographies within less than 1 minute. Thus in-situ observations of deformation within a loaded specimen can be performed. The three-dimensional internal architecture of materials can be determined quantitatively and correlated to their properties. Machines are mostly made of metals, the performance of which depends on the composition and spatial arrangement of the constituents, which extend over several magnitudes in size: from nanometres to centimetres. Cast conditions of steel, aluminium alloys and metal matrix composites were investigated to analyse their internal, three-dimensional architecture. Radiographic methods of different size scales were applied successively: microradiography at Vienna University of Technology, micro- and macro-focus X-ray tomography at the Upper Austrian University of Applied Sciences in Wels, Synchrotron radiation at the European Radiation Source Facility (ESRF) in Grenoble. At ESRF pores smaller than 1 micrometre could be located in steel samples for the first time. The 3-dimensional images allow to determine pore contents down to 1 pore per cubic-centimetre. The solidified structure of cast alloys could be imaged in dimensions of centimetres. The solidification process of an aluminium alloy could be recorded in-situ. Metal matrix composites were submitted to thermal cycling. The progress of damage could be quantified. The best tomographic methods available in Europe could be assessed regarding their applicability to describe the multi-scale internal architecture of technical alloys. The appropriate tools can be selected to contribute to alloy and processing development as well as to provide a tool for non-destructive quality control. Examples for steel and aluminium alloy slabs are presented. In-situ tests of solidification of an aluminium alloy and of cyclic thermal exposure of metal matrix composites demonstrate the potential to analyse the processing - structure - property relationship of metallic materials.