F. Garcia-Moreno

X-ray and neutron imaging – Complementary techniques for materials science and engineering

Abstract Using X-ray and neutron radiography and tomography, images of material and component inhomogeneities and their development with time can be obtained. Due to their non-destructiveness and non-invasive nature both methods give insight into the function of devices and their decay processes. Fundamentals of X-ray and neutron radiography and tomography are briefly outlined, examples for both techniques are given, their complementarities are highlighted and emerging techniques and frontiers are discussed.

Fast processes in liquid metal foams investigated by high-speed synchrotron x-ray microradioscopy

Rupture of an individual film in an evolving liquid metal foam is investigated by means of high-speed x-ray radioscopy using white synchrotron radiation. At a frame rate of 5000frames∕s, the rupture event is spread over three to four images. The images show that the remnants of the rupturing film are pulled into the surrounding plateau borders in 600±100μs which conforms well with a liquid movement governed by inertia and not by viscosity. Within one order of magnitude, the viscosity of the liquid involved must be similar to the viscosity of pure liquid aluminium.

Synchrotron-based radioscopy employing spatio-temporal micro-resolution for studying fast phenomena in liquid metal foams

High-speed synchrotron-based radioscopy is applied to study a coalescence event (which lasts ∼2 ms) in situ in a liquid metal foam.

On the possibilities of hard X-ray imaging with high spatio-temporal resolution using polychromatic synchrotron radiation.

Time-resolved imaging with penetrating radiation has an outstanding scientific value but its realisation requires a high density of photons as well as corresponding fast X-ray image detection schemes. Bending magnets and insertion devices of third generation synchrotron light sources offer a polychromatic photon flux density which is high enough to perform hard X-ray imaging with a spatio-temporal resolution up to the μm-μs range. Existing indirect X-ray image detectors commonly used at synchrotron light sources can be adapted for fast image acquisition by employing CMOS-based digital high speed cameras already available on the market. Selected applications from life sciences and materials research underline the high potential of this high-speed hard X-ray microimaging approach.

Particle-stabilised foams: structure and aging

We show that aqueous foams stabilised by nanoparticles can be easily imaged using an X-ray laboratory source. We have used hydrophobically modified silica nanoparticles that confer to the foam a remarkable stability. The X-ray tomography observations were compared with the information obtained using a multiple light scattering technique. Both techniques confirm that provided the concentration of particles in bulk water is high enough, the bubble size evolves little with time. X-Ray tomography revealed the presence of two populations of bubbles, small bubbles which size tends to decrease with time and large bubbles which number tends to increase with time. This behaviour could arise from an arrested coarsening process. The results demonstrate the great potential of the two techniques and of their combination for foam studies.

Analysis of the internal structure of monodisperse liquid foams by X-ray tomography

Bench-top X-ray tomography has been successfully applied to determine the detailed internal structure of wet and dry aqueous foams. We find extensive fcc crystallisation at surfaces and random (Bernal) packing in the interior of large samples.

Kinetics of coalescence in liquid aluminium foams

The cell wall rupture kinetics for a collection of AlSi9 foam samples kept at a constant temperature over the melting point for a long time period (700 s) has been characterized by in situ X-ray radioscopy. The images obtained have been computed by using image analysis algorithms allowing the identification of the cell wall ruptures in the molten state. The time evolution, the spatial distribution and the number of cell wall rupture events are studied in this work. The cell wall rupture rate was determined for the first time in liquid aluminium foams. Part of the results obtained has been explained in terms of the spatial distribution of temperature obtained by different heating configurations and measured by thermographic techniques. The progressive coarsening and density redistribution of the cellular structure and the drainage due to gravity have been also considered. The local temperature seems to be critical for the rupture phenomena and the temperature distribution clearly affects the foam density distribution.

Study on aluminium-based single films

In the present paper the authors studied isolated metallic films made from the same material used for making metallic foams, and then characterised their properties. Metal films were made from a liquid aluminium alloy reinforced with ceramic particles of known concentration. Melts without such particles were also investigated. It is shown that stable films could not be made from Al-Si alloy having no particles, and just extremely thin and fragile films could be made from commercially-pure Al. In contrast, aluminium alloys containing particles such as SiC and TiB(2) allowed pulling thin, stable films, which did not rupture. Significant thinning of films was observed when the particle concentration in the melt decreased. By in situ X-ray monitoring of liquid films during pulling, film thickness and drainage effects within the liquid film could be studied. The morphology and microstructure of films was characterised after solidification. Our work shows that the question of how foams are stabilised can be studied using a simplified system such as a film, instead of having to deal with the multitude of different structural elements present in a foam.

Metal foams – towards microcellular materials

Abstract Various techniques to manufacture low-density metallic foams containing sub-millimetre or even micrometre-sized pores are discussed and first trial experiments presented. Three strategies are evaluated: use of an intrinsic blowing agent, foaming under high pressure and foam control by mechanical pressure manipulation. In all three cases, average pore diameters well below 1 mm could be achieved for some aluminium or zinc-based foams while keeping the relative density in a range between 20 % and 50 % of the full metal density.

White-beam X-ray radioscopy and tomography with simultaneous diffraction at the EDDI beamline.

A set-up for simultaneous imaging and diffraction that yields radiograms with up to 200 frames per second and 5.6 µm effective pixel size is presented. Tomograms and diffractograms are acquired together in 10 s. Two examples illustrate the attractiveness of combining these methods at the EDDI beamline for in situ studies.