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The in situ investigation of dendrite deformation during solidification processing is an ongoing challenge because of the technical difficulties in carrying out experimental observations and direct measurements in metallic systems that are opaque to visible light and have a high melting temperature. Over the last 20 years, x-ray imaging has been established as a method of choice to overcome this experimental barrier by taking advantage of the increased capabilities of third-generation synchrotron x-ray sources, providing information on growth process and semisolid deformation that is not available otherwise. In this article, we present results showing that the unique combination of synchrotron x-ray radiography and synchrotron white beam x-ray topography can help to reveal the deformations that dendritic microstructures undergo during the upward directional solidification of Al - 7wt.%Si alloys. Particular focus will be placed on the bending phenomena of dendrites because of gravity, which may precede fragmentation in the case of well-developed secondary arms.

A high-quality CdZnTe single crystal has been imaged with micrometer resolution using monochromatic x-ray Bragg diffraction at the bending magnet (BM)-05 beamline of the European Synchrotron Radiation Facility. Since this material is used to produce substrates for the epitaxial growth of infrared sensitive HgCdTe photon detectors, imaging the defects is of strong interest to visualize their arrangement within the bulk. We show that large field-of-view rocking curve projection maps can be acquired in transmission geometry using samples as thick as 0.5 mm thanks to the Borrmann effect. Furthermore, section topography can be applied to provide information on the depth of the observed defects allowing for a complete 3D localization of lattice distortions. Examples of 1.5 mm × 1.5 mm full width at half-maximum maps are given to illustrate the spatial arrangement of dislocations in a number of samples cut with respect to different crystallographic planes in a particular CdZnTe ingot.

Alloy solidification was investigated in situ and real time by using a unique experimental setup developed at the European Synchrotron Radiation Facility (ESRF) combining both synchrotron X-ray radiography and topography. Although synchrotron X-ray radiography enables the investigation of the solid-liquid interface of metallic alloys, white-beam synchrotron X-ray topography enables the investigation of the formation of strains and defects formation in the growing solid microstructure. In this article, we present results obtained during directional solidification experiments performed with Al-3.5 wt pct Ni samples. First, the initial state after thermal stabilization is characterized. Next, the interface morphological instability and the transition to the columnar growth regime are thoroughly investigated. Topography observation shows that several parts of each dendrite become disoriented while the microstructure is developing. Disorientations are quantified and the aluminum yield stress at the melting point is estimated from the bending of secondary arms. Last, coupled growth of eutectic and dendrites settles with the formation of the eutectic phase. The eutectic grains grow strained and the dendrites concomitantly undergo additional stress.

The deformation behaviour of an open-cell flexible polyurethane foam was observed using X-ray microtomography on the ID19 beamline at the ESRF in Grenoble, France. Tomographs, consisting of 1024 voxels cubed, were collected with a voxel size of 6.6 μm from a small region near the centre of the foam at a range of compressive strains between 0 and 80%. The results show that the initial stages of compression are taken up by small amounts of elastic bending in struts that are inclined to the compression direction. At 23% strain, entirely collapsed bands were observed in the structure. By 63% strain, there was evidence of struts impinging on each other, corresponding to the densification regime. The compression of an irregular foam (i.e. one with strut length and cell size distributions) appears to involve a sudden change in modulus, accompanied by localised increases in density. Observations of this nature would have been extremely difficult to interpret unambiguously without the ability to carry out sequential microtomographic imaging under realistic in situ loading conditions. The process of finite element analysis (FEA) was begun by constructing node-strut models from the experimental data by a mathematical skeletonisation process. These were used to derive node coordination, strut-length and cell-size distributions. However, direct comparison of the elastic properties with FEA was hampered by the absence of periodicity in the experimentally determined foam structures.

In this article, we present a review of observations during Al-3.5 wt pct Ni alloy solidification experiments performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble. These experiments provide direct access to dynamical phenomena during columnar growth (initial transient and breakdown of a planar solid-liquid interface), and for the first time to the transition from columnar-to-equiaxed microstructure (nucleation ahead of a columnar front and blocking of a columnar front by an equiaxed microstructure) and fully equiaxed growth (propagation of an effective front). Based on these experimental observations, critical parameters such as columnar growth velocity variation during the transition or equiaxed-grain diameter are measured and discussed. [PUBLICATION ABSTRACT]

The mechanical properties of a cast product, and therefore its application, depend strongly on its inner microstructure. During the solidification step a change from columnar to equiaxed grain structure can occur. It is thus critical to understand the physical mechanisms of this transition in order to accurately predict and control its occurrence and the final grain structure morphology. This article reports on observations of the CET (Columnar to Equiaxed Transition) induced by a sudden increase of the pulling velocity during the directional solidification on a refined Al-3.5wt%Ni alloy by using synchrotron X-ray radiography. The influence of the pulling velocity on the blocking of the columnar structure is described. Next, the distribution of surface area and longitudinal asymmetry of the grains after CET are quantitatively characterized and discussed.

Three-dimensional X-ray microtomography is used to obtain three-dimensional images of the microstructure of two types of brick. The images are processed to remove the noise (random and circular pattern) and then thresholded to match the porosity determined experimentally. The 3-D binary images are then analyzed to estimate their vapor diffusivity and air permeability to compare to experimental data published in part one of this report. Care must be taken in obtaining the tomographic images at a resolution that both enables isolation and quantification of the pores of interest and provides a representative elementary volume for the transport property calculations. In general, the agreement between computed and measured properties is reasonable, suggesting that X-ray microtomography can provide valuable information on the characteristics and properties of the pore networks developed in these porous building materials. A preliminary evaluation indicates that the Katz-Thompson relationship between permeability, diffusivity, and pore size is valid for these materials.Original Abstract: La microtomographie a rayons X synchrotron, est utilisee pour obtenir des images tridimensionnelles de la microstructure de deux types de briques. Les images sont tout d'abord traitees pour eliminer le bruit (anneaux aleatoires_ et ensuite seuillees par ajustement avec la porosite determinee experimentalement. A partir des images binaires 3D, on estime numeriquement la diffusivite a la vapeur et la permeabilite a l'air, les valeurs obtenues sont ensuite comparees avec les donnees experimentales publiees dans la partie I de cette communication. Dans le cadre d'une telle procedure, la resolution des images doit a la fois rendre possible la discrimination et la quantification de tous les pores importants vis-a-vis du phenomene etudie et fournir un volume elementaire representatif pour le calcul des proprietes de transport. L'accord satisfaisant obtenu entre les valeurs calculees et mesurees montre que la microtomographie X peut fournir des informations pertinentes sur les caracteristiques et les proprietes du reseau poreux des materiaux de construction. Une evaluation preliminaire indique que la relation de Katz-Thompson entre la permeabilite, la diffusivite et la taille des pores est applicable pour ces materiaux.

With advances in x-ray microtomography, it is now possible to obtain three-dimensional representations of a material's microstructure with a voxel size of less than one micrometer. The Visible Cement Data Set represents a collection of 3-D data sets obtained using the European Synchrotron Radiation Facility in Grenoble, France in September 2000. Most of the images obtained are for hydrating portland cement pastes, with a few data sets representing hydrating Plaster of Paris and a common building brick. All of these data sets are being made available on the Visible Cement Data Set website at http://visiblecement.nist.gov. The website includes the raw 3-D datafiles, a description of the material imaged for each data set, example two-dimensional images and visualizations for each data set, and a collection of C language computer programs that will be of use in processing and analyzing the 3-D microstructural images. This paper provides the details of the experiments performed at the ESRF, the analysis procedures utilized in obtaining the data set files, and a few representative example images for each of the three materials investigated.

It has been shown in the last decade that in situ and real-time observation of metallic alloy solidification is possible by using X-ray monitoring conducted at third generation synchrotron sources. A detailed analysis of a Bridgman experiment carried out at ESRF with an Al - 3.5 wt% Ni alloy was presented earlier [1]. This article proposes a direct simulation of the solidification of the entire sample for this experiment, in which all the dendritic grains are individually represented as they nucleate and grow in the experiment. This is possible by extracting from the radiographs a list of all the nucleated grains, including the positions and orientations of their main trunks. Simulation is performed using a two-dimensional (2D) Cellular Automaton (CA) – Finite Element (FE) model. As a result of the coupling between the CA and FE methods, consequences of the macroscopic transport of heat, liquid momentum and solute mass on the development of the dendritic grain structure are accounted for, and vice versa. The macroscopic deformation of the columnar front observed during the experiment is reproduced, as well as the columnar-to-equiaxed transition. The influence of flow patterns on macrosegregation is also discussed.

We report on the in situ, time resolved, observation of solidification cracking in a thin sample of an Al-15wt%Sn alloy at ESRF BM05. During the experiment, solidification cracking was seen to occur during natural cooling of the sample at a solid fraction of ∼95%, between directionally agglomerated dendritic networks. Through detailed analysis, three stages of crack growth were observed: crack initiation, which typically occurs asymmetrically with the liquid film separating from one side of the dendritic network prior to full detachment; crack propagation, where the liquid film generally detaches from both sides simultaneously; and crack coalescence. We correlate our observations using scanning electron microscopy, which shows that voids between grains, and also spikes, can appear asymmetrically on either side of the crack surface.