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In conventional melting experiments of pure monocrystalline metals, the phase transformation starts at the sample surface and progresses inwards according to thermal gradients. In solutionized alloys, traces of internal melting are usually observed after reheating and quenching from the semi-solid state. The formation and development of these liquid pockets are not fully understood despite their significance in semi-solid processing, where the formability is greatly influenced by the distribution of liquid within the feedstock material. In situ X-ray microtomography was performed in this study to shed light on this phenomenon. We report in detail the melting and isothermal holding of a model binary alloy where a remarkable number of liquid droplets were observed to develop and coalesce. Various computational tools have been used to study their statistical evolution as well as the local ripening mechanisms involved. We analysed an interesting case of particle coarsening which differs from classical case studies by the fact that the fast-diffusing liquid phase is entrapped within the slow-diffusing solid medium. [ABSTRACT FROM AUTHOR]

Abstract: Recent developments of more powerful synchrotron sources have led to vast improvements in the performance of X-ray imaging. This is manifested by a continuous increase in the impact of synchrotron experiments in many research areas on materials, in particular solidification science. X-ray imaging has been established as a method of choice for in situ and real-time studies of solidification microstructure formation in metallic alloys, with spatio-temporal resolutions at the scales of relevance. In this article, we present illustrative results of the current capabilities of synchrotron X-ray imaging in this field of research, each of them using different X-ray techniques (radiography, topography and tomography). Those results demonstrate the high potential of these techniques for the investigation of dynamical phenomena in materials processing. [Copyright &y& Elsevier]

Abstract: The cyclic behaviour of 400μm pore size replicated aluminium foam is assessed in tension–tension fatigue with a stress ratio equal to 0.1, keeping the load amplitude constant, for relative density values comprised between 0.175 and 0.220. The number of cycles to failure ranges from 6×102 (lowest relative density) to 5×106 (highest relative density). The foams display cyclic creep coupled with a strong influence of relative density on their general fatigue performance. Data analysis shows that the foam fatigue behaviour is dominated by cyclic creep, which governs both the deformation and the fatigue life of the cycled specimens, yielding characteristics globally in line with what is expected knowing the metal making the foam. [Copyright &y& Elsevier]

Abstract: In the field of materials science, it is common to relate mechanical or physical behaviour to microstructure in order to optimize materials. Usually the observation of the microstructure is made using an optical or electronic microscope to obtain images in two dimensions. However, the measurable parameters in 2D are limited and often the techniques involved to obtain these images are destructive. Thanks to X-ray micro-tomography it is possible to overcome these two difficulties. The purpose of this article is to show the enormous contribution of this tool in the understanding and the modelling of some physical phenomena involved in aluminium alloys. [Copyright &y& Elsevier]

The aim of this article is to present the results of a partial remelting experiment carried out on a cold-rolled Al–8 wt.% Cu alloy. The evolution of the semi-solid mixture has been investigated using in situ X-ray microtomography with a particular emphasis on nucleation and coarsening of the liquid droplets entrapped inside the solid globules. [Copyright &y& Elsevier]

The microstructural evolution of Al–15.8wt.% Cu slurry held at 555°C was characterized in situ by X-ray microtomography. The average solid grain coordination and size measured on the reconstructed three-dimensional (3D) objects are compared with the values derived via image analysis carried out on 2D sections of the same objects. A new method is proposed for obtaining the average grain coordination by 2D image analysis. The method proves to be more accurate than the procedure commonly used in the literature. [Copyright &y& Elsevier]

Abstract: The paper is concerned with the study of the microstructural changes occurring during holding of an Al–15.8wt.% Cu alloy in the semi-solid state. These changes are investigated in 3D by in situ X-ray tomography carried out at the temperature of the treatment. The studies are classified in two categories: overall changes by measuring average values of characteristic parameters, and local changes by considering the evolution of individual necks between particles. It is shown in particular that the size of the solid particles or the surface area of the solid–liquid interfaces do not follow the classical power laws but rather evolve in a slower manner. Local observations confirm that these results are due to the competition of two coarsening mechanisms of the solid particles that occur simultaneously: dissolution of a small particle to the benefit of one or several bigger ones by an Ostwald-type mechanism and the growth of necks between solid particles due to their coalescence. Complex variations of neck size result from these mechanisms which can be explained only by considering the neighbourhood of the particles under investigation. These observations confirm that in situ X-ray tomography is a very powerful tool to provide data that are representative of the semi-solid state and to observe in real time the mechanisms that act on the microstructure. [Copyright &y& Elsevier]

Applying atmospherically plasma-sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) protective coatings on interconnector steels minimized the chromium-related degradation within solid oxide fuel cell stack-tests successfully. Post-test characterization of the coatings disclosed a severe microstructural and phase evolution. A self-healing of micro-cracks, the formation and agglomeration of small pores, the occurrence of a dense spinel layer at the surface and a strong elemental de-mixing were reported in ex situ experiments. In the present publication, we prove for the first time these mechanisms by tracking the microstructure in situ at a single APS coating using synchrotron X-ray nano-tomography at the European Synchrotron Radiation Facility. Therefore, a 100-µm-long cylindrical sample with a diameter of 123 µm was cut from an APS-MCF free-standing layer and measured within a high-temperature furnace. All microstructural changes mentioned above could be verified. Porosity measurements reveal a decrease in the porosity from 9 to 3% during the annealing, which is in good accordance with the literature. Additionally, a partial detachment of an approximately 5-µm-thick layer at the sample surface is observed. The layer is dense and does not exhibit any cracks which are penetrating the layer. This kind of shell is assumed to be gastight and thus protecting the bulk from further oxidation. [ABSTRACT FROM AUTHOR]

We describe our miniature laser powder bed fusion (L-PBF) system for in situ synchrotron x-ray micro-computed tomography (XCT) at the European Synchrotron Radiation Facility. This replicator was designed to extend the characterization of L-PBF to 3D. This instrument fills in a technical gap because the existing replicators were mostly designed to shed light on the dynamic mechanisms involved in molten pool formation but, therefore, suffered from a lack of 3D information. Technical details regarding the setup and beamline integration are given. Experimental validations via post-mortem XCT scans and in situ scans acquired during experiments conducted at the BM05 beamline of the European Synchrotron Radiation Facility are provided. Based on a few illustrative examples, we show that such a replicator opens the path to collect key 3D information that to date could not be available. Our miniature instrument complements the other replicators developed in the world by other research groups that enable operando x-ray imaging (radiography) and operando x-ray diffraction. [ABSTRACT FROM AUTHOR]

Evolution of the inner microstructure of uniaxially pressed copper powders was investigated by in situ X-ray microtomography. Experiments were carried out at the European Synchrotron in Grenoble, France. Sintering was performed under reducing atmosphere at 1050 °C. Qualitative and quantitative information from the 3D images acquired along the whole sintering cycle were analyzed. From that, macro- and microstructural features of the evolving microstructure were obtained. We found that large pores can be created during sintering and then they can grow during the whole thermal cycle by reducing the initial relative density of the sample by 10 %. The effect of the pore shape and the heterogeneous distribution of the pore volume fraction inside the sample are responsible for the behavior observed during sintering. The heterogeneous deformation measured is controlled by the evolution of the porosity. [ABSTRACT FROM AUTHOR]

Abstract: A method is presented that extends the replication process for the manufacture of open-celled metallic foams to include foams with spherical cells. Such foams are produced to a wide range of relative densities, tested for mechanical behaviour and resistance to fluid flow in the Darcian regime, and characterized using synchrotron X-ray microtomography for their internal architecture. It is shown that the effect of cell shape on uniaxial mechanical properties is negligible if the cells are roughly equiaxed. Spherical and angular pore foam mechanical properties differ, however, if the angular particles are more irregular and their aspect ratio deviates from unity; current micromechanical models assimilating pores to spheroids do not account for this difference. The permeability is, on the other hand, relatively insensitive to details of pore shape, matching well predictions of a simple spherical pore model in all cases. [Copyright &y& Elsevier]

Abstract: Samples of binary Al–Cu alloys quenched from the mushy state during permeability measurements [Ø. Nielsen, L. Arnberg, A. Mo, H. Thevik, Metall. Mater. Trans. A 30A (1999) 2455–2462] were mapped in 3D using microtomography at the European Synchrotron Radiation Facility. Using 3D image analysis, the tomograms were binarised in order to separate the Al dendrite skeleton from the quenched Al–Al2Cu eutectic, i.e., the interdendritic liquid during the permeability measurement. Subsequently, the complete permeability tensor was calculated for each sample by solving Stokes equations in the 3D tomogram domain constituted by the interdendritic liquid. In general, there is a reasonable agreement between the trace of the calculated permeability tensors and the experimental reference data. Some deviations have been attributed to a tendency for preferential flow channels in one of the samples and to a too small calculation volume compared with the characteristic lengths of the microstructure in one of the samples. [Copyright &y& Elsevier]

The analyses of several materials exhibiting a cellular structure have been carried out using X-ray tomography. This new technique allows the three dimensional and non destructive visualisation of the studied materials at the scale of their cellular microstructure. Qualitative examples are given for metal foams, bread and cellular concrete. The similarity between these materials is striking. It has been measured by quantitative 3D image processing. The different Finite Element Methods available today to produce meshes from these images are presented and discussed in the final part of this paper. [Copyright &y& Elsevier]

Metallic materials processing such as rolling, extrusion or forging often involves high‐temperature deformation. Usually under such conditions the samples are characterized post mortem, under pseudo in situ conditions with interrupted tests, or in situ with a limited strain rate. A full in situ 3D characterization, directly during high‐temperature deformation with a prescribed strain‐rate scheme, requires a dedicated sample environment and a dedicated image‐analysis workflow. A specific sample environment has been developed to enable highly controlled (temperature and strain rate) high‐temperature deformation mechanical testing to be conducted while performing in situ tomography on a synchrotron beamline. A dedicated digital volume correlation algorithm is used to estimate the strain field and track pores while the material endures large deformations. The algorithm is particularly suitable for materials with few internal features when the deformation steps between two images are large. An example of an application is provided: a high‐temperature compression test on a porous aluminium alloy with individual pore tracking with a specific strain‐rate scheme representative of rolling conditions. [ABSTRACT FROM AUTHOR]

Time-resolved in situ microtomography is employed to track the nucleation and growth of individual pores during solidification of a commercial nickel-based superalloy. Three cooling rates (0.1, 0.5 and 1 °C/s) are investigated to evaluate the effect of this key processing parameter on the formation of porosity. Phase contrast obtained with a coherent X-ray beam is used to visualize the evolution of dendritic structures in absence of a sufficient absorption contrast. Two mechanisms leading to shrinkage pores have been identified. The first mechanism (mechanism A) is associated with the coalescence of secondary dendrite arms at temperature during the early stages of solidification. The second mechanism (mechanism B) is related to insufficient liquid feeding in the interdendritic region during the last stages of solidification, at lower temperatures. A variation of cooling rate by a factor 2 does not affect the nucleation rate of pores generated through mechanism B. However, it seems to affect the nucleation rate of small pores obtained through the mechanism A. The kinetics of growth for the majority of individual pores can be described using an exponential-like function. This kinetics is faster for mechanism B compared to mechanism A. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this paper, the Discrete Element Method (DEM) is used as a tool to phenomenologically model large compressive viscoplastic strain in metallic composites. The model uses pairwise attractive and repulsive forces between spherical particles. Large packings of particles collectively cope with the prescribed strain, the changes of neighbors model the irreversible strain in the material. Using the proposed calibration method of the model parameters, the macroscopic behavior mimics perfect plasticity in compression, with a strain rate sensitivity approximating a viscoplastic Norton law. The error on flow stress and volume conservation is estimated for single material. Three bi-material geometrical configurations are built: parallel, series and spherical inclusion. Macroscopic metrics (flow stress and shape factor of inclusion) are confronted to Finite Element Method (FEM) simulations. The potential of the model, from a computing power point of view, is tested on a complex geometry, using a real microstructure of a crystalline/amorphous metallic composite, obtained by X-ray tomography. [ABSTRACT FROM AUTHOR]

The performance of many advanced materials is determined by the arrangement of their nanostructure which requires ever more precise characterization. In this respect, X-ray computed tomography (CT) is a powerful technique to investigate material properties as it provides non-destructive direct access to three-dimensional morphology with nanoscale resolution. However, challenges remain in clearly understanding physical mechanisms involved during their processing in real time and real conditions. So far, beam and sample stabilities, effective spatial resolution and tomography scan time have hindered the development of nanoscale in situ 4D imaging (3D plus time), and especially at high temperatures. Here, we report on the development of fast X-ray nanotomography at temperatures up to 700°C with an unprecedented combination of nanometer pixel size and acquisition times of a few tens of seconds. The great potential of the method is demonstrated by following the early stages of two thermally driven phenomena: neck curvature evolution in sintering and nucleation of liquid droplets in light alloys. The reported real time observations will benefit the fundamental understanding of the underlying physics and provide useful data to build new models. The novel aspects of this synchrotron based technique offer a powerful imaging tool for a wide variety of heterogeneous nanoscale dynamics in materials and open new perspectives for the investigation of advanced materials under realistic conditions. [ABSTRACT FROM AUTHOR]

Using in situ synchrotron tomography, we investigate the coarsening dynamics of barium borosilicate melts during phase separation. The 3-D geometry of the two interconnected phases is determined thanks to image processing. We observe a linear growth of the size of domains with time, at odds with the sublinear diffusive growth usually observed in phase-separating glasses or alloys. Such linear coarsening is attributed to viscous flow inside the bicontinuous phases, and quantitative measurements show that the growth rate is well explained by the ratio of surface tension over viscosity. The geometry of the domains is shown to be statistically similar at different times, provided that the microstructure is rescaled by the average domain size. Complementary experiments on melts with a droplet morphology demonstrate that viscous flow prevails over diffusion in the large range of domain sizes measured in our experiments (1–80 μ m). [ABSTRACT FROM AUTHOR]

Abstract: The features associated with modern synchrotron radiation machines (intense and coherent beams) result in a substantial extension of X-ray imaging capabilities in terms of spatial and temporal resolution, phase contrast and 3D images. This allows crystal growth-related information to be obtained which is not available otherwise. After briefly describing the main synchrotron radiation based imaging techniques of interest, we give original examples illustrating the new capabilities for crystal growth: characterisation of crystals grown for applications, such as ice tri-crystals produced for mechanical deformation studies; SiC; crystalline silicon for solar photovoltaic cells; in situ and in real time studies of quasicrystal growth (AlPdMn); and ultrafast tomography for the study of the growth of dendrites in metallic alloys. [Copyright &y& Elsevier]

Ultrafast synchrotron microtomography has been used to study in situ and in real time the initial stages of silicate glass melt formation from crystalline granular raw materials. Significant and unexpected rearrangements of grains occur below the nominal eutectic temperature, and several drastically different solid-state reactions are observed to take place at different types of intergranular contacts. These reactions have a profound influence on the formation and composition of the liquids produced, and control the formation of heterogeneities. [ABSTRACT FROM AUTHOR]

Abstract: Titanium and its alloy foams have been receiving a growing interest for biomedical industry due to their biocompatibility and high potential in dental, spinal, joint and hip implant applications. Near net shape production and reproducibility of a product require a full understanding of the processing parameters’ effect on manufacturing and final properties. The shape complexity of titanium surgical implants, for example dental implants, necessitates a good control in compaction and sintering of metal skeletons containing macropores for near-net shape manufacturing. In this study, effects of processing parameters such as spacer powder size and amount on compaction, sintering and final structural properties were investigated. Commercially pure cellular titanium with porosities ranging between 35 and 75% and pore sizes between 100μm and a few millimeters were produced via powder metallurgy with spacers. Architectural parameters of the foams, before and after sintering, were quantitatively characterized via X-ray computed microtomography. Addition of fine spacer powders enhanced compaction and densification of the foam structure. Pore walls were observed to get thicker during sintering. The use of coarser space holders resulted in thicker pore walls and a wider pore wall thickness distribution. The results indicated that strength and stiffness tend to increase with increasing pore size. Among the structural features that spacer size altered, pore wall thickness and pore face roughness were found to be the most dominant effects on strength and stiffness. [Copyright &y& Elsevier]

Abstract: Titanium foams have been of interest in dental and orthopedic implants over the past few decades on account of their excellent mechanical properties, chemical stability, and biocompatibility. A powerful tool, X-ray computed microtomography was used to measure quantitatively the effect of pore morphology on foam architecture. Mechanical properties of titanium foams with varying pore structure were investigated. Aspect ratio of the pores was quantitatively demonstrated to affect strength, degree of anisotropy and strain-rate sensitivity of the produced titanium foams. Needle-like pored foams showed 30–55% lower strength when compared to the foams having lower aspect ratio pores. Lower aspect ratio pored foams were 3–11%, higher aspect ratio pored foams were 17–34% weaker in the direction parallel to the compaction direction when compared to the perpendicular one. High aspect ratio pores also resulted in more pronounced strain-rate sensitivity. [Copyright &y& Elsevier]

This work analyses the microstructure changes of various copper-based powder systems during sintering from 3D images provided by in situ synchrotron microtomography. The investigated systems include a copper powder with a wide particle size distribution of 0-63 µm poured into a quartz capillary, a pre-sintered copper compact with artificially created large pores and a mixture of copper and alumina particles. The experiments were carried out at the European Synchrotron in Grenoble, France. Powders were sintered up to 1060°C under reducing atmosphere in a furnace located between the X-ray source and the detector. During each experiment, 3D images were taken at various times of the thermal cycle. We have obtained images with a resolution of 1.5 µm and the time of acquisition of every image was ∼1 min. Quantitative analysis of these images allowed the changes of various important parameters to be followed. Such parameters characterise the sintering process at the particle length scale: interparticle coordination, pore size distribution and particle centre-to-centre distance. Moreover, by tracking the displacement of each particle centre and comparing it to the displacement predicted by classical mean field assumption, we have been able to assess the magnitude of particle rearrangement occurring during sintering. From these data, the sintering behaviour of heterogeneous powder systems is discussed with particular emphasis of collective particle phenomena. [ABSTRACT FROM AUTHOR]

Abstract: The use of synchrotron radiation X-ray microtomography rests on the exploitation of the beam coherence, the high spatial (submicron) or temporal resolution (s), in situ and real time experiments and quantitative measurements. Selected original applications showing these capabilities are presented. They include the phase contrast imaging of Si particles in Al alloys, the nucleation of fatigue cracks in structural materials, the visualization of hollow silica spheres within polymer foams, and the real time observation of the semi-solid state in Al–Cu alloys. [Copyright &y& Elsevier]

Abstract: In replication processing of open-pore aluminium foams, the relative density can be varied by densifying the NaCl preform before infiltration. This can be done either by cold pressing or by sintering; we compare the two processes, show that the former yields superior foam modulus and flow stress, and explain why. [Copyright &y& Elsevier]