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The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol%Al2O3 MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images. [ABSTRACT FROM AUTHOR]

In twin-wire arc spraying (TWAS), the in-flight particles are atomized from a melting bath which generates an inhomogeneous spraying plume. This inhomogeneity is due to the fact that these particles are generated by the impingement of fast continuous flowing air upon the melting tips of electrically conductive wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. For this purpose, cored wires filled with W-rich particles were sprayed. After interrupting the TWAS process, the tips of these cored wires were imaged by 3D µCT and scanning electron microscopy in order to analyze how the filling powder interacts with the melted part of the sheath. The analysis of the 3D tomograms shows that the resolidified melting bath of the cored wires is interspersed with both spherical and irregular-shaped W-rich particles. This irregular shape suggests a partial melting of the W-rich particles. [ABSTRACT FROM AUTHOR]

Abstract: The impact of very high cycle fatigue (VHCF) load conditions on the microstructure of specimens consisting of nodular cast iron is analyzed by means of micro-computed tomography utilizing both monochromatic synchrotron radiation and polychromatic X-ray tube radiation. Using 3D , the microstructure in the region of the smallest cross-sections of shouldered round specimens is imaged in different stages of the VHCF loading. By digital image correlation (DIC) of these tomograms strain fields are analyzed three-dimensionally. Strain levels in the range of a few percent were detected. It is proven that a localization of strain allows to predict the site of the crack which precedes and induces the macroscopic failure of the specimens. [Copyright &y& Elsevier]

The formation of crack patterns in drying starch-water slurries is studied by means of in-situ radiography (measuring of the crack front velocity) and X-ray microtomography as an example of crack patterns driven by inhomogeneous shrinkage. The tomograms show the 3D crack networks forming columns with polygonal cross-sections. After crack initiation, the average crack spacing increases with growing depth, even if the crack front velocity is constant. A constant velocity is obtained by maintaining a constant evaporation rate using a feedback control. When the crack front has propagated at a constant evaporation rate over a distance of some millimeters, the average crack spacing approaches a stable value which depends on crack front velocity according to a power law. This relationship is compared to corresponding results of other authors and model predictions. The increase of crack spacing before stable values are achieved, is interpreted as a result of successive crack front instabilities. [ABSTRACT FROM AUTHOR]

Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes. [ABSTRACT FROM AUTHOR]

Abstract: Soil deformation is a perpetual process in the pedosphere where besides physicochemical stresses primarily alternating hydraulic and mechanical stresses continuously re-arrange the configuration of solid particles. In this study we present a local strain analysis and changes in soil structure resulting from hydraulic and mechanical stresses based on X-ray microtomography data. Digital image reconstructions were used to quantify local structural pore space characteristics and local soil deformation by 3D morphological and correlation analysis of grayscale tomograms. Swelling and shrinkage resulted in a complex heterogeneous soil structure which proofed to be very stable when mechanical loads were applied. The mechanism of soil deformation for both structure formation by internal hydraulic stresses and structure degradation by external mechanical stresses were in both cases controlled by pre-existing (micro)-structures. Especially during wetting such structures served as a nucleus for subsequent structure evolution. The results demonstrate the potential of more detailed non-invasive micromechanical analysis of soil deformation processes which could improve the conceptual understanding of the physical behavior of soil systems. [Copyright &y& Elsevier]

In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have a major impact on the microstructure as well as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings. [ABSTRACT FROM AUTHOR]

The objective of this article is to develop and apply a model for the design and evaluation of X-ray diffraction experiments to measure phase-specific residual stress profiles in multilayer systems. Using synchrotron radiation and angledispersive diffraction, the stress measurements are performed on the basis of the sin2 method. Instead of the traditional Ω or X mode, the experiments are carried out by a simultaneous variation of the goniometer angles X, Ω and ψG to ensure that the penetration and information depth and the measuring direction ψ remain unchanged when the polar angle ψ is varied. The applicability of this measuring and evaluation strategy is demonstrated by the example of a multilayer system consisting of Ti and TiAlN layers, alternately deposited on a steel substrate by means of physical vapour deposition. [ABSTRACT FROM AUTHOR]

The objective of this article is to develop and apply a model for the design and evaluation of X-ray diffraction experiments to measure phase-specific residual stress profiles in multilayer systems. Using synchrotron radiation and angledispersive diffraction, the stress measurements are performed on the basis of the sin²Ψ method. Instead of the traditional Ω or χ mode, the experiments are carried out by a simultaneous variation of the goniometer angles χ, Ω and φG to ensure that the penetration and information depth and the measuring direction φ remain unchanged when the polar angle ψ is varied. The applicability of this measuring and evaluation strategy is demonstrated by the example of a multilayer system consisting of Ti and TiAlN layers, alternately deposited on a steel substrate by means of physical vapour deposition. [ABSTRACT FROM AUTHOR]

Inconel 718 specimens were additively manufactured via selective laser melting (SLM) and subjected to different post-process heat treatments. Performance under monotonic and cyclic loading in the low-cycle fatigue (LCF) regime was investigated at room temperature (RT). Hot isostatic pressing (HIP) was employed in order to reduce the porosity typically present after SLM. Prior functional encapsulation with a Ni-20Cr coating applied by cathodic arc deposition (Arc-PVD) was applied to achieve full densification. The results show that after Arc-PVD and HIP, the mechanical properties are not only affected by reduced porosity but also by substantial microstructural changes. Precipitates, i.e. Laves phase, already present in the as-built condition can be dissolved by solution heat treatment. HIP leads to recrystallization and thus significantly changes microstructural appearance. Elongated grains and sub-micron sized cell structures stemming from SLM processing are eliminated by HIP. Aging leads to evolution of strengthening γ″ precipitates. Concurrently, δ phase forms upon HIP and aging. In comparison to aged conditions that were not subjected to HIP, microstructure upon HIP and aging results in improved ductility under monotonic loading, however, concomitantly deteriorates fatigue properties at RT. [ABSTRACT FROM AUTHOR]

Because of their excellent properties aluminum and its alloys cover a wide range of applications especially in the lightweight construction sector. In order to reach a higher strength and wear resistance metal matrix composites (MMCs) are used. Typically Al MMCs are manufactured by casting or extrusion processes. The disadvantage of these production routes is a cost-intensive and time-consuming finishing in terms of grinding and milling. The technique of thermal spraying provides the possibility to coat aluminum parts with MMCs close to their final shape. In addition to the shape accuracy the ductility and toughness of the coated parts are generally higher compared to extruded or casted parts. This study describes the development of detonation-sprayed boron carbide reinforced aluminum coatings on aluminum (EN AW 5754) substrates. The optimization of the coatings was focused on a homogeneous coating structure, a low coating porosity, a high deposition efficiency, a high number of embedded carbides, and a small percentage of oxides. In continuous tensile tests the influence of the MMC coating on the tensile strength was determined. Furthermore, the tensile strength was investigated in a discontinuous tensile test step by step. The different stages of deformation were analyzed by using micro computed tomography. This method enables the observation of tensile specimens in 3D, and consequently the site and moment of crack formation. [ABSTRACT FROM AUTHOR]

The powder injection parameters, the location of the injection port, as well as the metal matrix composites are important features, which determine the deposition efficiency and embedding behavior of hard materials in the surrounding matrix of the twin wire arc-spraying process. This study investigates the applicability of external powder injection and aims to determine whether the powder injection parameters, the location, and the material combination (composition of the matrix as well as hard material) need to be specifically tailored. Therefore, the position of the injection port in relation to the arc zone was altered along the spraying axis and perpendicular to the arc. The axial position of the injection port determines the thermal activation of the injected powder. An injection behind the arc, close to the nozzle outlet, seems to enhance the thermal activation. The optimal injection positions of different hard materials in combination with zinc-, nickel- and iron-based matrices were found to be closer to the arc zone utilizing a high-speed camera system. The powder size, the mass of the particle, the carrier gas flow, and the electric insulation of the hard material affect the perpendicular position of the radial injection port. These findings show that the local powder injection, the wetting behavior of particles in the realm of the molten pool as well as the atomization behavior of the molten pool all affect the embedding behavior of the hard material in the surrounded metallic matrix. Hardness measurement by means of nanoindentation and EDX analysis along transition zones were utilized to estimate the bonding strength. The observation of a diffusion zone indicates a strong metallurgical bonding for boron carbides embedded in steel matrix. [ABSTRACT FROM AUTHOR]