Synchrotron Radiography Research Articles

Dilatancy in semi-solid steels at high solid fraction

The study of mushy-zone deformation in steels is important for limiting defect formation in continuous casting. Here, we use in situ synchrotron radiography to quantify the shear deformation mechanisms of an equiaxed carbon steel at a solid fraction >0.9 and to understand how these mechanisms lead to casting defects. We show that the grain assembly undergoes shear-induced dilation (Reynolds' dilatancy) which opens liquid-filled fissures and cracks at high solid fraction. We further show a complex interaction between grain rearrangement, grain deformation and local coarsening, where rearrangement reduces the grain-grain contact area and coordination number which alters the stress network and also drives coarsening as grains move apart.

Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging

The influence of artificial ageing of gas diffusion layers (GDLs) on the cell performance was investigated using high resolution synchrotron radiography. State-of-the-art GDLs of the type SIGRACET® SGL 25BC were aged for 0 h, 16 h and 24 h in a hydrogen peroxide solution before they were assembled in the fuel cells. In-operando radiographic measurements were combined with voltage and contact angle measurements. Correlations between applied ageing conditions, GDL water saturation and cell performance were revealed. Hereby, all cell operating conditions were tested several times to estimate the reproducibility of in-operando radiographic fuel cell measurements.Water films at the GDL-membrane and at the GDL-flow field interfaces were found and attributed to MPL cracks and large pores in the GDL structure. The combination of these cracks and pores are assumed to play a crucial role for blocked gas paths, leading to an undersupply with reactants and an increased humidification of the membrane. It is shown that water agglomerations directly impact the membrane resistance. We assume that the hydrophobicity of the fibers inside the GDL is more important for the cell performance than water agglomerations at the membrane-GDL interface.

Imaging of Structure Evolution in Solidifying Al–Bi Immiscible Alloys by Synchrotron Radiography

Using synchrotron X-ray imaging technique, the segregation evolution in solidifying Al–10 wt% Bi immiscible alloys was investigated at different cooling rates. Irrespective of the cooling rate, most of the Bi solute appeared at the upper part of the sample after solidification. The reason for this Bi enrichment phenomenon is different for different cooling rates. Besides Marangoni motion, positive segregation, which has rarely been noticed before, can also make Bi solute transfer to the hot top zone. It is also found that, bubbles (or pores) appear in solidifying Al–10 wt% Bi alloys, and the number of bubbles (or pores) increases with the increase of the cooling rate, while the size of the bubbles (or pores) decreases.

The Impact of Force Transmission on Narrow-Body Dental Implants Made of Commercially Pure Titanium and Titanium Zirconia Alloy with a Conical Implant-Abutment Connection: An Experimental Pilot Study.

The purpose of this study was to visualize the mode and impact of force transmission in narrowdiameter implants with different implant-abutment designs and material properties and to quantify the displacement of the abutment. Narrow-diameter implants from two manufacturers were examined: Astra 3.0-mm-diameter implants (Astra OsseoSpeed TX; n = 2) and Straumann Bone Level implants with a 3.3-mm diameter made of commercially pure titanium (cpTi) Gr. 4 (n = 2) and 3.3-mm TiZr-alloy (n = 2; Bone Level, Straumann) under incremental force application using synchrotron radiography (absorption and inline x-ray phase-contrast) and tomography. During loading (250 N), Astra 3.0 and Bone Level 3.3- mm implants showed a deformation of the outer implant shoulder of 61.75 to 95 μm independent of the implant body material; the inner implant diameter showed a deformation of 71.25 to 109.25 μm. A deformation of the implant shoulder persisted after the removal of the load (range, 42.75 to 104.5 μm). An angulated intrusion of the abutment (maximum, 140 μm) into the implant body during load application was demonstrated; this spatial displacement persisted after removal of the load. This study demonstrated a deformation of the implant shoulder and displacement of the abutment during load application in narrow-diameter implants.

Impact of Liquid Water on Oxygen Transport Resistance in a PEMFC Cathode Via Limiting Current Methods and Concurrent Synchrotron X-Ray Radiography

The accumulation of liquid water formed by the electrochemical reaction of hydrogen and oxygen in a polymer electrolyte membrane (PEM) fuel cell causes a gas diffusion layer (GDL) to become effectively less porous and more tortuous. To achieve designs directed for water and gas transport in PEMFC diffusion media, a nuanced understanding of the nature and extent of GDL water as an impedance to oxygen flux is required. In this work, a limiting current approach was used to measure the mass transport resistance in a custom PEM fuel cell. Using a small active area at high stoichiometric ratios, it was assumed that the concentration of gases was constant along the length of the reactant flow channels, creating a uniform concentration gradient across the entire diffusion medium. If the fuel cells are then operated at the limiting current density, where cell potential approaches zero, it can also be assumed that all the reactants are being consumed and the concentration of reactants at the reaction site is effectively zero. Consequently, oxygen transport resistance in these conditions can be inferred directly from the limiting current density using the following equation, where RT is the transport resistance in s/cm, c0 is the oxygen concentration at the reactant channel and iL is the limiting current: RT = 4Fc0/iL Previously, through a thorough set of limiting current experiments at a range of operating conditions, Baker et al.1 showed that the carbon-fiber diffusion medium is the dominant contributor to oxygen transport resistance. Owejan et al.2 combined limiting current methods with in situ neutron radiography to demonstrate a strong qualitative correlation between GDL saturation and oxygen transport resistance. Synchrotron radiography has been established as a powerful tool for visualizing water distribution in PEMFC diffusion media3. In this work, synchrotron X-ray radiographs were obtained with a pixel size of 6.5 µm with a spatial resolution of 10 μm. GDL materials were imaged in operando at limiting current over a range of relative humidities (set in the inlet cathode gas stream). Figure 1 (a) is an example radiograph of a cell operating at limiting current density processed according to the Beer-Lambert law5,6. A distinct in-plane variation in liquid water distributions is visible between regions over ribs and regions over channels (Figure 1 (b)). This work exploits the high spatial resolution for capturing liquid water accumulation trends, information which is essential for building towards a direct, quantitative empirical relation between oxygen transport resistance and GDL saturation.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers: Mass Transport Resistance and Liquid Water Accumulation at Limiting Current Density with in operando Synchrotron X-ray Radiography

The polymer electrolyte membrane fuel cell (PEMFC) continues to develop as a viable alternative to the combustion engine in automotive applications. As this technology advances, it is critical that the PEMFC is capable of maintaining performance for long term operation. In an effort to understand the degradation of fuel cell performance over time, a study into the development of an accelerated gas diffusion layer degradation protocol was completed. As part of an effort to quantify the effects of this accelerated degradation, in situ synchrotron radiography imaging techniques were coupled with performance testing in order to quantify the effects of this degradation on water saturation profiles, transport resistance, and limiting current in an operating fuel cell. Gas diffusion layer samples of SGL 25 BC and SGL 29 BC were artificially aged in a concentrated solution of hydrogen peroxide (30% wt.) for a period of 12 hours at an elevated temperature of 90 degrees Celsius. Hydrogen peroxide facilitates chemical corrosion of the carbon material in the gas diffusion layer and was found to significantly affect the wettability of the degraded samples. Hydrogen peroxide was selected to facilitate the degradation mechanism due to the fact that it is a recognized chemical species found in operating fuel cells and produced at the catalyst layer (1, 2). The accelerated degradation procedure that was used in this study was found to primarily affect the wettability of the tested gas diffusion layers. Consequently, it was expected that the most significant impact of this degradation mechanism would be in the mass transport losses at high current densities. For this reason, a limiting current investigation was performed. Unlike other studies of limiting current in which the cathode oxygen concentration is varied (3), a limiting current study based on varying the relative humidity of reactant gases was performed. In this study, limiting current was measured for relative humidities of 0%, 50%, 80%, 90%, and 100% for several fuel cells with fresh and degraded GDLs. Simultaneous synchrotron imaging was used to quantify the distribution of liquid water in the anode and cathode of the operating cell during these limiting current studies. Trends with respect to reactant transport resistance, water saturation profiles, and limiting current were quantified as a function of relative humidity and degree of degradation. For all tested samples, as the relative humidity was reduced, the corresponding limiting current increased. It was also found that the limiting current liquid water saturation profile was independent of the reactant gas relative humidity. Additionally, artificial degradation led to increased liquid water saturation levels in the operating fuel cell. This work illustrates the potential impacts of long term fuel cell operation on the water management of PEMFC gas diffusion layers.

Observation of Bi Coarsening and Dissolution Behaviors in Melting Al–Bi Immiscible Alloy

Using synchrotron radiography, a melting process of the Al-10 wt% Bi immiscible alloy was fully observed, and Bi solute coarsening and dissolution behaviors as well as their effect on the microstructural evolution were also investigated. We found that two different steps including coarsening and dissolution processes can be used to describe the structure evolution in melting Al-10 wt% Bi immiscible alloy. And diffusion coalescence controlled the coalescence of Bi droplets at the early stage of the melting Al-10 wt% Bi immiscible alloy, while the coagulation during movement did at later stage. Also, the larger the droplet, the larger the distance of the steady-state concentration distribution around the droplet.

Interaction between L2 droplets and L1/L interface in solidifying Al–Bi immiscible alloy

Using synchrotron radiography, we observed the advancing L1/L interface and its interaction with L2 droplets during directional solidification of the Al-10wt% Bi immiscible alloy. Five forces act upon L2 droplets in front of the L1/L interface. The repulsive force that pushes the droplets away from the interface is the largest force when the droplet radius (r) is smaller than 20.5µm, while Marangoni force is the largest one when 20.5µm <r<89µm. And the resultant force that is responsible for the pushing of the droplets is larger than the drag force when r<141.5µm, leading to upward movement of L2 droplets.

In Situ Radiographic Investigation of (De)Lithiation Mechanisms in a Tin-Electrode Lithium-Ion Battery.

The lithiation and delithiation mechanisms of multiple-Sn particles in a customized flat radiography cell were investigated by in situ synchrotron radiography. For the first time, four (de)lithiation phenomena in a Sn-electrode battery system are highlighted: 1) the (de)lithiation behavior varies between different Sn particles, 2) the time required to lithiate individual Sn particles is markedly different from the time needed to discharge the complete battery, 3) electrochemical deactivation of originally electrochemically active particles is reported, and 4) a change of electrochemical behavior of individual particles during cycling is found and explained by dynamic changes of (de)lithiation pathways amongst particles within the electrode. These unexpected findings fundamentaly expand the understanding of the underlying (de)lithiation mechanisms inside commercial lithium-ion batteries (LIBs) and would open new design principles for high-performance next-generation LIBs.

In situ synchrotron study of liquid phase separation process in Al-10 wt.% Bi immiscible alloys by radiography and small angle X-ray scattering

Liquid phase separation process of immiscible alloys has been repeatedly tuned to create special structure for developing materials with unique properties. However, the fundamental understanding of the liquid phase separation process is still under debate due to the characteristics of immiscible alloys in opacity and high temperature environment of alloy melt. Here, the liquid phase separation process in solidifying Al-Bi immiscible alloys was investigated by synchrotron radiography and small angle X-ray scattering. We provide the first direct evidence of surface segregation prior to liquid decomposition and present that the time dependence on the number of Bi droplets follows Logistic curve. The liquid decomposition results from a nucleation and growth process rather than spinodal decomposition mechanism because of the positive deviation from Porod's law. We also found that the nanometer-sized Bi-rich droplets in Al matrix melt present mass fractal characteristics.

Synchrotron radiographic studies of ultrasonic melt processing of metal matrix nano composites

Fast synchrotron radiography was used to investigate ultrasonic cavitation bubble formation and their dynamics during liquid metal processing of Al–Cu metal matrix nano composites (MMNC) in comparison with conventional alloys. The experimental observations showed enhanced cavitation potential in MMNC melts, due to the presence of Al2O3 nano particles which were believed to be acting as heterogeneous nuclei for bubble formation. Quantitative image analysis demonstrates that the addition of nano particles increases melt agitation partially, while introducing higher flow velocity variations across the melt. This suggests that the presence of nano particles may substantially alter propensity for ultrasonic treatment effects during solidification processing of MMNCs.

Quantifying Percolation Events in PEM Fuel Cell Using Synchrotron Radiography

The distribution of independent water clusters within the gas diffusion layer (GDL) is an important, yet poorly understood, characteristic of polymer electrolyte membrane fuel cells. A better understanding of these water clusters would provide ex-situ invasion experiments and two-phase models with a set of validation criteria that is currently absent from the literature. Synchrotron based X-ray radiography was employed visualize liquid water emerging from the polymer electrolyte membrane fuel cell GDL. Droplet formations, entitled “breakthrough” events originated from either the channel or landing regions of the GDL. The number of breakthrough events in a given area (breakthrough density) provides insight into the size and number of independent water clusters evolving within the GDL. Water clusters were found under the flow field landings more frequently than under the gas channels. Each 1mm2 of projected GDL area was found to have 1-2 individual water clusters during most conditions studied, regardless of the GDL substrate or MPL type. The existence of percolating water clusters under flow field channels depended on the combination of GDL type and operating conditions employed.

In-Operando Neutron Radiography Studies of Polymer Electrolyte Membrane Water Electrolyzers

Polymer Electrolyte Membrane Water Electrolysis (PEMWE) is increasingly attracting interest from academia and industry as it offers an option for storing renewable energy from sources such as wind or solar [1]. Further, electrolysis is considered one of the few available pathways to achieve a 100% renewable electricity supply [2]. In PEMWE, water is typically supplied via flow channels and distributed across the catalyst layer through a porous transport layer (PTL). Gas evolution occurs at the catalyst layer, and the gas is transported back through the PTL and discharged into the flow channels where it is then transported out of the cell as a two-phase mixture with the feed water. This evolution of oxygen and hydrogen in the surrounding water leads to distinct two-phase flow phenomena, which are investigated in this study in-operando inside operating PEMWE cells using synchrotron X-ray radiography as well as neutron radiography. In the past, both methods have allowed to visualize processes inside operating fuel cells, elucidating water management issues [3], the formation of liquid water, its accumulation rate and transport in polymer electrolyte membrane fuel cells (PEFC) [4], as well as the carbon dioxide evolution in liquid fed direct methanol fuel cells (DMFC) [5]. PEM water electrolysis systems are currently scaled to the megawatt range, necessitating an increase of the active area of an individual cell. This scale-up goes along with challenges concerning media distribution on large cell areas. Information about the oxygen saturation and distribution inside the PTL at different points of operation but also at different locations a large area PTL is therefore of special interest. In this work, operating PEMWE cells are examined using synchrotron X-ray radiography at BESSY II and using neutron radiography at BER II (both Helmholtz-Zentrum Berlin). In both setups, a synchrotron X-ray beam or a neutron beam is directed at the running cell, and the attenuation dependence of the beam on the elements inside the cell allows visualizing the processes occurring inside the cells. Both methods are to a certain degree complimentary – the synchrotron radiography setup allows inspecting small areas of some mm² with a spatial resolution of a few µm, while the neutron radiography allows inspecting areas of several cm² with a spatial resolution of lower than 100µm. The especially strong neutron attenuation in hydrogen offers the possibility to visualize the gas/water distribution on large areas, while the comparably low neutron attenuation in metals allows using a typical cell setup with only minor adjustments. The PEMWE cells examined using neutron radiography contained different PTLs, which were compared in terms of the gas/water distribution under different operating conditions. On the anode side, an additional injection of oxygen from the bottom of the cell was used to simulate the effects occurring in a scaled cell exhibiting a larger area. This setup allowed examining the dynamics in the transport and quantifying the distribution along the cell area. In the PEMWE cells examined using synchrotron radiography, oscillations in the gas bubble discharge from the PTL into the flow channel were investigated in terms of their frequency and the gas discharge volumes. Furthermore, the implications of this on the gas transport within the PTL are discussed, and the number of gas bubble discharge sites is correlated with the current density. Thereby it is demonstrated that both synchrotron X-ray and neutron radiography represent valuable tools to study transport processes and to gain insights into the gas-water distribution inside running PEMWE cells. These results pose important implications for the design of cell components with facilitated gas removal, for the modeling of two-phase flow in PEMWE cells and for the modeling of mass transport losses in PEMWE.

In situ analysis of voltage degradation in a polymer electrolyte membrane fuel cell with a dead-ended anode

In this paper, the impact of dead-ended anode (DEA) operation on the liquid water management of polymer electrolyte membrane fuel cells (PEMFCs) was investigated using X-ray synchrotron radiography. The fuel cell voltage and concurrent through-plane liquid water saturations in anode and cathode gas diffusion layers (GDLs) were measured during DEA and open anode operation. It was observed that DEA operation led to significant increases in liquid water content in both anode and cathode GDLs, along with fuel cell power output degradation. A strong correlation was observed between the buildup of the liquid water in the anode and the cathode, and as a result, the liquid water transport in the cathode was also significantly impacted by this DEA operation.

Direct observation of the segregation driven by bubble evolution and liquid phase separation in Al–10 wt.% Bi immiscible alloy

The effects of the growth behaviors of bubbles and minor droplets on the segregation in solidifying Al–10 wt.% Bi immiscible alloy were investigated by synchrotron radiography. The Bi atoms were effectively trapped at the surface of bubbles, favorable to the bubble growth. With cladding Bi layer, the stochastic growth of bubbles remains, but the scaled size distribution follows neither Gaussian nor Lifshitz–Slyozov–Wagner (LSW) controlled distributions. The variation in Bi droplet number over time obeys a logistic curve.

The Effect of Nitrate on Salt Layers in Pitting Corrosion of 304L Stainless Steel

Current oscillations were observed during one-dimensional pitting corrosion of 304 L stainless steel in neutral 1 M NaCl solutions with varying NaNO3 concentrations. Synchrotron X-ray diffraction was used to identify the salt layer at the corrosion front. It was found that, although current oscillations were induced in solutions with higher concentrations of NaNO3, the salt species in the pit did not change and a nitrate-free salt was present in all solutions. At higher NaNO3 concentrations, a change of salt crystal morphology was detected. Electrochemical oscillations were seen to coincide with secondary pitting on the pit surface indicating that two corrosion regimes were operating in parallel. Synchrotron radiography was used on artificial pits to measure the change in corrosion front and material loss in situ. Before nitrate was added, the corrosion front showed non-uniform material loss across the interface when beneath the salt layer. Nitrate addition induced a local region of passivation that propagated across the pit surface. Surface roughness was quantified using R-values and seen to vary without a clear trend until passivation, after which it stayed constant. A mechanism is suggested in which partial passivation occurs in these systems, where passivated areas are undercut as the corrosion front moves, generating surges in current.

In Situ Synchrotron Radiography and Spectrum Analysis of Transient Cavitation Bubbles in Molten Aluminium Alloy

The melt processing of conventional and advanced metallic materials with high-intensity ultrasonic vibrations significantly improves the quality and properties of molten metals during their solidification. These improvements are primarily attributed to ultrasonic cavitation: the creation, growth, pulsation, and collapse of bubbles in the liquid. However, the development of practical applications is limited by the lack of fundamental knowledge on the dynamics of the cavitation bubbles; it is very difficult to directly observe ultrasonic cavitation using conventional techniques in molten metals due their high temperature and opaqueness.In this study, an in situ synchrotron radiography experiment was performed to investigate bubble dynamics in an Al-10wt.% Cu alloy under an external ultrasound field at 30kHz. Radiographs with an exposure time of 78ms were collected continuously during the sonication of molten alloys at temperatures of 660±10°C. To the best of our knowledge, this is the first time that transient cavitation bubbles have been observed in liquid aluminium. Quantification of bubble parameters such as average size and time of collapse were evaluated from radiographs using advanced image analysis. Additionally, broadband noise associated with the acoustic emissions from shock waves of transient cavitation bubbles and estimation of the real-time acoustic pressure at the driving frequency were assessed using an advanced high-temperature cavitometer in separate bulk experiments.

Retrieval of three-dimensional spatial information from fast in situ two-dimensional synchrotron radiography of solidification microstructure evolution

In situ synchrotron X-ray radiography of columnar dendritic growth in Al–15wt.% Cu–9wt.% Si–0.015wt.% Sr alloy has been carried out with the temporal and spatial resolutions of 100ms and 0.65μm, respectively. Two-dimensional (2-D) projected images have been processed and analysed to retrieve three-dimensional (3-D) spatial information on the growing dendrite network, through X-ray transmission contrast differences that account for solute-composition variations in the liquid and thickness variations in the growing dendrites. The analysis elucidates the way in which gravity-driven natural convection and solute sedimentation can result in solid fractions that deviate markedly from Scheil and Lever-rule-based models, which could alter mushy zone permeability. Furthermore, the ability to render 3-D spatial details may lead to future opportunities to use 2-D radiography in the development and validation of more sophisticated 3-D models of dendritic growth.

Recent Experimental and Numerical Developments in Semisolid Deformation

Semisolid deformation in metallic alloys has been studied for many years, both experimentally and through numerical simulations, because mushy zone mechanics plays a key role in the quality of metalcasting processes. For researchers, the topic of semisolid deformation is particularly interesting due to the concomitant phenomena of liquid metal flow, the evolution in fraction solid, and grain coalescence. At the same time, the high temperatures, opacity of the melt, and the strong sensitivity to alloy composition and cooling rates pose strong challenges requiring sustained research effort. This section of JOM explores recent experimental and numerical developments in semisolid deformation, along with their applications for improving industrial processing. Five diverse articles are presented to highlight current cutting-edge research, from experiments that can directly observe grain rearrangement under various loading modes to models that simulate dendrite deformation. The first two articles showcase exciting developments in the use of x-ray radiography to directly observe semisolid micromechanics in thin-sample Al alloy and steel specimens. The article ‘‘In situ Investigation of Dendrite Deformation during Upward Solidification of Al-7wt.%Si’’ by Reinhart et al. examines the phenomena of dendrites bending due to gravity, which can precede fragmentation in the case of well-developed secondary arms. The article ‘‘Synchrotron Radiography Studies of ShearInduced Dilation in Semisolid Al Alloys and Steels,’’ by Gourlay et al. focuses on the phenomena of Reynolds dilatancy during shear deformation in alloys with morphologies ranging from equiaxeddendritic to globular. While x-rays can be used to observe local semisolid deformation, the technique known as neutron diffraction is also being used to provide exciting new experimental results to examine structural transitions within the semisolid region. In the article ‘‘Deformation of Coherency and Rigidity Temperatures in Al-Cu Alloys using In-Situ Neutron Diffraction during Casting,’’ by Drezet et al., measurements are reported that quantify the point at which thermally induced deformations start to generate internal stresses in a casting as a function of cooling rates. This knowledge is required for macro-scale casting process simulations. The final two articles usecomputational modeling to advance our fundamental knowledge of deformation and defects. The article by Yamaguchi and Beckermann, ‘‘Direct Numerical Simulation of Solid Deformation during Dendritic Solidification,’’ combines a polycrystalline phase-field model with a material point stress analysis to numerically simulate the fully coupled dendritic solidification and elasto-viscoplastic deformation behavior of a pure substance. The results indicate that deformation modifies the crystallographic orientation of a dendrite and the subsequent mushy zone solidification behavior. In ‘‘Prediction of Hot Tearing using a Dimensionless Niyama Criterion,’’ by Monroe and Beckermann, the well-known Niyama criterion is extended to include the effect of applied strain. This new criterion can be easily implemented in industrial casting simulation software to predict both shrinkage porosity and hot tearing susceptibility in a casting process. Collectively, this series of articles provides new insight into the effects of phenomena at various length-scales on mechanical deformation and failure during solidification. New experimental techniques and new modeling methods are providing exciting results that are key for improving both fundamental understanding of semisolid deformation and industrial casting processes. The reader is also referred to prior JOM issues for other articles on similar themes. Andre B. Phillion is the guest editor for the Solidification Committee of the TMS Materials Processing & Manufacturing Division, and coordinator of the topic Semisolid Deformation: Experimental and Numerical Developments in this issue. JOM, Vol. 66, No. 8, 2014

Synchrotron Radiography Studies of Shear-Induced Dilation in Semisolid Al Alloys and Steels

An improved understanding of the response of solidifying microstructures to load is required to further minimize casting defects and optimize casting processes. This article overviews synchrotron radiography studies that directly measure the micromechanics of semisolid alloy deformation in a thin-sample direct-shear cell. It is shown that shear-induced dilation (also known as Reynolds’ dilatancy) occurs in semisolid alloys with morphologies ranging from equiaxed-dendritic to globular, at solid fractions from the dendrite coherency point to ~90% solid, and it occurs in both Al alloys and carbon steels. Discrete-element method simulations that treat solidifying microstructures as granular materials are then used to explore the origins of dilatancy in semisolid alloys.