High-energy X-ray Diffraction Experiments Research Articles

Ferrite Recrystallization Characterization by Isolated Diffraction Spot Tracking During High-Energy X-ray Diffraction Experiments

Ferrite Recrystallization Characterization by Isolated Diffraction Spot Tracking During High-Energy X-ray Diffraction Experiments

Investigation of the high-temperature fatigue mechanism related to altered local short-range order in AL6XN austenitic stainless steel

An increase in crystal local short-range order (SRO), characterized by obvious diffuse intensity maxima at 1/3{422} in the [111]-axis selected area diffraction pattern, is observed in AL6XN austenitic stainless steel subjected to high-temperature low-cycle fatigue deformation. SRO is an ordered fcc structure with nanometer size, which can be well explained by the ordered arrangement of Cr/Mo atomic layers segregated on the {422} crystal plane in austenite. Many planar slip bands are observed in the fatigue sample, the distribution of SROs is highly related with the distribution of dislocations. In situ high-energy X-ray diffraction (HE-XRD) experiments are performed to investigate the deformation behavior of both as-received and fatigued samples under uniaxial loading. The lattice strain evolution reveals an interesting lattice expansion in the initial elastic stage, which provides direct evidence of a decrease in SRO during further uniaxial tensile deformation, indicating that the deformation modes greatly affect the SRO transition. We believe that the high-temperature fatigue damage originates mainly from the accumulation of planar slip bands, accompanied by a local increase in SRO, which promotes work hardening near the final fatigue fracture.

Impact of nano-scale cavities on hydrogen storage and retention in yttrium hydride

In situ synchrotron high-energy x-ray diffraction experiments and detailed transmission electron microscopy (TEM) characterization were conducted on as-fabricated and neutron-irradiated yttrium hydrides. The high-resolution synchrotron x-ray diffraction revealed minor α yttrium and major δ yttrium hydride phases in all specimens. Specimens were subject to heat treatments (heating-cooling cycles), and the intensity of α yttrium partially and completely disappeared in as-fabricated and neutron-irradiated specimens, respectively. The disappearance of α yttrium was unforeseen because hydrogen was expected to leave δ phase, causing an increase in α yttrium diffraction peak intensity. This observation indicated a surplus of hydrogen in the specimens where it was odd for hydride-forming early transition metal elements. The subsequent through-focus TEM characterization discovered nanometric cavities in both as-fabricated and neutron-irradiated yttrium hydride specimens for the first time. Two types of cavities were identified as fabrication-caused and irradiation-induced. The fabrication-caused cavities were associated with regions having linear deformation features, interfaces, and inclusions. The irradiation-induced cavities were observed as being formed isolated in the yttrium hydride phase. The presence of such nanometric cavities was considered as potential hydrogen storage pockets where the overall hydrogen storing capacity of yttrium hydride would be enhanced.

Probing lead acetate in solution using X-ray diffraction

For the purpose of water purification involving toxic metal sensing and removal, high energy X-ray diffraction experiments have been performed on lead acetate Pb(Ac)2 solutions, liquid and amorphous glutathione disulfide (GSSG), and their mixtures. The data have been interpreted using Empirical Potential Structure Refinement and pair distribution function analysis. At the highest concentration of 1 M Pb(Ac)2 in water, the lead molecules are found to cluster and the second shell in the water structure becomes slightly more ordered as the water molecules become compressed. Liquid and amorphous GSSG are found to hydrogen bond primarily via OH-O interactions, while NH-O bonds are much more distorted. Aqueous solutions of Pb(Ac)2 + GSSG show the closest Pb-O (carboxyl) and Pb-N (amine) bonds both at a distance of 2.5 ± 0.1 Å at a concentration of 1 M Pb(Ac)2 in water. At 0.75 M Pb(Ac)2 in water, strong Pb-S bonds are found at a distance of 2.8 ± 0.1 Å. The implications for lead removal using capacitive deionization are discussed.

Domain pattern formation in tetragonal ferroelectric ceramics

We present the results of high-resolution simulations of the ferroelectric domain evolution in polycrystalline lead zirconate titanate (PZT), using a phase-field framework that accounts for thermal fluctuations. Leveraging the parallel efficiency of a Fourier spectral scheme, we model micron-sized ceramic samples with thousands of grains at atomic-unit-cell spatial resolution. We introduce a method to automatically identify and track different types of domain walls from phase-field data, which we exploit to study their role during polarization reversal under applied electric fields. Results indicate that the density of domain walls and the domain width obey the Kittel-Mitsui-Furuichi-Roitburd square-root law with implications on the macroscopically observable piezo- and dielectric material properties. Moreover, analyzing the statistics of the domain pattern formation in simulated samples reveals correlations between the average polarization and strain within a grain and its crystallographic orientation, which is in agreement with high-energy x-ray diffraction experiments. Furthermore, we study the occurrence of the two predominant types of domain patterns – monodomain and laminate/twin domain structures – whose emergence within gains of a polycrystal is traced back to the grain orientation. Phase-field statistics are supported by a simple analytical model, which is based on minimizing the electric enthalpy and accurately predicts some of the reported correlations and allows us to further study the behavior of monodomains vs. laminate patterns in ferroelectric ceramics.

Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO3.

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.

In-situ XRD investigation of [formula omitted] phase precipitation kinetics during isothermal holding in a hyper duplex stainless steel

The precipitation kinetics and formation mechanisms of σ phase during isothermal holding of a hyper duplex stainless steel in the temperature range from 750 °C to 1025 °C were studied by means of in-situ high energy X-ray diffraction experiments. Rietveld analysis of the data determined the nose temperature as 900 °C where a σ phase fraction of 1 wt% is attained after 15 s. The measurements revealed a distinct change from initial interface controlled nucleation of σ phase to a later stage of diffusion controlled growth during holding at 1000 °C after the σ phase fraction reaches half its final value. At lower holding temperatures the change from interface control to diffusion control happens more gradual and at later stages of the transformation. At these lower temperatures σ phase precipitation is accompanied by the formation of secondary austenite. Changes in σ phase formation mechanisms and kinetics were correlated with variations in the morphology of the precipitates through SEM analysis of quenched samples. Isothermal holding at 1000∘C and 1025 °C as well as above the solvus temperature of σ phase at 1120 °C leads to local fluctuations of austenite and ferrite phase fractions and the concomitant movement of austenite/ferrite phase boundaries even when no overall change in the phase fractions occurs.

Effect of wire-arc directed energy deposition on the microstructural formation and age-hardening response of the Mg-9Al-1Zn (AZ91) alloy

In recent years, wire-arc directed energy deposition (waDED), which is also commonly known as wire-arc additive manufacturing (WAAM), has emerged as a promising new fabrication technique for magnesium alloys. The major reason for this is the possibility of producing parts with a complex geometry as well as a fine-grained microstructure. While the process has been shown to be applicable for Mg-Al-Zn alloys, there is still a lack of knowledge in terms of the influence of the WAAM process on the age-hardening response. Consequently, this study deals with the aging response of a WAAM AZ91 alloy. In order to fully understand the mechanisms during aging, first, the as-built condition was analyzed by means of high-energy X-ray diffraction (HEXRD) and scanning electron microscopy. These investigations revealed a fine-grained, equiaxed microstructure with adjacent areas of alternating Al content. Subsequently, the difference between single- and double-step aging as well as conventional and direct aging was studied on the as-built WAAM AZ91 alloy for the first time. The aging response during the various heat treatments was monitored via in situ HEXRD experiments. Corroborating electron microscopy and hardness studies were conducted. The results showed that the application of a double-step aging heat treatment at 325 °C with pre-aging at 250 °C slightly improves the mechanical properties when compared to the single-step heat treatment at 325 °C. However, the hardness decreases considerably after the pre-aging step. Thus, aging at lower temperatures is preferable within the investigated temperature range of 250-325 °C. Moreover, no significant difference between the conventionally aged and directly aged samples was found. Lastly, the specimens showed enhanced precipitation kinetics during aging as compared to cast samples. This could be attributed to a higher amount of nucleation sites and the particular temperature profile of the solution heat treatment.

Ni-Nb-P-based bulk glass-forming alloys: Superior material properties combined in one alloy family

Ni-Nb-based bulk glass-forming alloys are among the most promising amorphous metals for industrial applications due to their incomparable combination of strength, hardness, elasticity and plasticity. However, the main drawback is the limited glass-forming ability, narrowing the field of application to solely small components. In this study, we show that minor additions of P to the binary Ni-Nb system increase the glass-forming ability by 150 % to a record value of 5 mm. P can be easily added by using an industrial Ni-P pre-alloy which is readily available. The partial substitution of Nb by Ta further boosts the glass-forming ability to values 200 % higher than that of the binary base alloy. Besides conventional X-ray diffraction measurements, the amorphous nature of the samples is verified by high-energy synchrotron X-ray diffraction experiments. Moreover, the mechanical properties of the new alloy compositions are characterized in uniaxial compression tests and Vickers hardness measurements, showing a high engineering yield strength of 3 GPa, an extended plastic regime up to 10 % strain to failure and an increase of the hardness to a maximum value of 1000 HV5. Additionally, calorimetric measurements reveal that the modified alloys feature an extended supercooled liquid region up to 69 K upon heating, permitting thermoplastic micro molding of amorphous feedstock material.

In-situ investigation of strengthening and strain hardening mechanisms of Cu-added medium-Mn steels by synchrotron-based high-energy X-ray diffraction

A novel Cu-added medium-Mn steel with a chemical composition of Fe–0.27C–9.1Mn–1.86Al–3.3Cu (wt.%) was designed and subjected to intercritical annealing (IA) temperature range from 620 °C to 680 °C for 1 h. The ultimate tensile strength (UTS) increases and the yielding strength (YS) decreases with the IA temperature increasing. The YS of 824 MPa, UTS of 1222 MPa, total elongation (TE) of 55%, and product of strength and elongation (PSE) of 67.2 GPa·% are achieved after IA at 660 °C. Transmission electron microscopy confirmed that Cu-rich nanoparticles precipitate in the ferrite. The in-situ high-energy X-ray diffraction (HE-XRD) experiments show that at the beginning of plastic deformation, both austenite and ferrite bear the applied load. The load is mainly undertaken by martensite with effective transformation-induced plasticity (TRIP) effect triggered. The YS of ferrite is significantly higher than that of austenite. The individual contribution of solid solution strengthening, grain refinement strengthening, dislocation strengthening, and precipitation strengthening in ferrite and austenite is analyzed. The discrepancy between the YS of ferrite and austenite is mainly attributed to the precipitation strengthening due to the Cu-rich nanoparticles precipitation. The moderate mechanical stability and the collaboration of TRIP and twinning-induced plasticity (TWIP) effects of austenite contributed to the enhanced strain hardening capability and resulted in large ductility.

Sequence of phase transformations in metastable β Zr–12Nb alloy studied in situ by HEXRD and complementary techniques

Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, βZr and βNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable βZr phase and athermal ω particles are present in the material. At about 300 °C, Nb-rich βNb phase starts to form in the material and the original βZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ω particles, which is related to a relatively large misfit between the ω and β phases. At 560 °C, ω solvus was observed, identified by an abrupt dissolution of ω particles which was followed by growth of the α phase.

Quantifying Dynamic Signal Spread in Real-Time High-Energy X-ray Diffraction

Measured intensity in high-energy monochromatic X-ray diffraction (HEXD) experiments provides information regarding the microstructure of the crystalline material under study. The location of intensity on an areal detector is determined by the lattice spacing and orientation of crystals so that changes in the heterogeneity of these quantities are reflected in the spreading of diffraction peaks over time. High temporal resolution of such dynamics can now be experimentally observed using technologies such as the mixed-mode pixel array detector (MM-PAD) which facilitates in situ dynamic HEXD experiments to study plasticity and its underlying mechanisms. In this paper, we define and demonstrate a feature computed directly from such diffraction time series data quantifying signal spread in a manner that is correlated with plastic deformation of the sample. A distinguishing characteristic of the analysis is the capability to describe the evolution from the distinct diffraction peaks of an undeformed alloy sample through to the non-uniform Debye–Scherrer rings developed upon significant plastic deformation. We build on our previous work modeling data using an overcomplete dictionary by treating temporal measurements jointly to improve signal spread recovery. We demonstrate our approach in simulations and on experimental HEXD measurements captured using the MM-PAD. Our method for characterizing the temporal evolution of signal spread is shown to provide an informative means of data analysis that adds to the capabilities of existing methods. Our work draws on ideas from convolutional sparse coding and requires solving a coupled convex optimization problem based on the alternating direction method of multipliers.

Temperature-dependent constitutive modeling of a magnesium alloy ZEK100 sheet using crystal plasticity models combined with in situ high-energy X-ray diffraction experiment

A multiscale crystal plasticity model accounting for temperature-dependent mechanical behaviors without introducing a larger number of unknown parameters was developed. The model was implemented in elastic-plastic self-consistent (EPSC) and crystal plasticity finite element (CPFE) frameworks for grain-scale simulations. A computationally efficient EPSC model was first employed to estimate the critical resolved shear stress and hardening parameters of the slip and twin systems available in a hexagonal close-packed magnesium alloy, ZEK100. The constitutive parameters were thereafter refined using the CPFE. The crystal plasticity frameworks incorporated with the temperature-dependent constitutive model were used to predict stress–strain curves in macroscale and lattice strains in microscale at different testing temperatures up to 200 °C. In particular, the predictions by the crystal plasticity models were compared with the measured lattice strain data at the elevated temperatures by in situ high-energy X-ray diffraction, for the first time. The comparison in the multiscale improved the fidelity of the developed temperature-dependent constitutive model and validated the assumption with regard to the temperature dependency of available slip and twin systems in the magnesium alloy. Finally, this work provides a time-efficient and precise modeling scheme for magnesium alloys at elevated temperatures.

Dislocation density evolution in cold-rolled Zr-2.5%Nb pressure tubes under thermal treatments by high energy XRD and neutron TOF diffraction peak profile analysis

Diffraction line profile analysis was done to characterize the change in the dislocation density at different stages of the processing route of a Zr-2.5%Nb pressure tube. Two diffraction experiments were carried out: High Energy X-Ray Diffraction (HE-XRD) and Neutron Time of Flight Diffraction (TOF-ND). On samples in the extruded, cold-rolled and post-annealed conditions, the HE-XRD experiment in transmission geometry allows a detailed characterization of the dependence of the line shape on the direction of the scattering vector. Important variations of the FWHM (Full Width at Half Maximum) were observed, which was interpreted as the non-uniform distribution of dislocations among grains with different orientations. Average values of the dislocation density and subgrain size were obtained after applying the Warren-Averbach method.The effect of post deformation annealing for soaking temperatures from 400° to 450°C was studied using TOF-ND. Peak profile analysis was performed on in-situ neutron time-of-flight diffractograms to obtain peak positions and widths to calculate residual stresses and dislocation densities. The results show that the dislocation density decreases largely at early times and it is followed by a slow diminution over time. Dislocation density decreases from ∼8 × 1014 m−2 for the cold-rolled sample to values close or under 2 × 1014 m−2. A remnant dislocation density after long annealing times was detected, with a value that decreases as the soaking temperature increases. Dislocation type was also characterized. It was found that dislocations with 〈a〉 Burgers vectors represent approximately 90% of the population, while the other 10% are 〈c+a〉. An empirical recovery kinetics model was proposed where dislocation mobility is assumed as a thermally activated mechanism. This model can be used as a processing tool to estimate the dislocation density expected for different soaking temperatures and times. The effect of residual stresses on peak broadening was also discussed. It was estimated that this contribution represents around 10–20% to the observed physical line broadening.

Time-resolved in-situ dislocation density evolution during martensitic transformation by high-energy-XRD experiments: A study of C content and cooling rate effects

Recent studies on the mechanical behavior of martensitic steels show the importance of considering lath martensite as a “polycrystalline aggregate” with a large distribution of the local yield strength. The latter depends on local microstructural features, such as lath sizes, carbon distribution (in solid solution, segregated to defects, in carbides) and density of defects Internal stresses have to be considered as an additional contribution to the yield strength participating to enlarge the distribution of the local flow stresses at the microstructure scale.This study focuses on one of these microstructural features, the dislocation density. Its evolution is followed in situ upon martensitic transformation by High Energy X-Ray Diffraction experiments on a synchrotron beamline. A previously introduced method, based on modified Williamson-Hall (mWH) analysis, is improved in order to take account of the martensite lattice tetragonality when applying the mWH method.The influence of the steel carbon content (0.11, 0.21 and 0.31 wt.% C steels) and of the cooling rate (-10, -50, -100 °C/s and water-quench) on the dislocation density evolution in martensite during the martensitic transformation is established. The effect of the cooling rate on the dislocation density is low between -10 and -100 °C/s, but becomes visible after the water-quench. The higher the C content, the higher the dislocation densities at the end of the transformation. The steels with higher C content also show a wider distribution of the local dislocation density and, therefore, of the local yield strength.

Composition-transferable machine learning potential for LiCl-KCl molten salts validated by high-energy x-ray diffraction

Unraveling the liquid structure of multicomponent molten salts is challenging due to the difficulty in conducting and interpreting high-temperature diffraction experiments. Motivated by this challenge, we developed composition-transferable Gaussian approximation potential (GAP) for molten LiCl-KCl. A DFT-SCAN accurate GAP is active-learned from only $\ensuremath{\sim}$1100 training configurations drawn from 10 unique mixture compositions enriched with metadynamics. The GAP-computed structures show strong agreement across high-energy x-ray diffraction experiments, including for a eutectic not explicitly included in model training, thereby opening the possibility of composition discovery.

High-throughput investigation of ferrite growth kinetics in graded ternary Fe-C-X alloys

The composition dependence of ferrite growth kinetics in Fe-C-X ternary alloyed steels, where X = Ni, Mn, Mo, Cr, Si, was investigated using a high-throughput approach. Compositionally graded samples were subjected to in situ time- and space-resolved X-ray diffraction during intercritical annealing. To this end, diffusion couples were created between a binary Fe-C and different Fe-C-X alloys, using hot uniaxial compression and high temperature diffusion treatments. In-situ high-energy X-ray diffraction experiments were performed to collect ferrite growth kinetics along the composition gradient of the diffusion couples. A large dataset describing the austenite-to-ferrite phase transformation kinetics was generated using a very limited number of experiments. This dataset of unprecedented size was compared to the kinetics predicted by the classical local-equilibrium (LE) and para-equilibrium (PE) models as well as a modified version of the three-jump solute drag (SD) model, which accounts for the different interactions between the elements present at the austenite/ferrite interface. The comparison showed that both LE and PE models fail to capture the effect of both composition and temperature on the kinetics of ferrite growth for the different Fe-C-X systems. The SD model calculations matched experimental transformation kinetics at all investigated temperatures and over almost all the investigated composition ranges of Si, Cr, Mn, Ni, and Mo.

How Si affects the microstructural evolution and phase transformations of intermetallic γ-TiAl based alloys

Small additions of Si and C have been proven to efficiently improve the creep properties of intermetallic γ-TiAl based alloys. In order to exploit the full potential of these alloying elements, detailed studies of their influence on the phase transformations and the resulting microstructural evolution during processing and heat treatments are a necessity. This work presents a fundamental investigation of the alloying effect of Si in the composition range up to 0.65 at.% on a β-solidifying TiAl alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%). After casting and a subsequent heat treatment at 1200 °C, Si increases the amount of γ phase at the expense of the α2-Ti3Al phase. Due to solid solution strengthening and the precipitation of ζ-Ti5Si3 silicides in the highly Si-alloyed materials, an increase in hardness is observed. As the silicides act as effective obstacles for grain boundaries, these precipitates also control the grain growth kinetics of the α phase and are able to maintain a fine-grained microstructure at 1300°C for holding times up to 20 h. By utilizing in-situ high-energy X-ray diffraction experiments and differential scanning calorimetry, Si is found to exhibit similar effects as Al on the alloying system, effectively increasing solid-solid phase transition temperatures, while simultaneously decreasing the solidus temperature of the materials.

Impact of Carbon N-Doping and Pyridinic-N Content on the Fuel Cell Performance and Durability of Carbon-Supported Pt Nanoparticle Catalysts.

Cathode catalyst layers of proton exchange membrane fuel cells (PEMFCs) typically consist of carbon-supported platinum catalysts with varying weight ratios of proton-conducting ionomers. N-Doping of carbon support materials is proposed to enhance the performance and durability of the cathode layer under operating conditions in a PEMFC. However, a detailed understanding of the contributing N-moieties is missing. Here, we report the successful synthesis and fuel cell implementation of Pt electrocatalysts supported on N-doped carbons, with a focus on the analysis of the N-induced effect on catalyst performance and durability. A customized fluidized bed reduction reactor was used to synthesize highly monodisperse Pt nanoparticles deposited on N-doped carbons (N-C), the catalytic oxygen reduction reaction activity and stability of which matched those of state-of-the-art PEMFC catalysts. Operando high-energy X-ray diffraction experiments were conducted using a fourth generation storage ring; the light of extreme brilliance and coherence allows investigating the impact of N-doping on the degradation behavior of the Pt/N-C catalysts. Tests in liquid electrolytes were compared with tests in membrane electrode assemblies in single-cell PEMFCs. Our analysis refines earlier views on the subject of N-doped carbon catalyst supports: it provides evidence that heteroatom doping and thus the incorporation of defects into the carbon backbone do not mitigate the carbon corrosion during high-potential cycling (1-1.5 V) and, however, can promote the cell performance under usual PEMFC operating conditions (0.6-0.9 V).

Correlating temperature-dependent stacking fault energy and in-situ bulk deformation behavior for a metastable austenitic stainless steel

In-situ high-energy synchrotron X-ray diffraction experiments during uniaxial tensile loading are performed to investigate the effect of temperature (25, 45 and 70 °C) on the deformation behavior of a 301 metastable austenitic stainless steel. The micromechanical behavior of the steel at the three deformation temperatures is correlated with the stacking fault energy (γSF) experimentally determined through the same in-situ X-ray experiments. The applied measurements provide a unique possibility to directly interrogate the temperature-dependent γSF in relation to the active bulk deformation mechanism in a metastable austenitic stainless steel. The determined γSF is 9.4 ± 1.7 mJ m−2 at 25 °C, 13.4 ± 1.9 mJ m−2 at 45 °C and 25.0 ± 1.1 mJ m−2 at 70 °C. This relatively minor change of γSF and temperature causes a significant change of the dominant deformation mechanism in the alloy. At room temperature (25 °C) significant amounts of stacking faults form at 0.05 true strain, with subsequent formation of large fractions of deformation-induced α′- and ε-martensite, 0.4 and 0.05, at 0.4 true strain, respectively. With increasing temperature (45 °C) fewer stacking faults form at low strain and thereupon also smaller α′- and ε-martensite fractions form, 0.2 and 0.025, at 0.4 true strain, respectively. At the highest temperature (70 °C) plastic deformation primarily occurs by the generation and glide of perfect dislocations at low strain, while at higher strain these dislocations dissociate to form stacking faults. The α′-martensite fraction formed is significantly less at 70 °C reaching 0.1 at 0.4 strain, whilst ε-martensite is not found to form at any strain at this temperature. The temperature-dependent mechanical behavior of the alloy is consistent with the observed dominant deformation mechanisms; the strong work hardening from the TRIP effect at low temperature, and low γSF, decreases significantly with increasing temperature, and γSF.