High-energy X-ray Diffraction Research Articles

Effect of Al composition tuning on properties and structure in Zr-Co-Al metallic glass

The variation of glass forming ability, mechanical properties and atomic structure in Zr72−xCo28Alx (x = 14–19 at%) and Zr56Co44−xAlx (x = 13–20 at%) bulk metallic glasses derived from composition tuning has been studied by using copper-mold suction, high-energy X-ray diffraction, extended X-ray absorption fine structure experiments and ab initio molecular dynamics simulations, respectively. Both experimental and theoretical results indicate that glass forming ability was enhanced when composition approached Zr56Co28Al16, attributed to the densification in the first shell around Co atoms with the increase of 11-coordinated polyhedra. In addition, a ductile-to-brittleness transition induced by minor substitution of larger-sized atoms by smaller-sized atoms has been found. As good glass forming ability is usually accompanied by dense atomic arrangement, the increasing larger-sized atoms may tend to offer more free volume which is beneficial to plastic flow. Compositional tuning, along with structural tuning to influence both glass forming ability and plastic flow, offers a reliable method to develop ductile metallic glasses.

Phase Equilibria in the Ti-Rich Part of the Ti-Al-Nb System—Part I: Low-Temperature Phase Equilibria Between 700 and 900 °C

Precise knowledge of the phase equilibria in the Ti-Al-Nb system between 700 and 900 °C is of crucial importance for the urgently needed improvement of TiAl-based turbine materials already in industrial use to achieve further energy savings. As a result of the occurrence of the two ternary intermetallic phases ωo (“Ti4NbAl3”) and O (“Ti2NbAl”), which form in the solid state just in the range of the application-relevant temperatures, the phase relations are very complex and not well studied. In the present investigation, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 700, 800, and 900 °C were determined by a systematic study of 15 ternary alloys, one solid-solid diffusion couple, and three liquid-solid diffusion couples. Using scanning electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) investigations, type and composition of phases as well as phase transitions were determined. With these results, the phase equilibria were established. A focus of the investigations is on the homogeneity ranges of the two ternary phases ωo and O, which both are stable up to temperatures above 900 °C. Based on the compositions measured for the ωo phase and its crystal structure type, a new formula (Ti,Nb)2Al is suggested. The results also indicate that the phase field of the ωo phase is split into two parts at 900 °C because of the growing phase field of the ordered (βTi,Nb)o phase.

A combined rotating disk electrode-surface x-ray diffraction setup for surface structure characterization in electrocatalysis.

Characterizing electrode surface structures under operando conditions is essential for fully understanding structure-activity relationships in electrocatalysis. Here, we combine in a single experiment high-energy surface x-ray diffraction as a characterizing technique with a rotating disk electrode to provide steady state kinetics under electrocatalytic conditions. Using Pt(111) and Pt(100) model electrodes, we show that full crystal truncation rod measurements are readily possible up to rotation rates of 1200 rpm. Furthermore, we discuss possibilities for both potentiostatic as well as potentiodynamic measurements, demonstrating the versatility of this technique. These different modes of operation, combined with the relatively simple experimental setup, make the combined rotating disk electrode-surface x-ray diffraction experiment a powerful technique for studying surface structures under operando electrocatalytic conditions.

Microstructural characterization through grain orientation mapping with Laue three-dimensional neutron diffraction tomography

For polycrystalline materials, key material properties, such as mechanical anisotropy or transformation behavior, and magnetic properties strongly depend on the crystallographic texture of the crystalline material. Assessment of texture is generally destructive and highly local. Only high energy X-ray diffraction at synchrotron sources and neutrons enable to study, non-destructively, the microstructure in the bulk of materials. Here, we report how progress in Laue three-dimensional neutron diffraction tomography enables to index the crystallographic orientation of several hundred grains and, thus, enables grain-resolved characterization of texture in the volume of centimeter-sized coarse-grained samples with statistical significance. To demonstrate the neutron technique for characterizing the crystallographic microstructure, we investigate a Fe-Ni-Mn austenitic alloy. A total number of 481 grains within a 1 cm3 of material is indexed and the results in assessing the crystallographic texture are compared with electron backscatter diffraction measurements. The short exposure times and non-destructive nature of the Laue three-dimensional neutron diffraction render it a novel promising method for corresponding characterization.

Origin of the low formation energy of oxygen vacancies in CeO2

Oxygen vacancies play a crucial role in determining the catalytic properties of Ce-based catalysts, especially in oxidation reactions. The design of catalytic activity requires keen insight into oxygen vacancy formation mechanisms. In this work, we investigate the origin of oxygen vacancies in CeO2 from the perspective of electron density via high-energy synchrotron powder x-ray diffraction. Multipole refinement results indicate that there is no obvious hybridization between bonded Ce and O atoms in CeO2. Subsequent quantitative topological analysis of the experimental total electron density reveals the closed-shell interaction behavior of the Ce–O bond. The results of first-principles calculation indicate that the oxygen vacancy formation energy of CeO2 is the lowest among three commonly used redox catalysts. These findings indicate the relatively weak bond strength of the Ce–O bond, which induces a low oxygen vacancy formation energy for CeO2 and thus promotes CeO2 as a superior catalyst for oxidation reactions. This work provides a new direction for design of functional metal oxides with high oxygen vacancy concentrations.

Growth and characterization of (Sc2O3)x(Ga2O3)1−x by molecular beam epitaxy

(Sc2O3)x(Ga2O3)1−x was grown by molecular beam epitaxy at low temperatures (100 °C) using a variety of growth sequences to avoid surface segregation of Ga. Continuous and digital growth techniques always produced Ga segregation. This surface segregation was attributed to the stronger bond between the Sc and O compared to the Ga and O. A digital growth technique (alternate opening of Sc and Ga shutters with the O shutter open continuously during the growth) was unsuccessful in eliminating this effect. The segregation was eliminated using a growth technique in which the Ga shutter was closed for a set amount of time toward the end of the growth while the O and Sc shutters remained open. Characterization with reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy revealed the growth of a fine-grained polycrystalline film under these conditions. A third growth technique was used that involved closing the Ga shutter for a set amount of time toward the end of the growth while the O and Sc shutters were open continuously. This technique was successful in depositing a uniform film. However, the breakdown field was only 1.40 MV/cm (at 1 mA/cm2). The addition of Ga to Sc2O3 diminished the insulating properties of the film. These initial experiments indicate that phase segregation is likely to be a major issue with most growth techniques and that alloying Ga2O3 with elements other than Sc, such as Gd or Al, might be a more successful approach.

Atomic structure of an FeCrMoCBY metallic glass revealed by high energy x-ray diffraction

Amorphous bulk metallic glasses with the composition Fe48Cr15Mo14C15B6Y2 have been of interest due to their special mechanical and electronic properties, including corrosion resistance, high yield-strength, large elasticity, catalytic performance, and soft ferromagnetism. Here, we apply a reverse Monte Carlo technique to unravel the atomic structure of these glasses. The pair-distribution functions for various atomic pairs are computed based on the high-energy x-ray diffraction data we have taken from an amorphous sample. Monte Carlo cycles are used to move the atomic positions until the model reproduces the experimental pair-distribution function. The resulting fitted model is consistent with our ab initio simulations of the metallic glass. Our study contributes to the understanding of functional properties of Fe-based bulk metallic glasses driven by disorder effects.

Calculating the grain boundary inclination of voxelated grain structures using a smoothing algorithm

We have developed a flexible method for calculating the grain boundary (GB) inclinations of voxelated grain structure data using smoothing algorithms. We compared the performance of four algorithms: the linear interpolation, Allen–Cahn, level-set, and vertex algorithms. We assessed their accuracy using 2D and 3D cases with known inclinations. The vertex algorithm provided the best balance between accuracy and efficiency for 2D structures while the linear interpolation algorithm provided the best balance for 3D structures. We compared the GB inclinations calculated using our smoothing method on a 3D high energy X-ray diffraction microscopy (HEDM) dataset to those determined by meshing the GBs. The two approaches determined similar GB plane distributions, though they varied significantly at triple junctions. Finally, the smoothing method was demonstrated for two sources of 3D voxelated grain structures: HEDM data and results from Monte Carlo Potts grain growth simulations.

High temperature micro-deformation behavior of continuous TiNb fiber reinforced TiAl matrix composite investigated by in-situ high-energy X-ray diffraction

The continuous TiNb fiber reinforced TiAl matrix composite possesses improved mechanical properties whereas the micro-deformation mechanism remains to be elucidated. This work dynamically analyzed the high-temperature compressive process of TiNbf/TiAl composite by in situ high-energy X-ray diffraction (HEXRD) method. Results indicated β-TiNb and interfacial α2 phase both contained two fiber textures whose formation obeyed the special sequences. Only <110> fiber texture in γ matrix and <0001> fiber texture in α-TiNb existed. The elastic-plastic (E-P) stresses of β-TiNb and α-TiNb grains were higher than macroscopic yield stress σy, especially α-TiNb presented significant strengthening effect. But E-P stresses of γ grains were all lower, especially the [200] and [202] orientated γ grains tended to yield easier along LD direction. Interfacial α2 grains would enter E-P stage earlier due to stress concentration, indicating to share loading effectively. Both fiber and interface can play an important reinforcing role at the initial deformation. However, the fiber degradation at the later stage of work hardening would make it lose the strengthening effect. The load capacity of α2 interface can maintain the strongest until the true strain reaching 22.16%. This work can provide a fundamental understanding of macro-deformation of TiNbf/TiAl composite from the perspective of micromechanical behavior.

Formation of lower bainite in a high carbon steel – an in-situ synchrotron XRD study

The microstructural evolution of and simultaneous dimensional changes in high-carbon SAE 52100 bearing steel were monitored continuously during austempering for 120 min at selected temperatures in the range of 210 °C-270 °C, and also during its subsequent tempering to 340 °C for an additional 120 min, via high-energy X-ray diffraction in real time and in-situ dilatometry. The austenite-to-bainitic ferrite transformation induces lattice defects and internal lattice stresses that increase with austempering time and at lower austempering temperatures. These changes are evidenced by the increase in the full-width half-maximum of the relevant reflections in X-ray diffraction. The lattice parameter of bainitic ferrite takes its highest value during the early stages of austempering, and then gradually decreases as the transformation progresses. This observation points to an initial state of carbon supersaturation in the ferritic lattice that is likely reducing due to carbon segregation close to dislocations, fine carbide precipitation within the bainitic ferrite, and carbon partitioning into the surrounding austenite. The carbon partitioning into austenite is evidenced in particular at the higher austempering temperatures of 240 °C and 270 °C, at which there is a noticeable increase in the lattice parameter of the remaining austenite at longer times. The dimensions of the bearing steel specimens are governed by the volume change due to the formation of bainitic ferrite during austempering and by the relaxation of its lattice distortion during tempering at 340 °C in the absence of further phase transformation.

Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti–48Al–4Nb–2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ → α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γ equiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti–48Al–4Nb–2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γ grain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and Σ5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.

Shear banding-induced 〈c+a〉 slip enables unprecedented strength-ductility combination of laminated metallic composites

Shear bands in metallic materials have been reported to be catastrophic because they normally lead to non-uniform plastic deformation. Ductility of laminated metallic composites deteriorates with increasing processing strain, particularly for those having hexagonal-close-packed (hcp) constituents due to inadequate slip systems and consequently prominent shear banding. Here, we propose a design strategy that counterintuitively tolerates the bands with localized strains, i.e. the shear banded laminar (SBL) structure, which promotes 〈c+a〉 dislocation activation in hcp metals and renders unprecedented strength-ductility combination in hcp-metal-based composites fabricated by accumulative roll bonding (ARB). The SBL structure is characterized with one soft hcp metal constrained by adjacent hard metal in which dislocations have been accumulated near the bimetal interfaces. High-energy X-ray diffraction astonishingly reveals that more than 90% of dislocations are non-basal in Ti layers of the SBL Ti/Nb composite processed by eight ARB cycles. Moreover, 〈c+a〉 dislocations occupy a high fraction of ∼30%, promoting further 〈c+a〉 cross slip. The unique stress field tailored by both shear banding and heterophase interface-mediated deformation accommodation triggers important 〈c+a〉 slip. This SBL design is of significance for developing hcp-based laminates and other heterostructured materials with high performances.

Unveiling Non-isothermal Crystallization of CaO-Al2O3-B2O3-Na2O-Li2O-SiO2 Glass via In Situ X-ray Scattering and Raman Spectroscopy.

The crystallization in glasses is a paradoxical phenomenon and scarcely investigated. This work explores the non-isothermal crystallization of a multicomponent alumino-borosilicate glass via in situ high-energy synchrotron X-ray diffraction, atomic pair distribution function, and Raman spectroscopy. Results depict the crystallization sequence as Ca3Al2O6 and CaSiO4 followed by LiAlO2 with the final compound formation of Ca3B2O6. These precipitations occur in a narrow temperature range and overlap, resulting in a single exothermic peak in the differential scanning calorimetry thermogram. The concurrent nucleation of Ca3Al2O6 and CaSiO4 is intermediated by their corresponding hydrates, which have dominantly short-range order. Moreover, the crystallization of LiAlO2 and Ca3B2O6 is strongly linked with the changes of structural units during the incubation stage in non-isothermal heating. These findings clarify the crystallization of multicomponent glass, which have been inferred from ex situ reports but never evidenced via in situ studies.

Thermal stability of a cathodic arc evaporated Cr0.74Ta0.26N coating

CrTaN coatings have recently received increasing industrial interest due to their combination of high hardness, promising fracture toughness and excellent oxidation resistance. However, up to now no thorough investigation on the thermal stability of this coating system is available. Thus, within this work, the evolution of the microstructure and phase composition of an arc evaporated CrTaN coating were illuminated in inert atmosphere up to 1400°C. The coating crystallizes in an fcc-Cr0.74Ta0.26N solid solution with a preferred <311> orientation. Alternating Cr-enriched and Ta-enriched layers are identified in the cross-section of the as-deposited coating, which arise from the three-fold rotation during deposition. In-situ high energy X-ray diffraction showed that powdered CrTaN is stable in inert atmosphere up to ∼1250°C, where fcc-CrxTa1-xN starts to transform into t-Cr1.2Ta0.8N. Upon further increasing the temperature to values exceeding 1300°C, h-Cr2N and h-Ta5N4 start to form. Vacuum annealing of a CrTaN coating on a sapphire substrate at 1000°C results in the homogenization of the synthesis-related compositional fluctuations. While still maintaining the fcc-CrxTa1-xN solid solution, a texture change to a preferred <100> orientation is observed after annealing at 1270°C. An annealing treatment at 1300°C results in the formation of t-Cr1.2Ta0.8N in addition to the fcc-Cr1-xTaxN.

Influence of interatomic potential and simulation procedures on the structures and properties of sodium aluminosilicate glasses from molecular dynamics simulations

The effects of various simulation parameters such as interatomic potential, cooling rate, system size, and glass-formation procedures on the short and medium range structures of sodium aluminosilicate glasses were investigated using molecular dynamics simulations. Two widely used interatomic potentials: a fixed-charge pair-wise potential and a diffusive charge reactive potential (DCRP) were investigated. To provide validation of the simulation results, high energy X-ray and neutron diffraction studies of the sodium aluminosilicate glasses were performed. The results show that the reactive potential provides reasonable structures consistent with those from the fixed charge potentials and experiments. But the fixed charge potential is more robust in various glass forming procedures such as constant pressure, constant volume, and the QSQ method, while DCRP is more sensitive to the ensemble choice and thermal history. The DCRP potential, however, has the advantage to allow study glass-water interactions and interfacial behaviors.

Nonaffine Strains Control Ductility of Metallic Glasses.

The origin of limited plasticity in metallic glasses is elusive, with no apparent link to their atomic structure. We propose that the response of the glassy structure to applied stress, not the original structure itself, provides a gauge to predict the degree of plasticity. We carried out high-energy x-ray diffraction on various bulk metallic glasses (BMGs) under uniaxial compression within the elastic limit and evaluated the anisotropic pair distribution function. We show that the extent of local deviation from the affine (uniform) deformation in the elastic regime is strongly correlated with the plastic behavior of BMGs beyond yield, across chemical compositions and sample history. The results suggest that the propensity for collective local atomic rearrangements under stress promotes plasticity.

Influence of La on the atomic structure of Al[sbnd]Cu alloy liquid

The influence of La element on the atomic structure of AlCu alloy liquid above the liquidus line was investigated through high-energy x-ray diffraction technology on synchrotron facility and constrained reverse Monte Carlo simulation. The addition of La induced a change in nearest neighbor environments and chemical short-range orders of the atoms, which makes the Cu and La atoms dispersed in the liquid and surrounded by Al atoms. The analysis results of atomic topological short-range order show La element does not promote the icosahedral short-range order in the Al86Cu9 liquid, but increases the number of cluster types and reduces the variance of the cluster fraction, leading to a more disordered structure. These results strongly support that the diversity and distribution of clusters are responsible for the glass-forming ability of alloys, not the specific cluster type. Moreover, La element affects the arrangement of clusters in space, enhancing structural heterogeneity of the liquid and the correlation between clusters with high five-fold symmetry, which can also result in an improvement in glass-forming ability. These findings reveal the role of La element in enhancing glass-forming ability, and have implications for understanding the liquid-glass transition mechanism.

The effect of Ni or Co additions on the structure of Zr60Cu30Al10 bulk metallic glass revealed by high-energy synchrotron radiation

The effect of substituting Cu by elemental additions of Ni or Co on the atomic structure of the Zr60Cu30Al10 ternary bulk metallic glass (BMG) is studied using high-energy synchrotron radiation X-ray diffraction. Analyses of the structural features in reciprocal and real space using the structure factors S(Q) and pair-distribution functions (PDF) point to an increase in the structural disorder for the Ni- or Co-bearing quaternary alloys. This is consistent with the “confusion principle” since upon alloying the initially nearly identical atomic sizes of Cu, Ni and Co diversify due to local electronic interactions. In real space, the disordering is manifested by a reduced deviation from the average particle density visible in the nearest-neighbour (NN) atomic shell structure over the complete short- and medium-range order region. Despite their similar atomic size, enthalpies of mixing with the main alloy elements and apparent disordering of the structure, the additions of Ni or Co have different effects on thermal stability of the ternary “mother” alloy.

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).

Interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile deformation

This study investigated the interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile loading by means of in-situ high-energy X-ray diffraction. Nb nanowires have significant influence on the transformation behavior, annealed texture, and martensite variant reorientation behavior including texture evolution and load transfer among martensite variants of NiTi matrix. Meanwhile, the existence of densely distributed Nb nanowires increases the internal stress in NiTi matrix generated during martensite reorientation process, and causes the negative axial lattice stresses of all unfavored NiTi martensite variants after reorientation when the composite wire is under axial tensile stress. In addition, an exceptionally large elastic strain of 4.7% was produced over Nb nanowires after tensile deformation . Meanwhile, heterogeneous strain distribution exists in Nb nanowires, and it is associated with heterogeneous martensite reorientation strain of NiTi matrix. • Interactions between martensitic NiTi alloy and Nb nanowires is investigated. • Nanowires inhibit NiTi martensitic transformation and deformation twinning. • Nanowires induce the negative lattice stresses of all unfavored NiTi variants. • Inhomogeneous strain distribution exists in nanowires.