Evolution Of Atomic Structure Research Articles

Molecular dynamics simulation of the γ’ phase deformation behaviour in nickel-based superalloys

The effects of γ′ phase volume fraction, morphology and size on tensile deformation behaviour of Ni-based superalloys at 300 K were studied using molecular dynamics simulation. The differences caused by different parameters were discussed in terms of atomic structure evolution, dislocation motion and stress–strain curve. The results showed that when the volume fraction of the γ′ phase was between 50% and 70%, the tensile strength and elongation increased with the volume fraction, but the variation trend was opposite when it was less than 50%. The cubic γ′ phase produced larger resistance to plastic deformation. Models with different-sized γ′ phases contributed to different fracture modes. Dislocation pile-up was avoided owing to the progressive movement of partial dislocations.

Ab initio study of local structures during cooling of liquid Fe-C and Fe-Cr-C alloys

The cooling process of liquid Fe-C and Fe-Cr-C systems from 2100 K to 1100 K was studied by ab initio molecular dynamics simulations. The evolution of local atomic structure was statistically analyzed by the pair correlation function, structure factor, coordination number, bond angle distribution, and the Honeycutt-Anderson (HA) index as well as the Voronoi index. We found that C atoms tend to be surrounded by metal atoms and the strong chemical bonds of Fe-C and Cr-C make the C-centered clusters more stable than Fe-centered clusters. The two alloy systems are dominated by icosahedral order in the liquid state but show different evolutionary trends of atomic structures during the cooling process. The Fe-C system undergoes a bcc-like crystalline phase transition at 1500 K, while the Fe-Cr-C system can be supercooled to an amorphous state with decreasing disclination density in the polytetrahedral packing. The analyses of clusters indicated that doping Cr can introduce new local topological and chemical orderings and thus renders a better glass-forming ability. Furthermore, to understand the mechanism of carbides precipitation from undercooled liquid of high-carbon chromium bearing steel, we simulated the local structures of several typical Fe and Cr carbides. We identified a close connection in the short-range order between the carbides and metallic liquids, which provides a theoretical basis for the formation of primary carbides.

Pressure-induced local structural crossover in a high-entropy metallic glass

Achieving effective tuning of glass structures between distinct states is intriguing for fundamental studies and applications, but has previously turned out to be challenging in practice. High-entropy metallic glasses (HEMGs), as an emerging type of metallic glasses (MGs) based on the high-entropy effect, are expected to have more disordered and frustrated chemical short-range structure compared with conventional MGs. Therefore, HEMGs may offer possibilities for structure and properties tuning in glasses. In this work, we employ pressure as a tuning parameter and monitor the atomic structural evolution of a senary HEMG, ${\mathrm{Ti}}_{16.7}{\mathrm{Zr}}_{16.7}{\mathrm{Hf}}_{16.7}{\mathrm{Cu}}_{16.7}{\mathrm{Ni}}_{16.7}{\mathrm{Be}}_{16.7}$, up to \ensuremath{\sim}40 GPa using in situ synchrotron x-ray diffraction. Analysis of its structure factor in reciprocal space and reduced pair distribution function in real space both reveal a pressure-induced structural crossover at \ensuremath{\sim}20 GPa with a dramatic change in short-range order (SRO), while no similar phenomenon is observed in a conventional MG, ${\mathrm{Cu}}_{36}{\mathrm{Zr}}_{64}$, as a control sample, suggesting the pressure-induced highly tunable SRO in HEMGs originates from the local chemical complexity, namely, the high-entropy effect. These results confirm that enhanced flexibility and tunability of atomic structures could be achieved by introducing the high-entropy effect into MGs. Therefore, configurational entropy could be another dimension for exploring MGs with highly tunable structures and properties for various potential applications.

Atomic Composition and Structure Evolution of the Solid-Liquid Boundary in Al-Si System During Interfacial Diffusion and Contact Melting

Atomic Composition and Structure Evolution of the Solid-Liquid Boundary in Al-Si System During Interfacial Diffusion and Contact Melting

Atomic structure evolution related to the Invar effect in Fe-based bulk metallic glasses

The Invar effect is universally observed in Fe-based bulk metallic glasses. However, there is limited understanding on how this effect manifests at the atomic scale. Here, we use in-situ synchrotron-based high-energy X-ray diffraction to study the structural transformations of (Fe71.2B24Y4.8)96Nb4 and (Fe73.2B22Y4.8)95Mo5 bulk metallic glasses around the Curie temperature to understand the Invar effect they exhibit. The first two diffraction peaks shift in accordance with the macroscopically measured thermal expansion, which reveals the Invar effect. Additionally, the nearest-neighbor Fe–Fe pair distance correlates well with the macroscopic thermal expansion. In-situ X-ray diffraction is thus able to elucidate the Invar effect in Fe-based metallic glasses at the atomic scale. Here, we find that the Invar effect is not just a macroscopic effect but has a clear atomistic equivalent in the average Fe–Fe pair distance and also shows itself in higher-order atomic shells composed of multiple atom species.

Synchrotron x-ray total scattering and modeling study of high-pressure-induced inhomogeneous atom reconfiguration in an equiatomic Zr50Cu50 metallic glassy alloy

We studied in situ the local atomic structure evolution of an equiatomic ${\mathrm{Zr}}_{50}{\mathrm{Cu}}_{50}$ metallic glassy alloy under high pressure compression inside a diamond anvil cell using synchrotron x-ray total scattering. The empirical potential structure refinement method was used to reconstruct the three-dimensional atomic models at each pressure step, and to analyze the spatially averaged local atomic structure configurations. The interatomic distances of different atomic pairs are reduced at different rates with increasing pressure and the Cu-Cu pairs exhibit the highest percentage reduction. Between ambient pressure and 36.85 GPa, the atomic separation of the Cu-Cu pairs is reduced by \ensuremath{\sim}12% compared to \ensuremath{\sim}5% for Zr-Zr and Zr-Cu pairs. Such disproportional decrease in interatomic distance results in inhomogeneous atom reconfiguration in the short atomic range. With the increase of pressure, the Zr atoms move preferentially towards the Zr-Zr pairs, while the Cu atoms move preferentially towards the Cu-Cu pairs, creating inhomogeneous atom reconfiguration with positive short-range order coefficients of 0.0309 and 0.0464 for Zr-Zr and Cu-Cu respectively, but a negative value of \ensuremath{-}0.0464 for Zr-Cu pairs. Voronoi tessellation method was also used to study the evolution of the short-range atom packing versus pressure, elucidating the cause for the bimodal distribution of the bond angle distributions. The research sheds light on understanding of the atomic reconfiguration of equiatomic alloys under high pressure.

Graphite microcrystals growth in naturally graphitized coal from Hunan, China

The transformation of coal to graphite is a process through which carbon atoms are arranged from amorphous into a crystalline form. However, there is still no study focusing on how the graphite single crystal grow up in graphitized coals during natural graphitization. Herein, a series of naturally graphitized coals were thoroughly studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to uncover the morphological and atomic structural evolution from anthracite to coaly graphite. The anthracite and meta-anthracite samples showed an amorphous carbon structure, whereas the carbon arrangement becomes more and more orderly from semi-graphite to coaly graphite as confirmed by XRD. The crushed particles of anthracite and meta-anthracite show granular and tabular texture with uneven surface under SEM. Small layered graphite flake appears from semi-graphite stage with co-occurrence of dense coal particles, HRTEM data shows the layered graphite flakes are the single graphite crystals. Almost all the dense coal particles change into layered microcrystalline graphite in the most graphitized coals (coaly graphite), and the flaky graphite microcrystals always show up as aggregates. The results will benefit the future applications of the naturally graphitized coals.

In situ study on the evolution of atomic and electronic structure of LaTiO3/SrTiO3 system

As with other transition-metal oxide interfaces, ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces exhibit interesting electronic properties, such as a two-dimensional electron gas. While the available Mott states and multiple pathways to metallicity in ${\mathrm{LaTiO}}_{3}$ can lead to a variety of applications in oxide electronics, the origin of the different properties remains uncertain. Utilizing real-time, in situ x-ray diffraction at the synchrotron, robust but thin ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures were fabricated using oxide molecular beam epitaxy. A thickness of six unit cells was determined to be critical, at which point the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures begin to sustain sharp, well-defined interfaces. The layers of ${\mathrm{LaTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ were sufficiently thin to benefit from the finite escape length of electrons at resonant soft x-ray photoemission, and the heterostructures were transferred in vacuo after growth to another chamber for synchrotron-based x-ray photoemission spectroscopy studies. With the x-ray energy tuned to the Ti-2p resonance, the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces could be probed with enough penetration depth and selectivity. It is shown that all of the heterointerfaces exhibit the two-dimensional electron gas.

3D local atomic structure evolution in a solidifying Al-0.4Sc dilute alloy melt revealed in operando by synchrotron X-ray total scattering and modelling

Using synchrotron X-ray total scattering and empirical potential structure refinement modelling, we studied systematically in operando condition the disorder-to-order local atomic structure transition in a pure Al and a dilute Al-0.4Sc alloy melt in the temperature range from 690 °C to 657 °C. In the liquid state, icosahedral short-range ordered Sc-centred Al polyhedrons form and most of them with Al coordination number of 10–12. As the melt is cooled to the semisolid state, the most Sc-centred polyhedrons become more connected atom clusters via vertex, edge and face-sharing. These polyhedrons exhibit partially icosahedral and partially face-centred-cubic symmetry. The medium-range ordered Sc-centred clusters with face-sharing are proved to be the “precursors” of the L12 Al3Sc primary phases in the liquid-solid coexisting state.

Structural origin of magnetic softening in a Fe-based amorphous alloy upon annealing

• Upon annealing, the microstructure, not only in short range order but also in medium range order, contributes to the magnetic softness in Fe-based amorphous alloy. • Exchange coupling of homogeneous weak and strong magnetic regions generate excellent magnetic softness. • A configuration with energetically stabilization is formed annealed at optimized temperature, resulting in heating-rate independence of coercivity. The underlying structural origin of magnetic properties is still elusive in Fe-based amorphous alloys. In this study, distinctive soft magnetic properties were developed in Fe 76 Si 9 B 10 P 5 amorphous ribbons through systematic design of annealing process. Combining with synchrotron radiation, high-resolution transmission electron microscopy and first principle ab initio molecular dynamic simulation, it is found that the atomic structural evolution both in short range order and medium range order is responsible for the magnetic softness at proper annealing temperature. In short range, formation of separated and densely coordinated Fe-metalloid clusters is instigated to adapt energy minimization, resulting in strengthening of ferromagnetic exchange interaction locally. In medium range, a homogeneous exchange-coupling from the uniformly strong and weak ferromagnetic regions is generated, which significantly weakens magnetic heterogeneity and leads to the excellent magnetic softness. Our findings may provide an effective/promising pathway to modulate the magnetic properties for Fe-based amorphous alloys, and give a comprehensive and quantitative understanding of the structure-properties relationship in amorphous materials.

Interface modulation and physical properties of heterostructure of metal nanoparticles and two-dimensional materials

<sec>Two-dimensional (2D) material has atomic smooth surface, nano-scale thickness and ultra-high specific surface area, which is an important platform for studying the interface interaction between metal nanoparticles (NPs) and 2D materials, and also for observing the surface atomic migration, structural evolution and aggregation of metal NPs in real time and <i>in situ</i>. By rationally designing and constructing the interfaces of metal NPs and 2D materials, the characterization of the interface structure on an atomic scale is very important in revealing the structure-property relationship. It is expected that the investigation is helpful in understanding the mechanism of interaction between metal and 2D materials and optimizing the performance of the devices based on metal-2D material heterojunctions.</sec><sec>In this review, the recent progress of interface modulation and physical properties of the heterostructure of metal NPs and 2D materials are summarized. The nucleation, growth, structural evolution and characterization of metal NPs on the surface of 2D materials are reviewed. The effects of metal NPs on the crystal structure, electronic state and energy band of 2D materials are analyzed. The possible interfacial strain and interfacial reaction are also included. Because of the modulation of electrical and optical properties of 2D materials, the performance of metal NPs-2D material based field effect transistor devices and optoelectronic devices are improved. This review is helpful in clarifying the physical mechanism of microstructure affecting the properties of metal NPs-2D material heterostructures on an atomic scale, and also in developing the metal-2D material heterostructures and their applications in the fields of electronic devices, photoelectric devices, energy devices, etc.</sec>

Spallation fracture dependence on shock intensity and loading duration in single-crystal aluminum

Non-equilibrium molecular dynamics simulations were carried out to explore the effects of the shock intensity and loading duration on the spallation fracture mechanisms for single-crystal aluminum. First, the dynamic applicability of the potential function was validated by the shock Hugoniot relation. It was then found that compression waveforms ranging from triangular to rectangular closely depended on the loading duration. Coupled with atomic structural evolution, the spatio-temporal planes characterized by the density and stress well reproduced the spallation process dominated by void nucleation, growth, and coalescence. A larger shock velocity could accelerate the increase in the void volume fraction, while a larger loading duration could postpone the void nucleation but did not affect the variation trend of the void volume fraction. Based on the void morphology evolution at shock velocities of 1–3 km/s, the void volume fraction in the nucleation and growth (NAG) stages were effectively demarcated and then obtained, and the numerical values agreed closely with theoretical values calculated via the NAG model. The NAG model parameters were subsequently acquired, wherein the nucleation rate threshold and pressure sensitivity parameter were basically independent of the loading duration and shock velocity. However, they largely affected the nucleation threshold, growth threshold, and viscosity coefficient. In addition, the variation trend of the spallation strength depended on the predominance of the strain hardening and temperature softening effects. Finally, the correlation between the slope of the velocity pullback and damage rate was preliminarily discussed based on the NAG model.

Modeling of phase transition in fabrication of polymer-derived ceramics (PDCs)

Polymer-derived ceramics’ (PDCs) route, which converts preceramic polymers to ceramics through heat treatment, presents a flexible and energy-efficient approach to fabricate ceramic composites with arbitrary geometries and tailorable properties. Due to the huge gaps of thermal and mechanical properties of polymers and ceramics, the current state phase composition and phase distribution largely affect the heat transfer behavior and temperature field evolution that ultimately determine the subsequent polymer decomposition and phase redistribution. In this paper, a computational framework is developed to predict the continuum-level phase transition and its effect on mechanical properties. Molecular dynamics (MD) simulations are carried out first to track the atomic structure evolution and mass loss associated with gas generation. It is found that pyrolysis temperature primarily determines the amount of gases that can be generated. Gas diffusion is triggered as a result of the non-uniform temperature field. Ceramic phase formation depends on the interplay between gas generation and gas diffusion. This computational framework allows real-time temperature field and phase composition map to be explicitly extracted. The phase composition map is incorporated into a finite element model for compression simulation. The effects of heating rate, pyrolysis temperature and pyrolysis time on mechanical response are systematically studied. Conclusions from this study can provide direct guidance for fabricating PDCs with tailored properties.

Probing Atomic-Scale Fracture of Grain Boundaries in Low-symmetry 2D Materials.

Grain boundaries (GBs) play a central role in the fracture of polycrystals. However, the complexity of GBs and the difficulty in monitoring the atomic structure evolution during fracture greatly limit the understanding of the GB mechanics. Here, in situ aberration-corrected scanning transmission electron microscopy and density functional theory calculations are combined to investigate the fracture mechanics in low-symmetry, polycrystalline, 2D rhenium disulfide (ReS2 ), unveiling the distinctive crack behaviors at different GBs with atomic resolution. Brittle intergranular fracture prefers to rip through the GBs that are parallel to the Re chains of at least one side of the GBs. In contrast, those GBs, which do not align with Re chains on either side of the GBs, are highly resistant to fracture, impeding or deflecting the crack propagation. These results disclose the GB type-dependent mechanical failure of anisotropic 2D polycrystals, providing new ideas for material reinforcement and controllable cutting via GB engineering.

Atomic Structural Evolution of Single-Layer Pt Clusters as Efficient Electrocatalysts.

The rational synthesis of single-layer noble metal directly anchored on support materials is an elusive target to accomplish for a long time. This paper reports well-defined single-layer Pt (Pt-SL) clusters anchored on ultrathin TiO2 nanosheets-as a new frontier in electrocatalysis. The structural evolution of Pt-SL/TiO2 via self-assembly of single Pt atoms (Pt-SA) is systematically recorded. Significantly, the Pt atoms of Pt-SL/TiO2 possess a unique electronic configuration with PtPt covalent bonds surrounded by abundant unpaired electrons. This Pt-SL/TiO2 catalyst presents enhanced electrochemical performance toward diverse electrocatalytic reactions (such as the hydrogen evolution reaction and the oxygen reduction reaction) compared with Pt-SA, multilayer Pt nanoclusters, and Pt nanoparticles, suggesting an efficient new type of catalyst that can be achieved by constructing single-layer atomic clusters on supports.

Multimodal cell with simultaneous electrochemical quartz crystal microbalance and in operando spectroscopic ellipsometry to understand thin film electrochemistry.

To inform the development of advanced electrodes for energy storage, water treatment, and catalysis, among other applications, we need to improve our understanding of how material structure evolves during electrochemical operation. Insight into the evolution of local atomic structure during electrochemical operation is accessible through a range of sophisticated in operando probes, but techniques for in operando observation of macroscale electrode phenomena (e.g., swelling, dissolution, and chemical degradation) are limited. This macroscale understanding is critical to establish a full picture of electrochemical material behavior. Here, we report a multimodal cell for simultaneous electrochemical quartz crystal microbalance (EQCM) and in operando spectroscopic ellipsometry (SE). This SE-EQCM cell allows for the measurement of mass, thickness, optical properties, and electrochemical properties together in one device. Using polyaniline (PANI) as a test case, we demonstrate the use of this SE-EQCM cell to rapidly measure known phenomena and reproduce a range of prior results during the electrodeposition, electrochemical cycling, and electrochemical degradation of PANI. In particular, the simultaneous mass and thickness measurement afforded by this cell allows us to distinguish known qualitative differences in the degradation of PANI under oxidative and reductive potentials. The SE-EQCM cell we report promises to reveal new insights into the electrochemical behavior of thin film materials for a range of applications.

Synchrotron X‐Ray Absorption Spectroscopy and Electrochemical Study of Bi2O2Se Electrode for Lithium‐/Potassium‐Ion Storage

AbstractElucidating the battery operating mechanism is important for designing better conversion‐type anodes as it determines the strategies used to improve electrochemical performances. Herein, the authors pioneered the electrochemical study of layered Bi2O2Se as anodes for lithium‐ion batteries (LIBs) and potassium‐ion batteries (PIBs). Surprisingly, the Bi2O2Se/graphite composite electrode shows even better cycle stability for PIBs. The electrochemical reaction mechanisms of the Bi2O2Se/graphite electrode for LIBs and PIBs are investigated by potential‐resolved in situ and ex situ X‐ray absorption spectroscopy based at the Bi LIII‐edge and Se K‐edge through characterizing the local atomic structure evolution, valence state change, and charge transfer. New insights are gained regarding the electrochemical process of Se2− anions in Bi2O2Se, where multiple Li–Se intermediates rather than the traditional single‐phase Li2Se are involved in this conversion‐type anode. The advanced understanding of anionic electrochemistry in conversion‐type anodes prompts one to find appropriate ways to suppress side‐reactions and improve the battery performances.

Evolution of atomic structure and electronic transport properties in n-type Bi2Te3 films via Bi2 planar defects

Bi2Te3 films always exhibit n-type transport characteristics even under the Bi-rich condition, which, however, was not clarified clearly. Herein, by virtue of advanced techniques such as scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, scanning transmission electron microscopy, and x-ray photoelectron spectroscopy, we are able to identify the structural evolution on the atomic scale for Bi-rich Bi2Te3 films. The excess of Bi content will lead to the formation of p-type BiTe antisite defects; however, there is a doping limit for the excess of Bi to form BiTe antisites. Beyond this limit, the excess of Bi will form the n-type Bi2 planar defects in the van der Waals gap, the excellent electron donors, which can enhance the electron density by over one order of magnitude and up to the 1021 cm−3 range for Bi-rich Bi2Te3 films. Benefiting from the remarkable increase in the electron density and the suppression of carrier intrinsic excitations, Bi2Te3 films with Bi2 planar defects possess a much improved thermoelectric power factor, with a maximum value of 1.4 mW m−1 K−2 at 450 K, showing about 130% enhancement compared to that of the film without Bi2 intercalations. The discovery opens a new avenue to improve the thermoelectric properties of Bi2Te3 films utilizing the Bi2 planar defects.

Origin of different thermal cycling effects in Fe80P20 and Ni60Nb40 metallic glasses

The effect of thermal cycling on structure and properties has been explored in many metallic glasses (MGs) within last 5 years. It is critical to establish the connection between macroscopic mechanical response and the underlying atomic level process during thermal cycling. The mechanisms underlying the complexity of thermal cycling effect are poorly understood. Here, we investigate the thermal cycling effect on structural evolution and property changes in brittle Fe80P20 and ductile Ni60Nb40 MGs. Our results demonstrate that thermal cycling treatment initially delays the cavitation behavior for Fe80P20 MG and induces local-to-non-localized deformation mode change with high probability for N60Nb40 MG, despite their opposite atomic structural evolution. Specifically, with initial thermal cycling, the Fe80P20 MG prefers to relax, while Ni60Nb40 MG prefers to rejuvenate. With further thermal cycling, the Fe80P20 MG rejuvenates while Ni60Nb40 relaxes back towards a relative stable state. To reveal the origin of such opposite thermal cycling effects in these MGs, we proposed ‘core-shell’ structural models of MGs. Our results shed light on the atomistic understanding of the relationship between the micro-structure and thermal cycling effect in MGs, which may offer useful insights for designing and processing MGs.

Two-orbital degeneracy lifted local precursor to a metal-insulator transition in MgTi2O4

The recent discovery of a local fluctuating t2g orbital-degeneracy-lifted (ODL) state in CuIr2S4 as a high temperature precursor to the metal-insulator transition (MIT) opens the door to a possible widespread presence of precursor states in scarcely studied high-temperature regimes of transition metal based quantum materials. Although in CuIr2S4 the ODL state comprises one orbital per Ir, there is no fundamental reason to exclude multi-orbital ODL states in general. The MgTi2O4 spinel exhibits a MIT on cooling at Ts ~250 K, accompanied by Ti t2g orbital ordering (OO) and spin dimerization with the average symmetry reducing to tetragonal. It shares with CuIr2S4 the pyrochlore transition metal sublattice with active t2g orbitals. This, together with its different orbital filling (t2g1 vs t2g5.5) make it a candidate for hosting a multi-orbital ODL precursor state. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the MIT we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions exist up to at least 500 K. Significantly, the high temperature local state is not continuously connected to the OO band insulator ground state, and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin singlet dimer bonds expand abruptly on warming across the transition but remain shorter than those seen in the cubic structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital t2g ODL precursor state. The ODL state in MgTi2O4 has a correlation length of about 1 nm at high temperature. We discuss that this extended character of the local distortions is consistent with the two-orbital nature of the ODL state imposed by the charge filling and the bond charge repulsion.