Evolution Of Atomic Structure Research Articles

Effects of γ/γ lamellar interfaces on translamellar crack propagation in TiAl alloys

Lamellar interfaces and their structures play a significant role in the fracture behavior of fully lamellar (FL) TiAl alloys. However, the effects of different lamellar interfaces on crack tip behavior are not clear. To this end, the interaction between three types of γ/γ lamellar interfaces, which are termed by true-twin, rotational-order, and pseudo-twin interfaces, and a translamellar crack in FL-TiAl alloy have been investigated by molecular dynamics simulation. By analyzing the atomic structure evolution of crack propagation and the effects of interfaces on the crack growth rate, we found that the introduction of γ/γ lamellar interfaces enhanced the crack resistance of a crack model without interfaces. Furthermore, the resistances of the γ/γ lamellar interfaces against translamellar crack propagation are in the order of rotational-order> pseudo-twin> true-twin. Specifically, crack growth is almost arrested at the rotational-order interface; the crack propagation nearly stops after crossing through the pseudo-twin interface; while the crack penetrates the true-twin interface and continues to propagate in adjacent lamellae. In addition, as crack resistance of the lamellar interface improves, the dislocation density in the corresponding model also increases. Among the three kinds of γ/γ lamellar interfaces, the true-twin interface effectively screens the stress field at the crack tip, whereas the pseudo-twin and rotational-order interfaces do not. • Existence of lamellar interfaces enhance the crack resistance of interface-free model. • Rotational order interface exhibits the most desirable crack resistance. • True twin interface has the worst resistance to crack propagation. • Interface migration has a positive effect on improving crack resistance of interface. • True twin interface has the effective screening on stress field at the crack tip.

Unveiling the mechanism of charge compensation in Li2RuxMn1−xO3 by tracking atomic structural evolution

The relationship between the structural evolution and redox of Li-rich transition-metal layered oxides (LLOs) cathodes remains ambiguous, obstructing the development of high-performance lithium-ion (Li+) battery. Herein, the coherent effects of local atomic and electronic structure in Li2RuxMn1−xO3 (LRMO) with a wide voltage window (1.3–4.8 V) is identified by in situ X-ray absorption fine spectroscopy (XAFS) and chemometrics. We not only skillfully separated the redox active structures to track the electrochemical path, but also visualized the coupling mechanism between the evolution of Ru-Ru dimer and the (de) excitation of cations and anions. Furthermore, introducing manganese triggers the “heterogeneity” of coordination environment and electronic structure between Ru and Mn after discharge to 3 V. The change of thermodynamic and kinetic paths affects the relithiation, and further leads to the hysteresis of the anion activation structure relaxation of Li2Ru0.4Mn0.6O3 relative to Li2RuO3 (LRO). Additionally, it is demonstrated that the high charge cut-off voltage restrains the relaxation of anionic active structure in LRO from a new perspective through comparative experiments. Our work associates the evolution of atomic structure with charge compensation and negative electrochemical reactions such as voltage hysteresis (VH) and capacity attenuation, deepening the understanding electrochemical reaction mechanism of LLOs during the first cycle and providing a theoretical support for the further design and synthesis of high-efficiency cathodes.

Four-dimensional emittance measurements of ultrafast electron diffraction optics corrected up to sextupole order

Ultrafast electron diffraction (UED) is a technique in which short-pulse electron beams can probe the femtosecond-scale evolution of atomic structure in matter driven far from equilibrium. As an accelerator physics challenge, UED imposes stringent constraints on the brightness of the probe electron beam. The low normalized emittance employed in UED, often at the 10-nm scale and below, is particularly sensitive to both applied field aberrations and space-charge effects. The role of aberrations is increasingly important in small probe systems that often feature multiple orders of magnitude variations in beam size during transport. In this work, we report the correction of normal quadrupole, skew quadrupole, and sextupole aberrations via dedicated corrector elements in an ultrafast electron microdiffraction beamline. To do this, we generate precise four-dimensional phase space maps of rf-compressed electron beams and find excellent agreement with aberration-free space-charge simulations. Finally, we discuss the role a probe-forming aperture can play in improving the brightness of bunches with appreciable space-charge effects.

Local atomic structure evolution of liquid gadolinium and yttrium during solidification: An ab initio study

Local atomic structure evolution of pure gadolinium (Gd) and yttrium (Y) during solidification was investigated by using ab initio molecular dynamics (AIMD) simulation. The calculated results indicate that the local short-range order (SRO) in liquid Gd and Y is similar to some transitional metals with an asymmetric shape of the second peak in static structure factors. Moreover, the formation of icosahedral local motifs as a function of temperature decreases the diffusivity, which explains the connection between structure evolution and dynamic properties. In examining the topological structures of both systems, we demonstrate that small atomic displacement leads to two different types of topological sixfold rings in liquid and solid states. All analyses yield a systematic study about rare earth metals Gd and Y at the atomic level.

Effect of pyrolysis parameters on mechanical properties of polymer-derived ceramics

Polymer-derived ceramics (PDCs) which are fabricated through pyrolysis of preceramic polymers have attracted increasing attention due to their versatility in structure architecture design and property tailoring. Shaping at the polymer state using 3D printing allows the final ceramic products to exhibit arbitrary shapes and complex architectures that are otherwise impossible to achieve through traditional processing routes. The polymer-to-ceramic phase transition also provides additional space for mechanical property tailoring. A multiscale computational model is developed to explore the phase transition mechanisms and their correlations with processing parameters and failure response. Calculations in this work concern PMHS/DVB preceramic polymers. Molecular dynamics (MD) simulations are carried out first to track the atomic structure evolution at different temperatures. Continuum-scale ceramic phase formation is calculated on the basis of the competition between gas generation and gas diffusion. The effect of processing parameters on mechanical properties of pyrolyzed PMHS/DVB is systematically studied. Conclusions from this study can provide direct guidance for fabricating PDC composites with tailored mechanical properties.

Evolution of geometric and electronic structures of oxygen-induced superstructures on Mo(110) surface: A LEED, ARPES, and DFT study

We have studied the evolution of atomic structure and electronic band dispersion of oxygen-induced superstructures on the Mo(110) surface by using low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory. Our experiments show that adsorption of oxygen at an elevated temperature (473 K) gets rid of the mixed-phase coexisting region observed at room temperature and exhibits three distinct superstructures: $p(2\ifmmode\times\else\texttimes\fi{}2$), $p(2\ifmmode\times\else\texttimes\fi{}1$), and $p(2\ifmmode\times\else\texttimes\fi{}6$) phases. Oxygenation of the Mo(110) surface leads to a confinement-induced gaplike opening at the zone center that can be tuned by the overlayer oxygen coverage. The ``hole'' pockets on the Fermi surface of the clean Mo(110) surface are found to persist upon oxygen adsorption with slightly changed volumes, while the ``electron'' pocket does not show any significant shift in momentum with no Fermi surface nesting behavior until saturation oxygen coverages. Apart from the significant modifications to the electronic states of the clean Mo(110) surface, we also observe the formation of new oxygen-induced bands. The evolution of the surface states near ${E}_{F}$ can be attributed to a change in the surface potential, while the evolution of oxygen-induced bands results from the hybridization between O-$2p\phantom{\rule{4pt}{0ex}}({m}_{j}=1/2$) and Mo-$4d$ (${m}_{j}=1/2$) orbitals at the interface. The interlayer separation, bond length, and lateral repulsive interactions between the adatoms are found to play a crucial role in the selective chemisorption of the Mo(110) surface.

Effect of water content and structural anisotropy on tensile mechanical properties of montmorillonite using molecular dynamics

The mechanical behavior of montmorillonite is sensitive to water content and structural anisotropy. In this study, the evolution of atomic structure and mechanical behavior of different hydrated montmorillonite under tension has been investigated using Molecular Dynamics simulations. External deformation has been applied to montmorillonite with a strain rate of 5 × 10−7 fs−1 for uniaxial tensile test, stretched along directions parallel (x- and y-direction) and perpendicular (z-direction) to the clay sheets. The bond-breakage criterion based on the radial distribution function of atom pairs and the evolution of broken bonds with tensile strain, were applied to explore the deformation and failure mechanism of the montmorillonite-water system. Simulation results indicated that the water content increased, causing a decrease in the ultimate tensile strength and Young's modulus, as well as more and more plasticity for montmorillonite, especially in z-direction. Moreover, the sequence of tensile mechanical strength was y > x > z. Stretching along the x- and y-direction, fractures occurred in the silicon‑oxygen tetrahedral and aluminum‑oxygen octahedral sheets, where the aluminum‑oxygen bond was easier to be broken, and the normalized number of total broken bonds in the y-direction was superior to the x-direction. Stretching along the z-direction, only the hydrogen bonds in the interlaminar space were broken. In terms of three directions, the anisotropy behavior was found in ultimate tensile strength, Young's modulus, residual tensile strength, and the evolution of broken bonds.

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.