Asymmetric magnetic levitation for density-based measurement and analysis

BACKGROUND

Magnetic levitation (MagLev) based on negative magnetophoresis represents a promising technology for density-based analysis and manipulation of nonmagnetic objects. This approach has garnered considerable interest across multiple fields, such as chemistry, materials science, and biochemistry, primarily due to its inherent simplicity, precision, and cost-effectiveness. However, it is essential to recognize that frequently used MagLev configurations, including standard MagLev and axial MagLev, are not without their limitations. These configurations often struggle to strike a balance between levitation performance, ease of operation, and visibility. Therefore, it is necessary to develop a new MagLev configuration to address the aforementioned issue.

RESULTS

This work describes the development of an innovative MagLev, termed "asymmetric MagLev", achieved by combining a ring magnet and a cylinder magnet as up-down asymmetric magnetic field sources. The asymmetric design overcomes the physical obstacles along the centerline of the standard MagLev, offering unique open-structure advantages, including easy handling of samples, the ability to observe samples from the top or bottom, and no restrictions on the container height. Meanwhile, comparative analysis reveals a considerable enhancement in the working distance of the asymmetric MagLev without significantly sacrificing the measurement range compared to the axial MagLev. Notably, the asymmetric MagLev achieves a remarkable sensitivity of up to about 1.8 × 104 mm (g cm-3)-1, surpassing the axial MagLev by approximately 30 times. Furthermore, experimental results validate the successful application of the asymmetric MagLev in density measurement and quality detection of small-sized objects.

SIGNIFICANCE

This pioneering configuration represents the first utilization of up-down asymmetric magnets in the field of MagLev. Through the integration of an axially magnetized ring magnet and a cylinder magnet, the asymmetric MagLev design overcomes the limitations associated with conventional MagLev configurations. This innovative design exhibits outstanding operational capabilities and levitation performance, making it suitable for a wide range of applications in density-based measurement and analysis.

Metallic Mimosa pudica: A 3D biomimetic buckling structure made of metallic glasses

Metallic Mimosa pudica, a three-dimensional (3D) biomimetic structure made of metallic glass, is formed via laser patterning: Blooming, closing, and reversing of the metallic M. pudica can be controlled by an applied magnetic field or by manual reshaping. An array of laser-crystallized lines is written in a metallic glass ribbon. Changes in density and/or elastic modulus due to laser patterning result in an appropriate size mismatch between the shrunken crystalline regions and the glassy matrix. The residual stress and elastic distortion energy make the composite material to buckle within the elastic limit and to obey the minimum elastic energy criterion. This work not only provides a programming route for constructing buckling structures of metallic glasses but also provides clues for the study of materials with automatic functions desired in robotics, electronic devices, and, especially, medical devices in the field of medicine, such as vessel scaffolds and vascular filters, which require contactless expansion and contraction functions.

Tandem catalysis with double-shelled hollow spheres

Metal-zeolite composites with metal (oxide) and acid sites are promising catalysts for integrating multiple reactions in tandem to produce a wide variety of wanted products without separating or purifying the intermediates. However, the conventional design of such materials often leads to uncontrolled and non-ideal spatial distributions of the metal inside/on the zeolites, limiting their catalytic performance. Here we demonstrate a simple strategy for synthesizing double-shelled, contiguous metal oxide@zeolite hollow spheres (denoted as MO@ZEO DSHSs) with controllable structural parameters and chemical compositions. This involves the self-assembly of zeolite nanocrystals onto the surface of metal ion-containing carbon spheres followed by calcination and zeolite growth steps. The step-by-step formation mechanism of the material is revealed using mainly in situ Raman spectroscopy and X-ray diffraction and ex situ electron microscopy. We demonstrate that it is due to this structure that an Fe₂O₃@H-ZSM-5 DSHSs-showcase catalyst exhibits superior performance compared with various conventionally structured Fe₂O₃-H-ZSM-5 catalysts in gasoline production by the Fischer-Tropsch synthesis. This work is expected to advance the rational synthesis and research of hierarchically hollow, core-shell, multifunctional catalyst materials.

Data-driven discovery of a universal indicator for metallic glass forming ability

Despite the importance of glass forming ability as a major alloy characteristic, it is poorly understood and its quantification has been experimentally laborious and computationally challenging. Here, we uncover that the glass forming ability of an alloy is represented in its amorphous structure far away from equilibrium, which can be exposed by conventional X-ray diffraction. Specifically, we fabricated roughly 5,700 alloys from 12 alloy systems and characterized the full-width at half-maximum, Δq, of the first diffraction peak in the X-ray diffraction pattern. A strong correlation between high glass forming ability and a large Δq was found. This correlation indicates that a large dispersion of structural units comprising the amorphous structure is the universal indicator for high metallic glass formation. When paired with combinatorial synthesis, the correlation enhances throughput by up to 100 times compared to today's state-of-the-art combinatorial methods and will facilitate the discovery of bulk metallic glasses.

Substantially enhanced plasticity of bulk metallic glasses by densifying local atomic packing

Introducing regions of looser atomic packing in bulk metallic glasses (BMGs) was reported to facilitate plastic deformation, rendering BMGs more ductile at room temperature. Here, we present a different alloy design approach, namely, doping the nonmetallic elements to form densely packed motifs. The enhanced structural fluctuations in Ti-, Zr- and Cu-based BMG systems leads to improved strength and renders these solutes' atomic neighborhoods more prone to plastic deformation at an increased critical stress. As a result, we simultaneously increased the compressive plasticity (from ∼8% to unfractured), strength (from ∼1725 to 1925 MPa) and toughness (from 87 ± 10 to 165 ± 15 MPa√m), as exemplarily demonstrated for the Zr₂₀Cu₂₀Hf₂₀Ti₂₀Ni₂₀ BMG. Our study advances the understanding of the atomic-scale origin of structure-property relationships in amorphous solids and provides a new strategy for ductilizing BMG without sacrificing strength.

Connecting glass-forming ability of binary mixtures of soft particles to equilibrium melting temperatures

The glass-forming ability is an important material property for manufacturing glasses and understanding the long-standing glass transition problem. Because of the nonequilibrium nature, it is difficult to develop the theory for it. Here we report that the glass-forming ability of binary mixtures of soft particles is related to the equilibrium melting temperatures. Due to the distinction in particle size or stiffness, the two components in a mixture effectively feel different melting temperatures, leading to a melting temperature gap. By varying the particle size, stiffness, and composition over a wide range of pressures, we establish a comprehensive picture for the glass-forming ability, based on our finding of the direct link between the glass-forming ability and the melting temperature gap. Our study reveals and explains the pressure and interaction dependence of the glass-forming ability of model glass-formers, and suggests strategies to optimize the glass-forming ability via the manipulation of particle interactions.

Glass-forming ability is an important parameter for manufacturing glassy materials, but it remains challenging to be characterized due to its nonequilibrium nature. Nie et al. provide a solution by linking it to the pressure dependence of melting temperature of constituent components in binary mixtures.

X-ray and neutron scattering measurements of ordering in a Cu46Zr54 liquid

The structural evolution of the equilibrium and supercooled Cu₄₆Zr₅₄ liquids was investigated with a combination of elastic neutron scattering (with isotopic substitution) and synchrotron x-ray scattering studies. The partial pair correlation functions were determined over a wide temperature range (∼270 °C). These show that the Cu-Cu and Zr-Zr ordering increases as the temperature decreases, while the Cu-Zr ordering decreases. This surprising result is in contradiction with the results from molecular dynamics studies.

Density, excess volume, and structure of Fe-Cr-Ni melts

The relationship between the excess volume and the structure of Fe-Cr-Ni melts is investigated using containerless levitation and in situ high-energy synchrotron x-ray diffraction techniques. The density of six hypoeutectic Fe-Cr-Ni alloys along the 72 wt. % Fe isopleth was measured in the stable and undercooled regions, and the excess volume was evaluated as a function of Cr concentration. It is found that the 72Fe-Cr-Ni alloys exhibit a positive sign of excess volume and the amount increases with increasing Cr concentration. Analysis of the structure factor and pair distribution function of the alloy family reveals that the short-range order in the melt becomes more pronounced with decreasing Cr concentration; this demonstrates a direct correlation between the excess volume and local liquid structure. A characteristic signature of the icosahedral structure is observed in the structure factor of the melts, and the potential origin of the positive excess volume of the 72Fe-Cr-Ni alloys is qualitatively discussed in relation to the icosahedral structure.

On Long-Term Stability of Metallic Glasses

Thermal stability of different types of metallic glasses and partially crystalline alloys stored for at least 15 years at ambient conditions was tested in the present work by differential scanning calorimetry in comparison with that of the original alloys tested in the as-cast state in the earlier works. The structure of the naturally aged alloys was also studied by X-ray diffractometry. The structure of a couple of selected alloys was also tested by transmission electron microscopy. Most of the alloys retained their initial structure and showed only a moderate decrease in the crystallization temperature. Only those alloys which showed visible surface oxidation (Cu-Zr-system based) were partly transformed into a crystalline state forming micron-scale Cu particles in air at ambient conditions.

Vitrification and New Phases in the Water:Pyrimidine Binary Eutectic System

The binary diagram for pyrimidine:water mixtures has been determined by differential scanning calorimetry, in situ single-crystal, and powder X-ray diffraction experiments. The eutectic point has been located near the 1:4 n/n ratio at 234.5 K. The eutectic and nearly eutectic mixtures easily vitrify, and the vitrification could be kinetically induced for 1:3 n/n mixtures, too. Depending on the cooling rate, the 1:4 mixture freezes in the glass state, as a conglomerate of the glass and crystalline phases, or as the eutectic mixture of pyrimidine phase I and hexagonal ice I_h. When heated above 160 K, the glass phase transforms to a novel crystalline phase, tentatively identified as a pyrimidine hydrate, which in turn at ca. 200-210 K transforms into a eutectic mixture of pyrimidine phase I and hexagonal ice I_h. The pyrimidine-water binary diagram and novel crystalline and amorphous phases are relevant to the thermodynamic behavior of hydrophilic pyrimidine and its natural and synthetic derivatives in humid environments. The presently determined binary diagram can be straightforwardly applied for assessing the contents of water in highly hygroscopic pyrimidine samples.

Atomic-level crystallization in selective laser melting fabricated Zr-based metallic glasses

As a promising additive manufacturing technique, selective laser melting (SLM) provides the possibility of fabricating metallic glassy components free of the constraints of geometrical complexity and dimensions. However, unexpected crystallization greatly affects the microstructure and degrades the mechanical performance of SLM-fabricated metallic glasses (MGs). To clarify the crystallization mechanism and the effect of laser processing on the crystallization, we investigate the atomic-level crystallization in the SLM Zr₉₀Cu₁₀ MG by using molecular dynamics simulations. The results show that crystallization highly related to scan speed lies in the atomic-level cluster changes. Lower scan speed leads to a dramatically increased fraction of the BCC crystal phase, accompanied by the nucleation of a few HCP and FCC crystal phases. As scan speed increases, more icosahedron-like clusters are formed, leading to the formation of the MG, while the nucleation of the crystal phase is suppressed. The suppression of crystallization is further attributed to a higher average temperature variation rate induced by higher scan speed, which reduces the relaxation time, preventing the nucleation and growth of crystal phases. This work contributes to the understanding of the crystallization in MGs during the SLM process at the atomic level, providing guidance to suppress the crystallization in the SLM process of desired metallic glassy components.

High-temperature bulk metallic glasses developed by combinatorial methods

Since their discovery in 1960¹, metallic glasses based on a wide range of elements have been developed². However, the theoretical prediction of glass-forming compositions is challenging and the discovery of alloys with specific properties has so far largely been the result of trial and error^3-8. Bulk metallic glasses can exhibit strength and elasticity surpassing those of conventional structural alloys^9-11, but the mechanical properties of these glasses are critically dependent on the glass transition temperature. At temperatures approaching the glass transition, bulk metallic glasses undergo plastic flow, resulting in a substantial decrease in quasi-static strength. Bulk metallic glasses with glass transition temperatures greater than 1,000 kelvin have been developed, but the supercooled liquid region (between the glass transition and the crystallization temperature) is narrow, resulting in very little thermoplastic formability, which limits their practical applicability. Here we report the design of iridium/nickel/tantalum metallic glasses (and others also containing boron) with a glass transition temperature of up to 1,162 kelvin and a supercooled liquid region of 136 kelvin that is wider than that of most existing metallic glasses¹². Our Ir-Ni-Ta-(B) glasses exhibit high strength at high temperatures compared to existing alloys: 3.7 gigapascals at 1,000 kelvin^9,13. Their glass-forming ability is characterized by a critical casting thickness of three millimetres, suggesting that small-scale components for applications at high temperatures or in harsh environments can readily be obtained by thermoplastic forming¹⁴. To identify alloys of interest, we used a simplified combinatorial approach^6-8 harnessing a previously reported correlation between glass-forming ability and electrical resistivity^15-17. This method is non-destructive, allowing subsequent testing of a range of physical properties on the same library of samples. The practicality of our design and discovery approach, exemplified by the identification of high-strength, high-temperature bulk metallic glasses, bodes well for enabling the discovery of other glassy alloys with exciting properties.

Structural origin of the high glass-forming ability of Ce70Ga10Cu20 alloys

The CeGaCu amorphous alloy has a good glass-forming ability and many special properties. However, its structure at the atomic scale is unclear. We systematically investigated the structure evolution of Ce70GaxCu30-x (x = 6, 10, 13) glass formation melts by ab initio molecular dynamics (AIMD) simulations. Based on the trajectories from the simulations, the pair-correlation function, coordination numbers, chemical short-range order, Voronoi polyhedra and electronic structures were discussed. Our results show that the concentration of Ga- and Cu-centered icosahedral (-like) clusters in Ce70Ga10Cu20 melts are larger than those in Ce70Ga6Cu24 and Ce70Ga13Cu17 melts. Furthermore, electronic analysis showed that the hybridization between Ga 4p and Cu 3d (Ce 5d) orbitals is strong and that of Cu 3d orbitals and Ga 4p orbitals was strengthened in Ce70Ga10Cu20 melts, which means that the interactions between Ga and Cu atoms nearby were enhanced in the Ce70Ga10Cu20 melts. The stability of the Ga- or Cu-centered icosahedral clusters increased accordingly, which favored their glass-forming ability. Our investigation helps people obtain an increased understanding of the glass-forming ability from the viewpoint of chemical interactions for metallic glasses.

High-Throughput Screening Solar-Thermal Conversion Films in a Pseudobinary (Cr, Fe, V)-(Ta, W) System

The properties and microstructure of (Cr, Fe, V)-(Ta, W) high-entropy films (HEFs) are studied using combinatorial strategies. The compositional library of (Cr_0.33Fe_0.33V_0.33) _x(Ta_0.5W_0.5)_{100- x}, (0 < x < 100), HEFs are fabricated by cosputtering to discover potential photothermal conversion materials. By verifying points in the compositional library, the structure and property variation according to the atomic content of elements are carefully studied. Results indicate that the films exhibit an amorphous structure when x ranges from 86.9 to 32.5, and high concentrations of Ta and W lead to the formation of a BCC structure in the films. The solar absorptivity of the films peaks at the transitional area from an amorphous to BCC structure. Our research provides an efficient combinatorial technique to discover HEFs with high performance.

Structural homogeneity and mass density of bulk metallic glasses revealed by their rough surfaces and ultra-small angle neutron scattering (USANS)

The ultra-small angle neutron scattering (USANS) measures the microscale structure of heterogeneity and the scattering from rough surfaces with small scattering volumes can be neglected. But this is not true in amorphous alloys. The small angle scattering from such surfaces is not negligible, regardless of scattering volume. However, we demonstrate that the unwanted rough surfaces can be utilized to determine the homogeneity and mass density of amorphous metallic glasses using the USANS and surface neutron contrast matching technique. The power law scattering of the homogeneous Cu₅₀Zr₅₀ amorphous alloy disappeared under the surface contrast-matched environment, a mixture of hydrogenated/deuterated ethanol having low surface tension against the metallic alloys, indicating that the scattering originated not from its internal structure but from the rough surface. This confirms the structural homogeneity not only at the atomic level but also on a larger scale of micrometer. On the other hand, the crystallized Cu₅₀Zr₅₀ alloy showed strong power-law scattering under the matching environment due to the structural heterogeneity inside the alloy. This technique can apply to the bulk samples when the transmission is high enough not causing multiple scattering that is easily detected with USANS and when the surface roughness is dominant source of scattering.

Crystal genes in a marginal glass-forming system of Ni50Zr50

The marginal glass-forming ability (GFA) of a binary Ni-Zr system is an issue to be explained considering numerous bulk metallic glasses (BMGs) found in a Cu-Zr system. Using molecular dynamics, the structures and dynamics of Ni₅₀Zr₅₀ metallic liquid and glass are investigated at the atomistic level. To achieve a well-relaxed glassy sample, a sub-T_g annealing method is applied and the final sample is closer to the experiments than the models prepared by continuous cooling. With the state-of-the-art structural analysis tools such as cluster alignment and pair-wise alignment methods, two glass-forming motifs with some mixed traits of a metastable B2 crystalline phase and a crystalline Ni-centered B33 motif are found to be dominant in the undercooled liquid and glass samples. A new chemical order characterization on each short-range order (SRO) structure is accomplished based on the cluster alignment method. The significant amount of the crystalline motif and the few icosahedra in the glassy sample deteriorate the GFA.

Anisotropic stress inhibits crystallization in Cu-Zr glass-forming liquids

Liquids attain a metastable state without crystallizing by cooling rapidly to a given temperature below the melting point. With increasing supercooling, the nucleation rate would show an increase based on the prediction of the classical nucleation theory. It is generally thought that the nucleation rate will reach the maximum upon approaching the glass transition temperature, T_g, for glass-forming liquids. We report that there exists a supercooled region above T_g in which the crystallization has actually been severely suppressed. Our molecular dynamics simulations show that the growth of embryos in the supercooled Cu₆₀Zr₄₀ melt is subjected to a strong anisotropic stress associated with the dynamic heterogeneity. Its long-range effect drives the embryo to grow into a ramified morphology so that the interface energy dominates over the embryo growth, leading to the suppression of nucleation.

Perspective on Structural Evolution and Relations with Thermophysical Properties of Metallic Liquids

The relationship between the structural evolution and properties of metallic liquids is a long-standing hot issue in condensed-matter physics and materials science. Here, recent progress is reviewed in several fundamental aspects of metallic liquids, including the methods to study their atomic structures, liquid-liquid transition, physical properties, fragility, and their correlations with local structures, together with potential applications of liquid metals at room temperature. Involved with more experimentally and theoretically advanced techniques, these studies provide more in-depth understanding of the structure-property relationship of metallic liquids and promote the design of new metallic materials with superior properties.

Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing

The glass forming ability (GFA) of metallic glasses (MGs) is quantified by the critical cooling rate (R _C). Despite its key role in MG research, experimental challenges have limited measured R _C to a minute fraction of known glass formers. We present a combinatorial approach to directly measure R _C for large compositional ranges. This is realized through the use of compositionally-graded alloy libraries, which were photo-thermally heated by scanning laser spike annealing of an absorbing layer, then melted and cooled at various rates. Coupled with X-ray diffraction mapping, GFA is determined from direct R _C measurements. We exemplify this technique for the Au-Cu-Si system, where we identify Au₅₆Cu₂₇Si₁₇ as the alloy with the highest GFA. In general, this method enables measurements of R _C over large compositional areas, which is powerful for materials discovery and, when correlating with chemistry and other properties, for a deeper understanding of MG formation.

Key factors affecting mechanical behavior of metallic glass nanowires

Both strengthening and weakening trends with decreasing diameter have been observed for metallic glass nanowires, sometimes even in the samples with the same chemical composition. How to reconcile the results has reminded a puzzle. Since the detailed stress state and microstructure of metallic glass nanowires may differ from each other significantly depending on preparation, to discover the intrinsic size effect it is necessary to study metallic glass nanowires fabricated differently. Here we show the complex size effects from one such class of metallic glass nanowires prepared by casting using molecular dynamics simulations. As compared with the nanowires of the same composition prepared by other methods, the cast nanowires deform nearly homogeneously with much lower strength but better ductility; and also show strengthening in tension but weakening in compression with decreasing wire diameter. The subtle size dependence is shown to be related to the key factors including internal and surface stress state, atomic structure variation, and presence of various gradients. The complex interplay of these factors at decreasing size leads to the different deformation behaviors.

Kinetic and structural fragility-a correlation between structures and dynamics in metallic liquids and glasses

The liquid phase remains poorly understood. In many cases, the densities of liquids and their crystallized solid phases are similar, but since they are amorphous they lack the spatial order of the solid. Their dynamical properties change remarkably over a very small temperature range. At high temperatures, near their melting temperature, liquids flow easily under shear. However, only a few hundred degrees lower flow effectively ceases, as the liquid transforms into a solid-like glass. This temperature-dependent dynamical behavior is frequently characterized by the concept of kinetic fragility (or, generally, simply fragility). Fragility is believed to be an important quantity in glass formation, making it of significant practical interest. The microscopic origin of fragility remains unclear, however, making it also of fundamental interest. It is widely (although not uniformly) believed that the dynamical behavior is linked to the atomic structure of the liquid, yet experimental studies show that although the viscosity changes by orders of magnitude with temperature, the structural change is barely perceptible. In this article the concept of fragility is discussed, building to a discussion of recent results in metallic glass-forming liquids that demonstrate the presumed connection between structural and dynamical changes. In particular, it becomes possible to define a structural fragility parameter that can be linked with the kinetic fragility.

The role of liquid–liquid transition in glass formation of CuZr alloys

The structure evolution during LLTs is beneficial to the glass forming ability (GFA) of Cu–Zr systems.

Critical scaling of icosahedral medium-range order in CuZr metallic glass-forming liquids

The temperature evolution of icosahedral medium-range order formed by interpenetrating icosahedra in CuZr metallic glassforming liquids was investigated via molecular dynamics simulations. Scaling analysis based on percolation theory was employed, and it is found that the size distribution of clusters formed by the central atoms of icosahedra at various temperatures follows a very good scaling law with the cluster number density scaled by S^−τ and the cluster size S scaled by |1 − T_c/T|^−1/σ, respectively. Here T_c is scaling crossover-temperature. τ and σ are scaling exponents. The critical scaling behaviour suggests that there would be a structural phase transition manifested by percolation of locally favoured structures underlying the glass transition, if the liquid could be cooled slowly enough but without crystallization intervening. Furthermore, it is revealed that when icosahedral short-range order (ISRO) extends to medium-range length scale by connection, the atomic configurations of ISROs will be optimized from distorted ones towards more regular ones gradually, which significantly lowers the energies of ISROs and introduces geometric frustration simultaneously. Both factors make key impacts on the drastic dynamic slow-down of supercooled liquids. Our findings provide direct structure-property relationship for understanding the nature of glass transition.

Spectral descriptors for bulk metallic glasses based on the thermodynamics of competing crystalline phases

Metallic glasses attract considerable interest due to their unique combination of superb properties and processability. Predicting their formation from known alloy parameters remains the major hindrance to the discovery of new systems. Here, we propose a descriptor based on the heuristics that structural and energetic 'confusion' obstructs crystalline growth, and demonstrate its validity by experiments on two well-known glass-forming alloy systems. We then develop a robust model for predicting glass formation ability based on the geometrical and energetic features of crystalline phases calculated ab initio in the AFLOW framework. Our findings indicate that the formation of metallic glass phases could be much more common than currently thought, with more than 17% of binary alloy systems potential glass formers. Our approach pinpoints favourable compositions and demonstrates that smart descriptors, based solely on alloy properties available in online repositories, offer the sought-after key for accelerated discovery of metallic glasses.

General Synthetic Strategy for Hollow Hybrid Microspheres through a Progressive Inward Crystallization Process

Hollow hybrid microspheres have found great potential in different areas, such as drug delivery, nanoreactors, photonics, and lithium-ion batteries. Here, we report a simple and scalable approach to construct high-quality hollow hybrid microspheres through a previously unexplored growth mechanism. Starting from uniform solid microspheres with low crystallinity, we identified that a hollowing process can happen through the progressive inward crystallization process initiated on the particle surface: the gradual encroachment of the crystallization frontline toward the core leads to the depletion of the center and forms the central cavity. We showed that such a synthetic platform was versatile and can be applicable for a large variety of materials. By using the production of Li4Ti5O12-carbon hollow hybrid microspheres as an example, we demonstrated that high-performance anode materials could be achieved through synthesis and structure control. We expect that our findings offer new perspectives in different areas ranging from materials chemistry, energy storage devices, catalysis, to drug delivery.

'Crystal Genes' in Metallic Liquids and Glasses

We analyze the underlying structural order that transcends liquid, glass and crystalline states in metallic systems. A genetic algorithm is applied to search for the most common energetically favorable packing motifs in crystalline structures. These motifs are in turn compared to the observed packing motifs in the actual liquid or glass structures using a cluster-alignment method. Using this method, we have revealed the nature of the short-range order in Cu64Zr36 glasses. More importantly, we identified a novel structural order in the Al90Sm10 system. In addition, our approach brings new insight into understanding the origin of vitrification and describing mesoscopic order-disorder transitions in condensed matter systems.

Ab initio molecular dynamics simulations of short-range order in Zr₅₀Cu₄₅Al₅ and Cu₅₀Zr₄₅Al₅ metallic glasses

Comparative analysis between Zr-rich Zr50Cu45Al5 and Cu-rich Cu50Zr45Al5 metallic glasses (MGs) is extensively performed to locate the key structural motifs accounting for their difference of glass forming ability. Here we adopt ab initio molecular dynamics simulations to investigate the local atomic structures of Zr50Cu45Al5 and Cu50Zr45Al5 MGs. A high content of icosahedral-related (full and distorted) orders was found in both samples, while in the Zr-rich MG full icosahedrons ⟨0, 0, 12, 0⟩ is dominant, and in the Cu-rich one the distorted icosahedral orders, especially ⟨0, 2, 8, 2⟩ and ⟨0, 2, 8, 1⟩, are prominent. And the ⟨0, 2, 8, 2⟩ polyhedra in Cu50Zr45Al5 MG mainly originate from Al-centered clusters, while the ⟨0, 0, 12, 0⟩ in Zr50Cu45Al5 derives from both Cu-centered clusters and Al-centered clusters. These difference may be ascribed to the atomic size difference and chemical property between Cu and Zr atoms. The relatively large size of Zr and large negative heat of mixing between Zr and Al atoms, enhancing the packing density and stability of metallic glass system, may be responsible for the higher glass forming ability of Zr50Cu45Al5.

High-Sensitivity Measurement of Density by Magnetic Levitation

This paper presents methods that use Magnetic Levitation (MagLev) to measure very small differences in density of solid diamagnetic objects suspended in a paramagnetic medium. Previous work in this field has shown that, while it is a convenient method, standard MagLev (i.e., where the direction of magnetization and gravitational force are parallel) cannot resolve differences in density <10(-4) g/cm(3) for macroscopic objects (>mm) because (i) objects close in density prevent each other from reaching an equilibrium height due to hard contact and excluded volume, and (ii) using weaker magnets or reducing the magnetic susceptibility of the medium destabilizes the magnetic trap. The present work investigates the use of weak magnetic gradients parallel to the faces of the magnets as a means of increasing the sensitivity of MagLev without destabilization. Configuring the MagLev device in a rotated state (i.e., where the direction of magnetization and gravitational force are perpendicular) relative to the standard configuration enables simple measurements along the axes with the highest sensitivity to changes in density. Manipulating the distance of separation between the magnets or the lengths of the magnets (along the axis of measurement) enables the sensitivity to be tuned. These modifications enable an improvement in the resolution up to 100-fold over the standard configuration, and measurements with resolution down to 10(-6) g/cm(3). Three examples of characterizing the small differences in density among samples of materials having ostensibly indistinguishable densities-Nylon spheres, PMMA spheres, and drug spheres-demonstrate the applicability of rotated Maglev to measuring the density of small (0.1-1 mm) objects with high sensitivity. This capability will be useful in materials science, separations, and quality control of manufactured objects.

General 2.5 power law of metallic glasses

Metallic glass (MG) is an important new category of materials, but very few rigorous laws are currently known for defining its "disordered" structure. Recently we found that under compression, the volume (V) of an MG changes precisely to the 2.5 power of its principal diffraction peak position (1/q1). In the present study, we find that this 2.5 power law holds even through the first-order polyamorphic transition of a Ce68Al10Cu20Co2 MG. This transition is, in effect, the equivalent of a continuous "composition" change of 4f-localized "big Ce" to 4f-itinerant "small Ce," indicating the 2.5 power law is general for tuning with composition. The exactness and universality imply that the 2.5 power law may be a general rule defining the structure of MGs.

Atomistic Design of Favored Compositions for Synthesizing the Al-Ni-Y Metallic Glasses

For a ternary alloy system promising for obtaining the so-called bulk metallic glasses (BMGs), the first priority issue is to predict the favored compositions, which could then serve as guidance for the appropriate alloy design. Taking the Al-Ni-Y system as an example, here we show an atomistic approach, which is developed based on a recently constructed and proven realistic interatomic potential of the system. Applying the Al-Ni-Y potential, series simulations not only clarify the glass formation mechanism, but also predict in the composition triangle, a hexagonal region, in which a disordered state, i.e., the glassy phase, is favored energetically. The predicted region is defined as glass formation region (GFR) for the ternary alloy system. Moreover, the approach is able to calculate an amorphization driving force (ADF) for each possible glassy alloy located within the GFR. The calculations predict an optimized sub-region nearby a stoichiometry of Al₈₀Ni₅Y₁₅, implying that the Al-Ni-Y metallic glasses designed in the sub-region could be the most stable. Interestingly, the atomistic predictions are supported by experimental results observed in the Al-Ni-Y system. In addition, structural origin underlying the stability of the Al-Ni-Y metallic glasses is also discussed in terms of a hybrid packing mode in the medium-range scale.

Cyclic Deformation in Metallic Glasses

Despite the utmost importance and decades of experimental studies on fatigue in metallic glasses (MGs), there has been so far little or no atomic-level understanding of the mechanisms involved. Here we perform molecular dynamics simulations of tension-compression fatigue in Cu50Zr50 MGs under strain-controlled cyclic loading. It is shown that the shear band (SB) initiation under cyclic loading is distinctly different from that under monotonic loading. Under cyclic loading, SB initiation takes place when aggregates of shear transformation zones (STZs) accumulating at the MG surface reach a critical size comparable to the SB width, and the accumulation of STZs follows a power law with rate depending on the applied strain. It is further shown that almost the entire fatigue life of nanoscale MGs under low cycle fatigue is spent in the SB-initiation stage, similar to that of crystalline materials. Furthermore, a qualitative investigation of the effect of cycling frequency on the fatigue behavior of MGs suggests that higher cycling frequency leads to more cycles to failure. The present study sheds light on the fundamental fatigue mechanisms of MGs that could be useful in developing strategies for their engineering applications.

Investigating the atomic level influencing factors of glass forming ability in NiAl and CuZr metallic glasses

Bulk metallic glasses are a relatively new class of amorphous metal alloy which possess unique mechanical and magnetic properties. The specific concentrations and combinations of alloy elements needed to prevent crystallization during melt quenching remains poorly understood. A correlation between atomic properties that can explain some of the previously identified glass forming ability (GFA) anomalies of the NiAl and CuZr systems has been identified, with these findings likely extensible to other transition metal-transition metal and transition metal-metalloid (TM-M) alloy classes as a whole. In this work, molecular dynamics simulation methods are utilized to study thermodynamic, kinetic, and structural properties of equiatomic CuZr and NiAl metallic glasses in an attempt to further understand the underlying connections between glass forming ability, nature of atomic level bonding, short and medium range ordering, and the evolution of structure and relaxation properties in the disordered phase. The anomalous breakdown of the fragility parameter as a useful GFA indicator in TM-M alloy systems is addressed through an in-depth investigation of bulk stiffness properties and the evolution of (pseudo)Gruneisen parameters over the quench domain, with the efficacy of other common glass forming ability indicators similarly being analyzed through direct computation in respective CuZr and NiAl systems. Comparison of fractional liquid-crystal density differences in the two systems revealed 2-3 times higher values for the NiAl system, providing further support for its efficacy as a general purpose GFA indicator.

A predictive structural model for bulk metallic glasses

Great progress has been made in understanding the atomic structure of metallic glasses, but there is still no clear connection between atomic structure and glass-forming ability. Here we give new insights into perhaps the most important question in the field of amorphous metals: how can glass-forming ability be predicted from atomic structure? We give a new approach to modelling metallic glass atomic structures by solving three long-standing problems: we discover a new family of structural defects that discourage glass formation; we impose efficient local packing around all atoms simultaneously; and we enforce structural self-consistency. Fewer than a dozen binary structures satisfy these constraints, but extra degrees of freedom in structures with three or more different atom sizes significantly expand the number of relatively stable, ‘bulk' metallic glasses. The present work gives a new approach towards achieving the long-sought goal of a predictive capability for bulk metallic glasses.

The atomic structure of metallic glasses is related to their properties such as the ability of materials to form glasses. Here, the authors develop a new approach to model the atomic structure of metallic glasses, to enable a better connection between atomic structure and glass properties.

Computer simulations of glasses: the potential energy landscape

We review the current state of research on glasses, discussing the theoretical background and computational models employed to describe them. This article focuses on the use of the potential energy landscape (PEL) paradigm to account for the phenomenology of glassy systems, and the way in which it can be applied in simulations and the interpretation of their results. This article provides a broad overview of the rich phenomenology of glasses, followed by a summary of the theoretical frameworks developed to describe this phenomonology. We discuss the background of the PEL in detail, the onerous task of how to generate computer models of glasses, various methods of analysing numerical simulations, and the literature on the most commonly used model systems. Finally, we tackle the problem of how to distinguish a good glass former from a good crystal former from an analysis of the PEL. In summarising the state of the potential energy landscape picture, we develop the foundations for new theoretical methods that allow the ab initio prediction of the glass-forming ability of new materials by analysis of the PEL.

Computation assisted design of favored composition for ternary Mg-Cu-Y metallic glass formation

With the aid of ab initio calculations, a realistic interatomic potential was constructed for the Mg-Cu-Y ternary system under the proposed formalism of smoothed and long-range second-moment approximation of tight-binding. Taking the potential as the starting base, an atomistic computation/simulation route was developed for designing favored and optimized compositions for Mg-Cu-Y metallic glass formation. Simulations revealed that the physical origin of metallic glass formation is the collapse of crystalline lattice when solute concentration exceeds a critical value, thus leading to predict a hexagonal region in the Mg-Cu-Y composition triangle, within which metallic glass formation is energetically favored. It is proposed that the hexagonal region can be defined as the intrinsic glass formation region, or quantitative glass formation ability of the system. Inside the hexagonal region, the driving force for formation of each specific glassy alloy was further calculated and correlated with its forming ability in practice. Calculations pinpointed the optimized stoichiometry in the Mg-Cu-Y system to be Mg64Cu16Y20, at which the formation driving force reaches its maximum, suggesting that metallic glasses designed to have compositions around Mg64Cu16Y20 are most stable or easiest to obtain. The predictions derived directly from the atomistic simulations are supported by experimental observations reported so far in the literature. Furthermore, Honeycutt-Anderson analysis indicated that pentagonal bipyramids (although not aggregating to form icosahedra) dominate in the local structure of the Mg-Cu-Y metallic glasses. A microscopic picture of the medium-range packing can then be described as an extended network of the pentagonal bipyramids, entangled with the fourfold and sixfold disclination lines, jointly fulfilling the space of the metallic glasses.

Metallic glass nanostructures of tunable shape and composition

Metals of hybrid nano-/microstructures are of broad technological and fundamental interests. Manipulation of shape and composition on the nanoscale, however, is challenging, especially for multicomponent alloys such as metallic glasses. Although top-down approaches have demonstrated nanomoulding, they are limited to very few alloy systems. Here we report a facile method to synthesize metallic glass nanoarchitectures that can be applied to a broad range of glass-forming alloys. This strategy, using multitarget carousel oblique angle deposition, offers the opportunity to achieve control over size, shape and composition of complex alloys at the nanoscale. As a consequence, nanostructures of programmable three-dimensional shapes and tunable compositions are realized on wafer scale for metallic glasses including the marginal glass formers. Realizing nanostructures in a wide compositional range allows chemistry optimization for technological usage of metallic glass nanostructures, and also enables the fundamental study on size, composition and fabrication dependences of metallic glass properties.