Characterization of nanoscale mechanical heterogeneity in a metallic glass by dynamic force microscopy

Designing Advanced Amorphous/Nanocrystalline Alloys by Controlling the Energy State

Amorphous alloys, also known as metallic glasses, exhibit many advanced mechanical, physical, and chemical properties. Owing to the nonequilibrium nature, their energy states can vary over a wide range. However, the energy relaxation kinetics are very complex and composed of various types that are coupled with each other. This makes it challenging to control the energy state precisely and to study the energy-properties relationship. This brief review introduces the recent progresses on studying the enthalpy relaxation kinetics during isothermal annealing, for example, the observation of two-step relaxation phenomenon, the detection of relaxation unit (relaxun), the key role of large activation entropy in triggering memory effect, the influence of glass energy state on nanocrystallization. Based on the above knowledge, a new strategy is proposed to design a series of amorphous alloys and their composites consisting of nanocrystals and glass matrix with superior functional properties by precisely controlling the nonequilibrium energy states. As the typical examples, Fe-based amorphous alloys with both advanced soft magnetism and good plasticity, Gd-based amorphous/nanocrystalline composites with large magnetocaloric effect, and Fe-based amorphous alloys with high catalytic performance are specifically described.

Competing Effects of Molecular Additives and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

The introduction of molecular additives into thermosets often results in changes in their dynamics and mechanical properties that can have significant ramifications for diverse applications of this broad class of materials such as coatings, high-performance composites, etc. Currently, there is limited fundamental understanding of how such additives influence glass formation in these materials, a problem of broader significance in glass-forming materials. To address this fundamental problem, here, we employ a simplified coarse-grained (CG) model of a polymer network as a model of thermoset materials and then introduce a polymer additive having the same inherent rigidity and polymer-polymer interaction strength as the cross-linked polymer matrix. This energetically "neutral" or "self-plasticizing" additive model gives rise to non-trivial changes in the dynamics of glass formation and provides an important theoretical reference point for the technologically more important case of interacting additives. Based on this rather idealized model, we systematically explore the combined effect of varying the additive mass percentage (m) and cross-link density (c) on the segmental relaxation dynamics and mechanical properties of a model thermoset material with additives. We find that increasing the additive mass percentage m progressively decreases both the glass-transition temperature Tg and the fragility of glass formation, a trend opposite to increasing c so that these thermoset variables clearly have a competing effect on glass formation in these model materials. Moreover, basic mechanical properties (i.e., bulk, shear, and tensile moduli) likewise exhibit a competitive variation with the increase of m and c, which are strongly correlated with the Debye-Waller parameter ⟨u2⟩, a measure of material stiffness at a molecular scale. Our findings prove beneficial in the development of structure-property relationships for the cross-linked polymers, which could help guide the design of such network materials with tailored physical properties.

Attractive Electron Delocalization Behavior of FeCoMoPB Amorphous Nanoplates for Highly Efficient Alkaline Water Oxidation

The rational design of high-performance and cost-effective electrocatalysts to overcome the kinetically sluggish water oxidation reaction is a grand challenge in water electrolysis. Transitional metals with incompletely filled d orbitals are expected to have intrinsic electronic interaction to promote the reaction kinetics, however, the construction of multiple active sites is still a bottleneck problem. Here, inspired by an amorphous alloy design strategy with chemical tunability, a noble-metal-free FeCoMoPB amorphous nanoplate for superior alkaline water oxidation is developed. The achieved overpotentials at current densities of 10, 100, and 500 mA cm^-2 are 239, 281, and 331 mV, respectively, while retaining a reliable stability of 48 h, outperforming most currently available electrocatalysts. Experimental and theoretical results reveal that the chemical complexity of the amorphous nanoplate leads to the formation of multiple active sites that is able to greatly lower the free energy of the rate-determining step during the water oxidation reaction. Moreover, the Mo element would result in an electron delocalization behavior to promote electron redistribution at its surrounding regions for readily donating and taking electrons. This amorphous alloy design strategy is expected to stimulate the development of more efficient electrocatalysts that is applicable in energy devices, such as metal-air batteries, fuel cells, and water electrolysis.

Spatial Resolution Limit for Nanoindentation Mapping on Metallic Glasses

Spatial heterogeneity, as a crucial structural feature, has been intensively studied in metallic glasses (MGs) using various techniques, including two-dimensional nanoindentation mapping. However, the limiting spatial resolution of nanoindentation mapping on MGs remains unexplored. In this study, a comprehensive study on four representative MGs using nanoindentation mapping with a Berkovich indenter was carried out by considering the influence of a normalized indentation spacing d/h (indentation spacing/maximum indentation depth). It appeared to have no significant correlation with the measured hardness and elastic modulus when d/h > 10. The hardness and elastic modulus started to increase slightly (up to ~5%) when d/h < 10 and further started to decrease obviously when d/h < 5. The mechanism behind these phenomena was discussed based on a morphology analysis of residual indents using scanning electron microscopy and atomic force microscopy. It was found that the highest spatial resolution of ~200 nm could be achieved with d/h = 10 using a typical Berkovich indenter for nanoindentation mapping on MGs, which was roughly ten times the curvature radius of the Berkovich indenter tip (not an ideal triangular pyramid) used in this study. These results help to promote the heterogeneity studies of MGs using nanoindentation that are capable of covering a wide range of length scales with reliable and consistent results.

Understanding the role of cross-link density in the segmental dynamics and elastic properties of cross-linked thermosets

Cross-linking is known to play a pivotal role in the relaxation dynamics and mechanical properties of thermoset polymers, which are commonly used in structural applications because of their light weight and inherently strong nature. Here, we employ a coarse-grained (CG) polymer model to systematically explore the effect of cross-link density on basic thermodynamic properties as well as corresponding changes in the segmental dynamics and elastic properties of these network materials upon approaching their glass transition temperatures (T_g). Increasing the cross-link density unsurprisingly leads to a significant slowing down of the segmental dynamics, and the fragility K of glass formation shifts in lockstep with T_g, as often found in linear polymer melts when the polymer mass is varied. As a consequence, the segmental relaxation time τ_α becomes almost a universal function of reduced temperature, (T - T_g)/T_g, a phenomenon that underlies the applicability of the "universal" Williams-Landel-Ferry (WLF) relation to many polymer materials. We also test a mathematical model of the temperature dependence of the linear elastic moduli based on a simple rigidity percolation theory and quantify the fluctuations in the local stiffness of the network material. The moduli and distribution of the local stiffness likewise exhibit a universal scaling behavior for materials having different cross-link densities but fixed (T - T_g)/T_g. Evidently, T_g dominates both τ_α and the mechanical properties of our model cross-linked polymer materials. Our work provides physical insights into how the cross-link density affects glass formation, aiding in the design of cross-linked thermosets and other structurally complex glass-forming materials.

A Review of the Preparation, Machining Performance, and Application of Fe-Based Amorphous Alloys

Amorphous alloy is an emerging metal material, and its unique atomic arrangement brings it the excellent properties of high strength and high hardness, and, therefore, have attracted extensive attention in the fields of electronic information and cutting-edge products. Their applications involve machining and forming, make the machining performance of amorphous alloys being a research hotspot. However, the present research on amorphous alloys and their machining performance is widely focused, especially for Fe-based amorphous alloys, and there lacks a systematic review. Therefore, in the present research, based on the properties of amorphous alloys and Fe-based amorphous alloys, the fundamental reason and improvement method of the difficult-to-machine properties of Fe-based amorphous alloys are reviewed and analyzed. Firstly, the properties of amorphous alloys are summarized, and it is found that crystallization and high temperature in machining are the main reasons for difficult-to-machine properties. Then, the unique properties, preparation and application of Fe-based amorphous alloys are reviewed. The review found that the machining of Fe-based amorphous alloys is also deteriorated by extremely high hardness and chemical tool wear. Tool-assisted machining, low-temperature lubrication assisted machining, and magnetic field-assisted machining can effectively improve the machining performance of Fe-based amorphous alloys. The combination of assisted machining methods is the development trend in machining Fe-based amorphous alloys, and even amorphous alloys in the future. The present research provides a systematic summary for the machining of Fe-based amorphous alloys, which would serve as a reference for relevant research.

Beyond the Average: Spatial and Temporal Fluctuations in Oxide Glass-Forming Systems

Atomic structure dictates the performance of all materials systems; the characteristic of disordered materials is the significance of spatial and temporal fluctuations on composition-structure-property-performance relationships. Glass has a disordered atomic arrangement, which induces localized distributions in physical properties that are conventionally defined by average values. Quantifying these statistical distributions (including variances, fluctuations, and heterogeneities) is necessary to describe the complexity of glass-forming systems. Only recently have rigorous theories been developed to predict heterogeneities to manipulate and optimize glass properties. This article provides a comprehensive review of experimental, computational, and theoretical approaches to characterize and demonstrate the effects of short-, medium-, and long-range statistical fluctuations on physical properties (e.g., thermodynamic, kinetic, mechanical, and optical) and processes (e.g., relaxation, crystallization, and phase separation), focusing primarily on commercially relevant oxide glasses. Rigorous investigations of fluctuations enable researchers to improve the fundamental understanding of the chemistry and physics governing glass-forming systems and optimize structure-property-performance relationships for next-generation technological applications of glass, including damage-resistant electronic displays, safer pharmaceutical vials to store and transport vaccines, and lower-attenuation fiber optics. We invite the reader to join us in exploring what can be discovered by going beyond the average.

Tension-Tension Fatigue Behavior of High-Toughness Zr61Ti2Cu25Al12 Bulk Metallic Glass

Bulk metallic glasses have application potential in engineering structures due to their exceptional strength and fracture toughness. Their fatigue resistance is very important for the application as well. We report the tension-tension fatigue damage behavior of a Zr₆₁Ti₂Cu₂₅Al₁₂ bulk metallic glass, which has the highest fracture toughness among BMGs. The Zr₆₁Ti₂Cu₂₅Al₁₂ glass exhibits a tension-tension fatigue endurance limit of 195 MPa, which is higher than that of high-toughness steels. The fracture morphology of the specimens depends on the applied stress amplitude. We found flocks of shear bands, which were perpendicular to the loading direction, on the surface of the fatigue test specimens with stress amplitude higher than the fatigue limit of the glass. The fatigue cracking of the glass initiated from a shear band in a shear band flock. Our work demonstrated that the Zr₆₁Ti₂Cu₂₅Al₁₂ glass is a competitive structural material and shed light on improving the fatigue resistance of bulk metallic glasses.

Dynamic heterogeneity, cooperative motion, and Johari-Goldstein [Formula: see text]-relaxation in a metallic glass-forming material exhibiting a fragile-to-strong transition

We investigate the Johari-Goldstein (JG) [Formula: see text]-relaxation process in a model metallic glass-forming (GF) material ([Formula: see text]), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG [Formula: see text]-relaxation time [Formula: see text] and the lifetime of the mobile particle clusters [Formula: see text], defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the [Formula: see text]-relaxation time [Formula: see text] and the lifetime of immobile particle clusters [Formula: see text]. Moreover, we find that the average diffusion coefficient D nearly coincides with [Formula: see text] of the smaller atomic species (Al) and that the 'hopping time' associated with D coincides with [Formula: see text] to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG [Formula: see text]-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG [Formula: see text]-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that [Formula: see text] exhibits fragile-to-strong (FS) glass formation, as found in many other metallic GF liquids, but this fact does not greatly alter the geometrical nature of DH in this material and the relation of DH to dynamical properties. On the other hand, the temperature dependence of the DH and dynamical properties, such as the structural relaxation time, can be significantly altered from 'ordinary' GF liquids.

Investigation of the Structural Heterogeneity and Corrosion Performance of the Annealed Fe-Based Metallic Glasses

This study investigated the structural heterogeneity, mechanical property, electrochemical behavior, and passive film characteristics of Fe-Cr-Mo-W-C-B-Y metallic glasses (MGs), which were modified through annealing at different temperatures. Results showed that annealing MGs below the glass transition temperature enhanced corrosion resistance in HCl solution owing to a highly protective passive film formed, originating from the decreased free volume and the shrinkage of the first coordination shell, which was found by pair distribution function analysis. In contrast, the enlarged first coordination shell and nanoscale crystal-like clusters were identified for MGs annealed in the supercooled liquid region, which led to a destabilized passive film and thereby deteriorated corrosion resistance. This finding reveals the crucial role of structural heterogeneity in tuning the corrosion performance of MGs.

Confinement effects on the mechanical heterogeneity of polymer fiber glasses

Both polymer fiber glasses and bulk polymer glasses exhibit nonlinear mechanical responses under uniaxial deformation. In polymer fibers, however, polymer chains are confined strongly and the surface area is relatively large compared to their volume. The confinement and the surface may lead to the spatially heterogeneous relaxation of chains in polymer fibers. In this work we perform molecular dynamics simulations and investigate the relation between the heterogeneous dynamics and the nonlinear mechanical responses at a molecular level. Our molecular simulations capture successfully not only the nonlinear mechanical response but also the dependence of mechanical properties on the strain rate of typical polymer glasses as in experiments. We find that the local elastic modulus and the nonaffine displacement are spatially heterogeneous in the pre-yield regime, which results in a lower elastic modulus for polymer fibers than bulk polymer glasses. In the post-yield regime, those mechanical properties become relatively homogeneous. Monomers with large nonaffine displacement are localized mainly at the interfacial region in the pre-yield regime while highly nonaffine monomers are distributed throughout the fibers in the post-yield regime. We show that the nonaffine displacement during deformation relates closely to the mechanical response of the polymer fibers. We also find that in the strain-hardening regime there is a significant difference in the energetic contribution to the stress between polymer fibers and bulk polymers, for which the modulus of the strain-hardening regime of the polymer fibers is smaller than that of bulk polymers.

Mapping the Viscoelastic Heterogeneity at the Nanoscale in Metallic Glasses by Static Force Spectroscopy

Nanoscale viscoelastic heterogeneity is an important concept for understanding the relationship between the microscopic atomic structure and the macroscopic mechanical behaviors in metallic glasses. However, the direct measurement of viscoelastic behavior at the nanoscale is still missing. Here we report a new measurement method based on static force microscopy to directly measure the viscoelastic properties at the nanoscale. The observed adhesive force and elastic modulus maps clearly display a typical hierarchical viscoelastic microstructure consisting of local hard and soft regions. Moreover, the adhesive force is more sensitive than the elastic modulus to viscoelastic heterogeneity and exhibits a bimodal distribution. In addition, we found that the structural relaxation and the rejuvenation effects induce the transition between the solid-like and liquid-like modes. The new measurement technique provides a powerful and quantative tool to investigate the nanoscale heterogeneity and build a connection between the microscopic structure and macroscopic mechanical behaviors in amorphous materials.

Dealloying Kinetics of AgAu Nanoparticles by In Situ Liquid-Cell Scanning Transmission Electron Microscopy

Understanding the formation and evolution of bicontinuous nanoporous structure during dealloying has been one of the most challenging subjects of dealloying research. However, previous in situ investigations either suffer from insufficient spatial resolution (e.g., X-ray tomography) or lack morphology visualization and mass information (e.g., scanning tunneling microscopy). In this work, we report the kinetics of the whole course of dealloying by utilizing liquid-cell aberration-corrected scanning transmission electron microscopy. With Z-contrast imaging analysis, the in situ sub-nanoscale characterization reveals two new phenomena, an initial period of dealloying indicative of an initial length scale for bulk dealloying and a large volume shrinkage in a nanoscale alloy precursor. We explain the particle-size-dependent volume shrinkage with the formation of a dense shell and quantify the dependence with a simple geometric model. These insights into the mechanisms of dealloying will enable deliberate designs of nanoporous structures.

Two-way tuning of structural order in metallic glasses

Metallic glasses are expected to have quite tunable structures in their configuration space, without the strict constraints of a well-defined crystalline symmetry and large energy barriers separating different states in crystals. However, effectively modulating the structure of metallic glasses is rather difficult. Here, using complementary in situ synchrotron x-ray techniques, we reveal thermal-driven structural ordering in a Ce₆₅Al₁₀Co₂₅ metallic glass, and a reverse disordering process via a pressure-induced rejuvenation between two states with distinct structural order characteristics. Studies on other metallic glass samples with different compositions also show similar phenomena. Our findings demonstrate the feasibility of two-way structural tuning states in terms of their dramatic ordering and disordering far beyond the nearest-neighbor shells with the combination of temperature and pressure, extending accessible states of metallic glasses to unexplored configuration spaces.

Universal nature of dynamic heterogeneity in glass-forming liquids: A comparative study of metallic and polymeric glass-forming liquids

Glass-formation is a ubiquitous phenomenon that is often observed in a broad class of materials ranging from biological matter to commonly encountered synthetic polymer, as well as metallic and inorganic glass-forming (GF) materials. Despite the many regularities in the dynamical properties of GF materials, the structural origin of the universal dynamical properties of these materials has not yet been identified. Recent simulations of coarse-grained polymeric GF liquids have indicated the coexistence of clusters of mobile and immobile particles that appear to be directly linked, respectively, to the rate of molecular diffusion and structural relaxation. The present work examines the extent to which these distinct types of "dynamic heterogeneity" (DH) arise in metallic GF liquids (Cu-Zr, Ni-Nb, and Pd-Si alloys) having a vastly different molecular structure and chemistry. We first identified mobile and immobile particles and their transient clusters and found the DH in the metallic alloys to be remarkably similar in form to polymeric GF liquids, confirming the "universality" of the DH phenomenon. Furthermore, the lifetime of the mobile particle clusters was found to be directly related to the rate of diffusion in these materials, while the lifetime of immobile particles was found to be proportional to the structural relaxation time, providing some insight into the origin of decoupling in GF liquids. An examination of particles having a locally preferred atomic packing, and clusters of such particles, suggests that there is no one-to-one relation between these populations of particles so that an understanding of the origin of DH in terms of static fluid structure remains elusive.

Atomic-scale viscoplasticity mechanisms revealed in high ductility metallic glass films

The fundamental plasticity mechanisms in thin freestanding Zr65Ni35 metallic glass films are investigated in order to unravel the origin of an outstanding strength/ductility balance. The deformation process is homogenous until fracture with no evidence of catastrophic shear banding. The creep/relaxation behaviour of the films was characterized by on-chip tensile testing, revealing an activation volume in the range 100-200 Å3. Advanced high-resolution transmission electron microscopy imaging and spectroscopy exhibit a very fine glassy nanostructure with well-defined dense Ni-rich clusters embedded in Zr-rich clusters of lower atomic density and a ~2-3 nm characteristic length scale. Nanobeam electron diffraction analysis reveals that the accumulation of plastic deformation at room-temperature correlates with monotonously increasing disruption of the local atomic order. These results provide experimental evidences of the dynamics of shear transformation zones activation in metallic glasses. The impact of the nanoscale structural heterogeneities on the mechanical properties including the rate dependent behaviour is discussed, shedding new light on the governing plasticity mechanisms in metallic glasses with initially heterogeneous atomic arrangement.

Flow units as dynamic defects in metallic glassy materials

In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass (MG) with unique properties in the absence of a long-range lattice, intensive efforts have focused on the search for similar 'defects'. The primary objective has been the elucidation of the flow mechanism of MGs. However, their atomistic mechanism of mechanical deformation and atomic flow response to stress, temperature, and failure, have proven to be challenging. In this paper, we briefly review the state-of-the-art studies on the dynamic defects in metallic glasses from the perspective of flow units. The characteristics, activation and evolution processes of flow units as well as their correlation with mechanical properties, including plasticity, strength, fracture, and dynamic relaxation, are introduced. We show that flow units that are similar to structural defects such as dislocations are crucial in the optimization and design of metallic glassy materials via the thermal, mechanical and high-pressure tailoring of these units. In this report, the relevant issues and open questions with regard to the flow unit model are also introduced and discussed.

Spatial heterogeneity as the structure feature for structure-property relationship of metallic glasses

The mechanical properties of crystalline materials can be quantitatively described by crystal defects of solute atoms, dislocations, twins, and grain boundaries with the models of solid solution strengthening, Taylor strain hardening and Hall-Petch grain boundary strengthening. However, for metallic glasses, a well-defined structure feature which dominates the mechanical properties of the disordered materials is still missing. Here, we report that nanoscale spatial heterogeneity is the inherent structural feature of metallic glasses. It has an intrinsic correlation with the strength and deformation behavior. The strength and Young's modulus of metallic glasses can be defined by the function of the square root reciprocal of the characteristic length of the spatial heterogeneity. Moreover, the stretching exponent of time-dependent strain relaxation can be quantitatively described by the characteristic length. Our study provides compelling evidence that the spatial heterogeneity is a feasible structural indicator for portraying mechanical properties of metallic glasses.

Disordered Nanoparticle Packings under Local Stress Exhibit Avalanche-Like, Environmentally Dependent Plastic Deformation

Nanoindentation experiments on disordered nanoparticle packings performed both in an atomic force microscope and in situ in a transmission electron microscope are used to investigate the mechanics of plastic deformation. Under an applied load, these highly porous films exhibit load drops, the magnitudes of which are consistent with an exponential population distribution. These load drops are attributed to local rearrangements of a small number of particles, which bear similarities to shear transformation zones and to the T1 process, both of which have been previously predicted for disordered packings. An increase in the relative humidity results in an increase in the number of observed load drops, indicating that the strength of the particle interactions has a significant effect on the modes of plastic deformation. These results suggest how disordered nanoparticle packings may be expected to behave in devices operating under varying environments.

Nanoscale Structural Evolution and Anomalous Mechanical Response of Nanoglasses by Cryogenic Thermal Cycling

One of the central themes in the amorphous materials research is to understand the nanoscale structural responses to mechanical and thermal agitations, the decoding of which is expected to provide new insights into the complex amorphous structural-property relationship. For common metallic glasses, their inherent atomic structural inhomogeneities can be rejuvenated and amplified by cryogenic thermal cycling, thus can be decoded from their responses to mechanical and thermal agitations. Here, we reported an anomalous mechanical response of a new kind of metallic glass (nanoglass) with nanoscale interface structures to cryogenic thermal cycling. As compared to those metallic glasses by liquid quenching, the Sc₇₅Fe₂₅ (at. %) nanoglass exhibits a decrease in the Young's modulus but a significant increase in the yield strength after cryogenic cycling treatments. The abnormal mechanical property change can be attributed to the complex atomic rearrangements at the short- and medium- range orders due to the intrinsic nonuniformity of the nanoglass architecture. The present work gives a new route for designing high-performance metallic glassy materials by manipulating their atomic structures and helps for understanding the complex atomic structure-property relationship in amorphous materials.

Configuration correlation governs slow dynamics of supercooled metallic liquids

The origin of dramatic slowing down of dynamics in metallic glass-forming liquids toward their glass transition temperatures is a fundamental but unresolved issue. Through extensive molecular dynamics simulations, here we show that, contrary to the previous beliefs, it is not local geometrical orderings extracted from instantaneous configurations but the intrinsic correlation between configurations that captures the structural origin governing slow dynamics. More significantly, it is demonstrated by scaling analyses that it is the correlation length extracted from configuration correlation rather than dynamic correlation lengths that is the key to determine the drastic slowdown of supercooled metallic liquids. The key role of the configuration correlation established here sheds important light on the structural origin of the mysterious glass transition and provides an essential piece of the puzzle for the development of a universal theoretical understanding of glass transition in glasses.

Correlation between Local Structure Order and Spatial Heterogeneity in a Metallic Glass

Although nanoscale spatial heterogeneity of metallic glasses has been demonstrated by extensive experimental and theoretical investigations, the nature of spatial heterogeneity remains poorly known owing to the absence of a structural depiction of the inhomogeneity from experimental insight. Here we report the experimental characterization of the spatial heterogeneity of a metallic glass by utilizing state-of-the-art angstrom-beam electron diffraction and scanning transmission electron microscopy. The subnanoscale electron diffraction reveals that the nanoscale spatial heterogeneity and corresponding density fluctuation have a close correlation with the local structure variation from icosahedronlike to tetragonal crystal-like order. The structural insights of spatial heterogeneity have important implications in understanding the properties and dynamics of metallic glasses.

Structural and dynamical characteristics of flow units in metallic glasses

The metallic glasses (MGs) are conjectured to be heterogeneous—their microscopic structures are embedded with localized, soft and loosely packed atomic regions, which are termed as flow units (FUs). Detailed knowledges on the structure and dynamical features of FUs are essential for understanding the plasticity of MGs. In our study, by performing dynamical tests on MGs in molecular dynamics simulations, we show that mechanical hysteretic loops are formed in the strain-stress curves due to the undergoing plastic events. By analyzing the activated times of each atom in different dynamical tests, we map the exact locations of FUs and the distribution of their activation probability in the initial structure of MGs. More importantly, we demonstrate that the FUs are indeed liquid-like according to the Lindemann criterion of melting.

Fragility and Strength in Nanoparticle Glasses

Glasses formed from nano- and microparticles form a fascinating testing ground to explore and understand the origins of vitrification. For atomic and molecular glasses, a wide range of fragilities have been observed; in colloidal systems, these effects can be emulated by adjusting the particle softness. The colloidal glass transition can range from a superexponential, fragile increase in viscosity with increasing density for hard spheres to a strong, Arrhenius-like transition for compressible particles. However, the microscopic origin of fragility and strength remains elusive, both in the colloidal and in the atomic domains. Here, we propose a simple model that explains fragility changes in colloidal glasses by describing the volume regulation of compressible colloids in order to maintain osmotic equilibrium. Our simple model provides a microscopic explanation for fragility, and we show that it can describe experimental data for a variety of soft colloidal systems, ranging from microgels to star polymers and proteins. Our results highlight that the elastic energy per particle acts as an effective fragility order parameter, leading to a universal description of the colloidal glass transition.

The role of configurational disorder on plastic and dynamic deformation in Cu64Zr36 metallic glasses: A molecular dynamics analysis

The varying degrees of configurational disorder in metallic glasses are investigated quantitatively by molecular dynamics studies. A parameter, the quasi-nearest atom, is used to characterize the configurational disorder in metallic glasses. Our observations suggest configurational disorder play a role in structural heterogeneity, plasticity and dynamic relaxations in metallic glasses. The broad configurational disorder regions distribution is the indicator of abundant potential deformation units and relaxations. Plastic flow, as well as relaxation, is believed to start at configurational disorder regions. The width of the shear bands and dynamic relaxations can then be regulated by the degree of configurational disorder regions in metallic glasses.

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.

Focus: Nucleation kinetics of shear bands in metallic glass

The development of shear bands is recognized as the primary mechanism in controlling the plastic deformability of metallic glasses. However, the kinetics of the nucleation of shear bands has received limited attention. The nucleation of shear bands in metallic glasses (MG) can be investigated using a nanoindentation method to monitor the development of the first pop-in event that is a signature of shear band nucleation. The analysis of a statistically significant number of first pop-in events demonstrates the stochastic behavior that is characteristic of nucleation and reveals a multimodal behavior associated with local spatial heterogeneities. The shear band nucleation rate of the two nucleation modes and the associated activation energy, activation volume, and site density were determined by loading rate experiments. The nucleation activation energy is very close to the value that is characteristic of the β relaxation in metallic glass. The identification of the rate controlling kinetics for shear band nucleation offers guidance for promoting plastic flow in metallic glass.

Fabrication of sharp atomic force microscope probes using in situ local electric field induced deposition under ambient conditions

We demonstrate a simple method to significantly improve the sharpness of standard silicon probes for an atomic force microscope or to repair a damaged probe. The method is based on creating and maintaining a strong, spatially localized electric field in the air gap between the probe tip and the surface of conductive sample. Under these conditions, nanostructure growth takes place on both the sample and the tip. The most likely mechanism is the decomposition of atmospheric adsorbate with subsequent deposition of carbon structures. This makes it possible to grow a spike of a few hundred nanometers in length on the tip. We further demonstrate that probes obtained by this method can be used for high-resolution scanning. It is important to note that all process operations are carried out in situ, in air and do not require the use of closed chambers or any additional equipment beyond the atomic force microscope itself.

Correlation between Viscoelastic Moduli and Atomic Rearrangements in Metallic Glasses

Dynamical moduli, such as storage and loss moduli, characterize the viscoelasticity of materials (i.e., time-dependent elasticity) and convey important information about the relaxation processes of glasses and supercooled liquids. A fundamental question is what ultimately determines them in glassy materials. Here, for several model metallic glasses, we demonstrate that both the storage and loss moduli are uniquely determined by the most probable atomic nonaffine displacements, regardless of temperature or frequency. Moreover, the fast-moving atoms (which contribute to dynamical heterogeneity) do not contribute explicitly to the moduli. Our findings provide a physical basis for the origin of viscoelasticity in metallic glasses.

Nanoscale Structure, Dynamics, and Aging Behavior of Metallic Glass Thin Films

Scanning tunnelling microscopy observations resolve the structure and dynamics of metallic glass Cu_100−xHf_x films and demonstrate scanning tunnelling microscopy control of aging at a metallic glass surface. Surface clusters exhibit heterogeneous hopping dynamics. Low Hf concentration films feature an aged surface of larger, slower clusters. Argon ion-sputtering destroys the aged configuration, yielding a surface in constant fluctuation. Scanning tunnelling microscopy can locally restore the relaxed state, allowing for nanoscale lithographic definition of aged sections.

Structural Signature of Plasticity Unveiled by Nano-Scale Viscoelastic Contact in a Metallic Glass

Room-temperature plasticity in metallic glasses (MGs) is commonly associated with local structural heterogeneity; however, direct observation of the subtle structural change caused by plasticity is vitally important but the data are extremely scarce. Based on dynamic atomic force microscopy (DAFM), here we show that plasticity-induced structural evolution in a Zr-Ni MG can be revealed via nano-scale viscoelastic contacts between an AFM tip and plastically deformed MG surface layers. Our experimental results clearly show a spatial amplification of the nano-scale structural heterogeneity caused by the distributed plastic flow, which can be linked to the limited growth, reorientation and agglomeration of some nano-scale energy-absorbing regions, which are reminiscent of the behavior of the defect-like regions with non-affine deformation as conceived in many theories and models. Furthermore, we are able to experimentally extract the thermodynamic properties of these nano-scale regions, which possess an energy barrier of 0.3–0.5 eV, about half of that for a typical shear transformation event that usually occurs at the onset of plasticity. The outcome of our current work sheds quantitative insights into the correlation between plasticity and structural heterogeneity in MGs.

Materials inspired by mathematics

Our world is transforming into an interacting system of the physical world and the digital world. What will be the materials science in the new era? With the rising expectations of the rapid development of computers, information science and mathematical science including statistics and probability theory, 'data-driven materials design' has become a common term. There is knowledge and experience gained in the physical world in the form of know-how and recipes for the creation of material. An important key is how we establish vocabulary and grammar to translate them into the language of the digital world. In this article, we outline how materials science develops when it encounters mathematics, showing some emerging directions.

Intrinsic correlation between β-relaxation and spatial heterogeneity in a metallic glass

β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.

Beta-relaxation in glasses is commonly attributed to the confined motions of constituent atoms in nanosized domains, but there is no direct evidence so far. Here, Zhu et al. show the correlation between the evolution of spatial heterogeneity at nanoscale and beta-relaxation below glass transition point.

Revealing β-relaxation mechanism based on energy distribution of flow units in metallic glass

The β-relaxation, which is the source of the dynamics in glass state and has practical significance to relaxation and mechanical properties of glasses, has been an open question for decades. Here, we propose a flow unit perspective to explain the structural origin and evolution of β-relaxation based on experimentally obtained energy distribution of flow units using stress relaxation method under isothermal and linear heating modes. Through the molecular dynamics simulations, we creatively design various artificial metallic glass systems and build a direct relation between β-relaxation behavior and features of flow units. Our results demonstrate that the β-relaxation in metallic glasses originates from flow units and is modulated by the energy distribution of flow units, and the density and distribution of flow units can effectively regulate the β-relaxation behavior. The results provide a better understanding of the structural origin of β-relaxation and also afford a method for designing metallic glasses with obvious β-relaxation and better mechanical properties.

Heterogeneity in the Small-Scale Deformation Behavior of Disordered Nanoparticle Packings

Atomic force microscopy-based nanoindentation is used to image and probe the local mechanical properties of thin disordered nanoparticle packings. The probed region is limited to the size of a few particles, and an individual particle can be loaded and displaced to a fraction of a single particle radius. The results demonstrate heterogeneous mechanical response that is location-dependent. The weak locations may be analogous to the "soft spots" previously predicted in glasses and other disordered packings.

Antibacterial Additive for Polystyrene Based on Silver Nanoparticles Supported on Titanium Dioxide

Silver nanoparticles supported on titanium dioxide nanoparticles (Ag/TiO2) were incorporated and evaluated as antibacterial additive for polystyrene materials. These particles were synthesized using a deposition-precipitation method by adding silver nitrate as metallic precursor, sodium hydroxide as reducing agent, and commercial TiO2 (P25) as support. Rectangular pieces of polystyrene (PS) containing 100, 300, 500, and 700 ppm (wt.%) of the additive were made using an extrusion-injection molding process, and they were evaluated for their antibacterial properties against Escherichia coli using the Pour Plate method. Particles were distributed on the PS surface, and PS pieces presented a good antibacterial efficiency at 100, 300, and 500 ppm and decreased for 700 ppm due to an additive agglomeration on the PS surface. These results validate the antibacterial properties of Ag/TiO2, determine a concentration limit at which the additive is well distributed on the PS surface, and assess the importance of Ag in this system.

Mechanisms of metastable states in CuZr systems with glass-like structures

The local structural inhomogeneity of glasses, as evidenced from broad bond-length distributions (BLDs), has been widely observed. However, the relationship between this particular structural feature and metastable states of glassy solids is poorly understood. It is important to understand the main problems of glassy solids, such as the plastic deformation mechanisms and glass-forming ability. The former is related to β-relaxation, the relaxation of a system from a subbasin to another in the potential energy landscape (PEL). The latter represents the stability of a metastable state in the PEL. Here, we explain the main reason why CuZr systems with glass-like structures exist in metastable states: a large strain energy. The calculation results obtained in this study indicate that a system with broad BLD has a large strain energy because of the nonlinear and asymmetric strain energy of bonds. Unstable polyhedra have larger volumes and more short and long bonds than stable polyhedra, which are most prone to form deformation units. The driving force for pure metal crystallization was also elucidated to be the decrease in strain energy. The results obtained in this study, which are verified by a series of calculations as well as molecular dynamics simulations, indicate the presence of metastable states in amorphous materials and elucidate the mechanisms of plastic deformation and the driving force for crystallization without chemical bonding.

Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy

The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

Inherent structure length in metallic glasses: simplicity behind complexity

One of the central themes in materials science is the structure-property relationship. In conventional crystalline metals, their mechanical behaviour is often dictated by well-defined structural defects such as dislocations, impurities, and twins. However, the structure-property relationship in amorphous alloys is far from being understood, due to great difficulties in characterizing and describing the disordered atomic-level structure. Herein, we report a universal, yet simple, correlation between the macroscopic mechanical properties (i.e., yield strength and shear modulus) and a unique characteristic structural length in metallic glasses (MGs). Our analysis indicates that this characteristic length can incorporate effects of both the inter-atomic distance and valence electron density in MGs, and result in the observed universal correlation. The current findings shed lights on the basic understanding of mechanical properties of MGs from their disordered atomic structures.