Characterization Of Metallic Glasses Research Articles

Evidence of intrinsic structural heterogeneity by monatomic metallic glass

Structural heterogeneity is generally considered to be an intrinsic feature of metallic glasses (MGs), playing a crucial role in their numerous exceptional properties. Prior studies on the structural and dynamic heterogeneities of MGs mainly focused on multi-element systems, which were possibly caused by chemical variations, challenging to exclude. This study employed a monatomic MG, as an ideal system, to investigate structural heterogeneities despite chemical factors using atomic force microscopy. Our results confirm the persistence of markedly structural heterogeneities in monatomic MGs, which cannot be entirely eliminated through annealing, thereby solidifying the intrinsic nature of structural heterogeneity in amorphous solids. Furthermore, we discover that tuning heterogeneity allows for tailoring the properties of monatomic MGs. This work provides conclusive evidence of the intrinsic property of heterogeneity in amorphous solids, reveals the hidden the complexity which covered by the chemical factors, and could advance the current understanding of the nature of glasses.

A fractal structural feature related to dynamic crossover in metallic glass-forming liquids.

The dynamic crossover in supercooled liquids initially predicted by model coupling theory has been widely accepted, but its underlying structural origin is still an open issue for glass-forming liquids. By molecular dynamics simulations of binary CuZr liquids, the present work verifies that high pressure could enhance this crossover, facilitating the studies on the structural features at the crossover temperature Tc. We discover that the topological connectivity of icosahedral clusters is responsible for this dynamic crossover, rather than all clusters. Tc is the temperature at which the connectivity degree between these clusters reaches a maximum and the dynamic heterogeneity begins to keep stable. Below Tc, the fractal topological structures appear in the medium-range order scale. The icosahedral clusters with a certain connectivity pattern can be regarded as a fractal structural unit. By employing the established fractal analysis method, the fractal dimension D of the icosahedral network is calculated. Our results indicate that the D value increases monotonically with increasing pressure and the fractal behavior of the icosahedral network is an inherent feature of metallic glasses. We also find similar fractal behavior in clusters with high local five-fold symmetry. Our findings shed light on the origin of a dynamic crossover in the deep supercooled region of metallic glasses and also demonstrate the important role of icosahedral clusters in uncovering the fractal behavior of metallic glass.

Atomistic study on simultaneous achievement of partial crystallization and rejuvenated glassy structure in thermal process of metallic glasses

ABSTRACT The relaxation state and partial crystallization are crucial factors that affect the material properties of metallic glasses. Rejuvenation induces a less relaxed glass state and hence facilitates the control of the relaxation state. The rejuvenation and crystallization during thermal processing are associated closely with the phase changes of metallic glasses upon heating. Most nanocrystalline metallic glasses formed via conventional thermal annealing have a relaxed glassy matrix. In this molecular dynamics study, we investigate the feasibility of thermal processing to simultaneously realise both partial crystallization and rejuvenation using model alloy system. Dynamic mechanical analyses reveal relaxation behaviours and local deformation features of metallic glasses composed of a crystalline phase and a rejuvenated glass matrix. The crystalline phase increases the macroscopic shear stiffness of the composite model over a wide temperature range, whereas it does not significantly affect the internal friction of the rejuvenated glass matrix. In addition, dispersed nanocrystals suppress the development of sharp and concentrated shear localisation in the rejuvenated glass matrix. The simultaneous adjustment of the relaxation state and crystalline phase in the glass matrix is discussed from a viewpoint of the microstructure design of metallic glasses.

The effects of Ni or Nb additions on the relaxation behavior of Zr55Cu35Al10 metallic glass

Many questions raised in the field of metallic glasses (MGs) are inherently attributed to β relaxation. Therefore, this study investigated the behavior of β relaxation in the Zr55Cu35Al10 MG by differential scanning calorimetry (DSC) under different heating rates. Minor additions of Ni and Nb into the base alloy and pre-annealing process were also carried out to highlight the features of β relaxation in the thermal curves. The results showed that the pre-annealing process led to the appearance of an endothermic peak under the glass transition temperature (Tg), which is a sign of reversible β relaxation in the glassy structure. Moreover, the peak position of the endothermic event was at 0.78 Tg, 0.83 Tg, and 0.87 Tg for Zr55Cu35Al10, (Zr55Cu35Al10)98Ni2, and (Zr55Cu35Al10)98Nb2, respectively, which indicated the crucial role of minor addition on the evolution of β relaxation. In addition, the pre-annealing process from 3 h to 48 h demonstrated that the endothermic peaks in the minor-added samples tended to shift to higher temperatures. Furthermore, structural characterization of MGs revealed various types of short-range and medium-range orders in the Ni- and Nb-added alloys, leading to a different relaxation behavior in the glassy structure.

Spatial heterogeneity as the structure feature for structure\u2013property 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.

Dilatancy induced ductile-brittle transition of shear band in metallic glasses.

Dilatancy-generated structural disordering, an inherent feature of metallic glasses (MGs), has been widely accepted as the physical mechanism for the primary origin and structural evolution of shear banding, as well as the resultant shear failure. However, it remains a great challenge to determine, to what degree of dilatation, a shear banding will evolve into a runaway shear failure. In this work, using in situ acoustic emission monitoring, we probe the dilatancy evolution at the different stages of individual shear band in MGs that underwent severely plastic deformation by the controlled cutting technology. A scaling law is revealed that the dilatancy in a shear band is linearly related to its evolution degree. A transition from ductile-to-brittle shear bands is observed, where the formers dominate stable serrated flow, and the latter lead to a runaway instability (catastrophe failure) of serrated flow. To uncover the underlying mechanics, we develop a theoretical model of shear-band evolution dynamics taking into account an atomic-scale deformation process. Our theoretical results agree with the experimental observations, and demonstrate that the atomic-scale volume expansion arises from an intrinsic shear-band evolution dynamics. Importantly, the onset of the ductile-brittle transition of shear banding is controlled by a critical dilatation.

Composition interpretation of binary bulk metallic glasses via principal cluster definition

It has been pointed out that a bulk metallic glass composition could be formulated as [cluster](glue atoms)x, where the cluster is derived from a devitrification phase. However, the selection rule of the so-called principal cluster should be specified because an alloy phase usually contains multiple clusters. In this paper, two important properties of the principal clusters are emphasized, i.e., spherical periodicity and cluster isolation, both being structural features of metallic glasses. According to these two criteria, the principal clusters are rigorously identified in devitrification phases and are used to construct cluster formulas to explain binary bulk-metallic glasses of Cu-(Zr,Hf), Ni-(Nb,Ta), Al-Ca, and Pd-Si.

Detecting Structural Features in Metallic Glass via Synchrotron Radiation Experiments Combined with Simulations

Revealing the essential structural features of metallic glasses (MGs) will enhance the understanding of glass-forming mechanisms. In this work, a feasible scheme is provided where we performed the state-of-the-art synchrotron-radiation based experiments combined with simulations to investigate the microstructures of ZrCu amorphous compositions. It is revealed that in order to stabilize the amorphous state and optimize the topological and chemical distribution, besides the icosahedral or icosahedral-like clusters, other types of clusters also participate in the formation of the microstructure in MGs. This cluster-level co-existing feature may be popular in this class of glassy materials.

Structural and Mechanical Characteristics of Metallic Glasses

Structural and Mechanical Characteristics of Metallic Glasses

Magnetisation dynamics of as quenched and pulse annealed soft metallic glasses

Pulse annealing method (PAM) is a practical and effective method to improve the magnetic characteristics of metallic glasses. Pulse annealing method overcomes oxidation problems during the annealing process. It minimises the difficulties in managing the crystallisation steps and causes low coercivity in contrast to high susceptibility in milliseconds. In this work, Fe77Cr2Si5B16 and Fe39Ni39Mo4Si6B12 metallic glasses were annealed by PAM. The ferromagnetic resonance spectra were measured, and the angular variation of resonance fields at the out of plane geometries was studied. The effective magnetisation and g factor fitting parameters were obtained as 3450 and 3750 Oe for Fe77Cr2Si5B16 and Fe39Ni39Mo4Si6B12 respectively from the angular variation of the resonance field. The M–H loops at room temperature were also plotted, and the changes of the magnetic properties were observed after pulse annealing. Amorphous to nanostructure steps can be seen from this method without any doubt.

Outer medium effects and fracture nucleation sites in model experiments to mimic fracture surface features of metallic glasses

Model experiments have been conducted in order to simulate the fracture morphology of metallic glasses at low temperatures or high strain rates. The effects of specimen geometry, stress state, and external pressure on the fracture surface features in model grease systems has been investigated. It is shown that the fracture surface feature size decreases with increasing viscosity of the viscous layer. Increasing the external pressure via testing samples submerged in media with different viscosities similarly produces decreases in the fracture surface feature sizes. Results on the model systems have been compared to metallic glass fracture surfaces.

Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications

Metallic glass microstructures with high aspect ratios for micro-electro-mechanical system applications have been fabricated by micro-electro-discharge machining and selective electrochemical dissolution methods. Micro-holes and three-dimensional microstructures machined on the La62Al14Ni12Cu12, Zr55Al10Ni5Cu30 and Cu46Zr44Al7Y3 bulk metallic glasses by micro-electro-discharge machining are evaluated by using X-ray diffraction, scanning electron microscopy, and nanoindentation. The experimental results demonstrate that the machined samples kept their amorphous structure without devitrification, and their machining characteristics are related to the thermo-physical properties of the alloys and the electrode diameters. Porous, single-pore and thin-walled Zr-based metallic glass tubes with micro-pore structures can be prepared by selective electrochemical dissolution method. The high aspect ratio microstructures fabricated by the two methods have the potential applications as micro-nozzles, polymer micro-injection molding tools, micro-channels or micro-flow meters in micro-electro-mechanical system devices.

Indentation characteristics of rapidly solidified Al-Cu-V and Al-Cu-Ti alloys

We report indentation characteristics of metallic glasses and nano-composites in Al-Cu-V as well as in Al-Cu-Ti systems. The melt spun ribbons of these alloys are employed to study the surface hardness characteristics of the specimen containing microstructural features at various length scales. The characterization of these materials has been done with the help of transmission electron microscope, scanning electron microscope and X-ray diffractometer. The surface hardness characteristics of melt spun ribbons with and without crystallization have been studied using a micro-hardness tester. We shall discuss the indentation behavior of ribbons in relation to their structures and microstructures.

On the Serrated Behavior during Plastic Deformation of a Zr-Based Bulk Metallic Glass

The uniaxial compression tests were conducted on an amorphous Zr-based alloy at room temperature. The results show that this alloy exhibited the typical features of metallic glasses and the stress-strain curve is extensively serrated within the plastic regime. The analysis reveals that for all the shear band events, the maximum stresses remained approximately constant during the course of plastic deformation, and the minimum stresses began to decrease when the deformation enters the late stage. This phenomenon is probably associated with the different shear-banding behaviors within various deformation stages. Calculations find that the stress accumulation sections in the stress-strain curve are parallel to each other, indicating that such reloading processes are elastic in nature. Therefore, there are elastic and plastic sections in the stress-strain curve. Further analysis discloses that the plastic strain component carried by each shear band event increases with the applied strain within the late deformation stage.

Glass-to-liquid transition in zirconium and palladium based metallic glasses

In metallic glasses, the presence of the glass-to-liquid transition upon heating is a measure for evaluating thermal or structural stability against crystallization. In some less-stable metallic glasses, the crystallization precedes the glass transition, while the distinct glass transition is observed prior to crystallization for some stable metallic glasses. This feature seems to be exceptional for metallic glasses and rarely observed for other kinds of glass materials, e.g., covalently bonded glasses. In this paper, focusing on this intriguing feature of metallic glasses, we investigate the glass-to-liquid transition of Zr 70 Ni 30, Zr 70 Cu 30, and Pd 42.5 Ni 7.5 Cu 30 P 20 (numbers indicate at.%) metallic glasses for varying the heating rate β in differential scanning calorimetry. Although in Zr 70 Ni 30 metallic glass, the glass-to-liquid transition is not detected for ordinary heating rates, sufficiently rapid heating exposes the glass-to-liquid transition by suppressing crystallization. Whereas two fitted curves for glass transition temperature, T g and crystallization temperature, T x against log ⁡ β intersect at a quite small β for Pd 42.5 Ni 7.5 Cu 30 P 20, their intersection occurs at a relatively large β for Zr 70 Cu 30 or Zr 70 Ni 30. The value of β at the intersection apparently reflects the thermal stability of metallic glasses.

Low-temperature specific-heat anomalies associated with the boson peak in CuZr-based bulk metallic glasses

We report the pronounced low-temperature specific-heat ${C}_{p}$ anomalies associated with the boson peak in the new CuZr-based bulk metallic glasses. The origin of the ${C}_{p}$ anomalies in the atomic glasses is interpreted with the harmonic localized mode based on the dense-packed atomic clusters structural model of metallic glass. The results might have important implications for understanding the origin of the boson peak and the structural features of metallic glasses.

Structure of Metallic Glasses: Experiments and Models

Abstract The topological and the chemical short range order in metallic glasses is discussed by means of representative examples from the systems Ni-P, Ni-B, Nb-Ni, and Dy-Ni. The characterization of the chemical short range order, e.g. in terms of a short range order parameter, depends on the definition of an appropriate hypothetic statistical reference system. The consideration of the size effect is essential. Some structural features of metallic glasses point to relationships with the related individual crystalline phases, whereas other features show that the different types of metallic glasses have certain construction principles in common. Small angle scattering with hydrogenated metallic glasses suggests the presence of extended two-dimensional structures. Reverse Monte Carlo models reveal rather broad distributions of the local structural properties and show that metallic glasses are not built as assemblies of defined structural units.

Crystallization of Amorphous Ribbon Observed by Means of Scanning Tunneling Microscope

Studying of crystallization processes in metallic glasses is very important from the point of view of the phase transformations occurring in extreme conditions far from equilibrium. These processes are also interesting from the point of view of the stability of metallic glasses in potential applications. Scanning tunneling microscopy (STM) allows one to observe the surface properties of investigated species, thus it can inform on the processes of surface crystallization. On the other hand, the surface crystallization may affect some specific surface features of metallic glasses like ability to corrosion or catalytic properties. For example, magnetic anisotropy of amorphous ribbon after annealing is assumed to originate from a compressive stress induced by the formation of a crystalline surface , layer with higher density [1]. Surface crystallization and its influence on magnetic properties of Fe-based metallic glass have been studied recently by Herzer and Hilzinger [2].