Binary Metallic Glasses Research Articles

Connectivity of icosahedral network and a dramatically growing static length scale in Cu-Zr binary metallic glasses

We report on and characterize, via molecular dynamics studies, the evolution of the structure of Cu${}_{50}$Zr${}_{50}$ and Cu${}_{64}$Zr${}_{36}$ metallic glasses (MGs) as temperature is varied. Interestingly, a percolating icosahedral network appears in the Cu${}_{64}$Zr${}_{36}$ system as it is supercooled. This leads us to introduce a static length scale, which grows dramatically as this three-dimensional system approaches the glass transition. Amidst interpenetrating connections, noninterpenetrating connections between icosahedra are shown to become prevalent upon supercooling and to greatly enhance the connectivity of the MG's icosahedral network. Additionally, we characterize the chemical compositions of the icosahedral networks and their components. These findings demonstrate the importance of noninterpenetrating connections for facilitating extensive structural networks in Cu-Zr MGs, which in turn drive dynamical slowing in these materials.

Room-temperature anelasticity and viscoplasticity of Cu–Zr bulk metallic glasses evaluated using nanoindentation

Anelastic and viscoplastic characteristics of Cu50Zr50 and Cu65Zr35 binary bulk metallic glasses at room temperature were examined through nanoindentation creep experiments. Results show that both the deformations are relatively more pronounced in Cu50Zr50 than in Cu65Zr35, and their amount increases with the loading rate. The results are analyzed in terms of the influences of structural defects and loading rate on the room temperature indentation creep.

The density and packing fraction of binary metallic glasses

The present work develops a physical model of metallic glass structure that gives a reasonable estimate of density. The efficient cluster-packing model is used as a starting point, and is refined by a high-fidelity estimate of the size of structure-forming clusters and cluster–cluster separations. These are predicted as continuous functions of composition and relative atom radii. Predicted densities are all are within ±10% of measured densities for 200 binary metallic glasses, representing a precision in cluster–cluster separations of ±3%. New structural insights from this work include the importance of acknowledging the unique cluster topologies to estimate cluster–cluster separations; an improved ability to estimate the higher packing efficiency of unlike atoms in the first coordination shell of atomic clusters; and an improved estimate of metalloid–metalloid separations. The unusual, bilinear influence of composition on density in Fe–B glasses is explained by considering the sizes of β and γ sites in different metallic glass structures. Global atom packing fractions derived from measured densities range from about 0.62 to 0.76, and the most stable binary glasses all have packing fractions in excess of 0.70, supporting the idea that atom packing efficiency influences glass stability.

The Deformation Mechanism of Ni–Ta Bulk Metallic Glasses After Tensile: Molecular Dynamics Study

The mechanical properties of Ni-Ta crystallizationand binary bulk metallic glasses (BMG) were investigated for this study at the nanoscale. First, the Ta9Ni3 crystals are formed by space group, and structures with different ratios (Ta1Ni1, BTa8Ni4, BTa9Ni3, BTa7Ni5) were put into unit cell randomly. The optimizations of BMG structures are performed by Density functional theory (DFT) calculation to find the stable amorphous structures and corresponding energy. The FMM is utilized to obtain the suitable parameters of tight-binding potential bystable amorphous structures and corresponding energies. Finally, we employ molecular dynamics (MD) simulation to study mechanical properties of Ni/Ta crystallization and BMG, such as atomistic stress-strain, plastic and elastic deformation, and elastic modulus.

Al<sub>0.5</sub>TiZrPdCuNi High-Entropy (H-E) Alloy Developed through Ti<sub>20</sub>Zr<sub>20</sub>Pd<sub>20</sub>Cu<sub>20</sub>Ni<sub>20</sub> H-E Glassy Alloy Comprising Inter-Transition Metals

An Al0.5TiZrPdCuNi high-entropy (H-E) alloy with a bcc single phase was found through a Ti20Zr20Pd20Cu20Ni20 H-E glassy alloy designed based on equi-atomicity inherent to H-E alloys. The constituent elements and the composition of the Ti20Zr20Pd20Cu20Ni20 alloy were determined by regarding a binary Cu64Zr36 bulk metallic glass as Cu60Zr40 alloy and subsequent replacements of the Cu and Zr atoms with other late- and early-transition metals, respectively. The Ti20Zr20Pd20Cu20Ni20 alloy in a ribbon shape forms into a glassy single phase. The addition of 0.5Al to the Ti20Zr20Pd20Cu20Ni20 H-E glassy alloy resulted in forming a bcc single phase for a rod specimen with a diameter of 1.5 mm. The analysis revealed that the Al0.5TiZrPdCuNi H-E alloy is characterized by mixing enthalpy of −46.7 kJ·mol−1 and Delta parameter of 8.8, which are considerably larger and negative for the former and larger for the latter against the conventional H-E alloys.

Superconducting State Parameters Of Binary Metallic Glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSPs) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature TC, isotope effect exponent αand effective interaction strength NOV of some binary metallic glasses based on the superconducting (S), conditional superconducting (S’) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The TC obtained from the H-local field correction function are in excellent agreement with available theoretical or experimental data. In the present computation, the use of the pseudo-alloy-atom model (PAA) was proposed and found successful. Present work results are in qualitative agreement with such earlier reported experimental values which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSPs of the metallic glasses on the valence ‘Z’ is identified.

Fractal growth of the dense-packing phase in annealed metallic glass imaged by high-resolution atomic force microscopy

Unlike crystalline metals, which have a well-understood periodical structure, the amorphous structure of metallic glasses (MGs) is still poorly understood, particularly when such a structure rearranges itself at the nanoscale under external agitations. In this article, we provide compelling evidence obtained from a recently developed high-resolution atomic force microscopy (HRAFM) technique that reveals the nanoscale structural heterogeneity after thermal annealing in a Zr–Ni metallic glass. Through the HRAFM technique, we are able to uncover the annealing-induced fractal growth of the dense-packing phases in the binary MG thin film, which exhibits a fractal dimension of ∼1.7, in line with a two-dimensional diffusion limited aggregation process. The current findings not only reveal the evolution process of atomic packing in the annealed MG thin film, but also shed light on the possible cooling rate effect on the atomic structure of MGs.

First-principles study of the binary Ni60Ta40 metallic glass: The atomic structure and elastic properties

Ni–Ta bulk metallic glass (BMG) with compositions around Ni60Ta40 is a newly found binary BMG with high glass forming ability and extraordinary mechanical strength. Using ab initio molecular dynamics, the local atomic structure, elastic properties and electronic structures of Ni60Ta40 glass have been explored. The pair-correlation functions, coordination numbers, and chemical compositions of the most abundant local clusters have been analyzed. We demonstrated the existence of icosahedral Ni7Ta6 clusters as the major Ni-centered clusters, while the most popular Ta-centered cluster is Ta7Ni8. These findings agree with our previous cluster model of Ni–Ta binary BMG. The elastic moduli of Ni60Ta40 glass were also computed and the experimental Young's modulus is well reproduced. Analysis of electronic structures further revealed that the interaction between d electrons of Ni and Ta atoms is responsible for the experimentally observed ultrahigh mechanical strength for the Ni–Ta BMGs.

Correlation between fragility and eutectic instability and glass-forming ability in binary metallic glasses under growth controlled conditions

We find that the fragility can be correlated to the eutectic instability and the glass forming ability in binary metallic glass formers under growth controlled conditions via a dimensionless parameter β defined as DfT0/2(Te − T0), where Df is fragility parameter, T0 is Vogel-Fulcher-Tammann temperature, and Te is eutectic temperature. It is shown that the large β value, which results from high Df and from that T0 is close to the eutectic temperature, can lead to small interface growth velocity and undercooling of eutectic structure, and good glass forming ability. This indicates that high β leads to small characteristic diffusion length resulting from sluggish material transport that governs the glass forming ability. The results might provide an in-depth insight into the glass formation mechanism and be helpful for searching new glasses in growth controlled process.

Chemical and topological order in shear bands of Cu64Zr36 and Cu36Zr64 glasses

Shear bands in binary Cu64Zr36 and Cu36Zr64 metallic glasses are studied by molecular dynamics simulations with respect to their chemical and topological short range order. In both glasses, shear band formation goes along with an increase in excess volume inside the shear band. Only in the Cu-rich alloy, where Cu-centered icosahedra represent the most abundant topological unit, the dilatation within the shear band is related to a decrease in the number of icosahedral units, while the degree of cross-linking and the cluster size decrease when a shear band forms. In the Cu36Zr64 glass, in contrast, no topological feature changes when the material starts yielding and a shear band is formed. The chemical short range order, however, is affected in both materials within the shear band, which is revealed by an increase in the number of Cu–Cu and Zr–Zr bonds. Since all structural modifications are rather the result of plastic deformation than a pre-requisite, we conclude that the presence of distinct topological features, such as icosahedral units, is not a pre-requisite for shear band formation.

三元合金体系金属玻璃成分区域的初步预测

The glass forming ability of binary alloy metallic glass was calculated and evaluated by using thermodynamics method. The Zr-Cu-Al ternary alloy glass formation component range was estimated by using the 1/χ parameter of the proposed early. Early results proved, when 1/χ>1.28, the component range of Zr-Cu-Al ternary alloy roughly coincides with the component range of Zr-Cu-Al ternary alloy obtained through abundant experiment. On that basis, the 1/χ value of the Nd-Al-Fe and Nd-Al-Co ternary alloy was calculated and about 1/χ>2.7 and 1/χ>2.9, respec- tively. Compared with Earlier Studies result, it knows that 1/χ can effectively make true Nd-Al-Fe and Nd-Al-Co ternary alloy metallic glass formation approximate range. At last, it regards as that the 1/χ value can effectively make true the glass formation component approximate range of ternary alloy metallic glass.

The nature of the atomic-level structure in the Cu–Zr binary metallic glasses

Ab initio simulations on the basic clusters in the best glass formers of Cu–Zr metallic glasses (MGs) provide the most straightforward evidence that a gap in the density of states (DOSs) at the Fermi level is observed. We establish a direct connection between the electronic structure of the basic clusters in MGs and the glass-forming ability (GFA) of MGs, providing a new avenue to examine the GFA of MGs. And our findings provide a check for the atomic structural models of MGs, and have implications for understanding the formation and properties of MGs.

Effect of temperature on the yield strength of a binary CuZr metallic glass: Stress-induced glass transition

Compression tests were conducted on the binary Cu50Zr50 metallic glass in a temperature range below glass transition temperature where deformation mode was inhomogeneous. The yield strength of the glass was found to decrease monotonically with the increase of testing temperature in a non-linear fashion. The strength–temperature relation for the binary glass, as well as several other metallic glass systems, could be well correlated through the concept of stress-induced glass transition. The viscosity in the propagating shear band of the binary glass was also measured and found to be insensitive to the sample size and test temperature.

Glass Forming Ability and Alloying Effect of a Noble-Metal-Based Glass Former

This work addresses the question on how the glass-forming ability (GFA) of a binary Pd-Ni metallic glass can be enhanced by the alloying effect of Pt. The structural features and slow dynamics of liquid and glassy states on both alloys are investigated by molecular dynamics simulations. Both alloys show typical features of glassy dynamics, namely, the non-Arrhenian behavior of diffusion and relaxation and the fractional Stokes-Einstein relation validity at low temperatures. On the basis of the analysis of the dynamical susceptibilities, we demonstrate that there is a strong influence of the alloying effect on the collective motion of the species, revealing that the GFA of the binary liquid increases with Pt alloying.

Critically-Percolated, Cluster-Packed Structure in Cu–Zr Binary Bulk Metallic Glass Demonstrated by Molecular Dynamics Simulations Based on Plastic Crystal Model

Molecular dynamics (MD) simulations based on a plastic crystal model (PCM) were performed for a Cu0.618Zr0.382 binary bulk metallic glass (BMG). The local atomic arrangements of the Cu0.618Zr0.382 BMG were demonstrated with MD-PCM under an assumption that the BMG is composed of randomly-rotated as well as distorted hypothetical clusters. The Cu-rich Cu0.618Zr0.382 alloy was computationally created from a tentatively-created Zr-rich Zr0.73Cu0.27 alloy through two steps. The first step includes the Zr0.73Cu0.27 alloy quenched from a liquid through conventional MD simulation, whereas the second step has a replacement of the Zr and Cu atoms in the Zr0.73Cu0.27 alloy with randomly-rotated icosahedral and tetrahedral clusters, respectively, and subsequent structural relaxation. The Cu0.618Zr0.382 alloy, thus created with MD-PCM, was formed in a noncrystalline state as a critically-percolated cluster-packed structure. The analyses with total pair-distribution and interference functions revealed that the calculation results tend to reproduce the experimental data in an as-quenched state. The results explain that the glassforming ability of the Cu0.618Zr0.382 BMG is due to (1) a critically-percolated and distorted tetrahedral clusters forming a network and (2) atomistic-level inhomogeneity for the local atomic arrangements with keeping macroscopic homogeneity in terms of the chemical composition and dense random atomic arrangements. [doi:10.2320/matertrans.M2012025]

Partial Coordination Numbers in Binary Metallic Glasses

A critical analysis of measured partial coordination numbers for binary metallic glasses as a function of composition shows a large scatter of ±1.5 but clear trends. The current work uses two topological models to predict the influence of relative atomic size and concentration on partial coordination numbers. The equations for partial coordination numbers derived from these two models can reproduce measured data within experimental scatter, suggesting that chemical effects on local structure, although present, may be relatively small. Insights gained from these models show that structural site-filling rules are different for glasses with solute atoms that are smaller than solvent atoms and for glasses where solute atoms are larger than solvent atoms. Specifically, solutes may occupy both β and γ intercluster sites when the solute-to-solvent radius ratio R is less than 1.26, but only β sites can be occupied by solutes when R > 1.26. This distinction gives a simple topological explanation for the observed preference for binary metallic glasses with solutes smaller than solvent atoms. In addition to structure-specific equations, simplified phenomenological equations for partial coordination numbers are given as a convenience.

Nanoindentation study of size effect and loading rate effect on mechanical properties of a thin film metallic glass Cu 49.3Zr 50.7

A binary metallic glass (MG) Cu 49.3Zr 50.7 in the form of thin film was successfully grown on a Si (1 0 0) substrate by magnetron sputtering. The mechanical properties, specifically, hardness and modulus at various peak loads and loading rates were characterized through instrumented nanoindentation. Unlike other metallic glasses showing an indentation size effect (ISE), the composition of this study does not have an ISE, which is phenomenologically the result of the negligible length scale according to the strain gradient plasticity model. The proportional specimen resistance model is applicable to the load–displacement behaviors and suggests that the frictional effect is too small to contribute to the ISE. The occurrence of plasticity depends on loading rates and can be delayed so that the displacement during the load holding segment increases logarithmically. In addition, the hardness and modulus are both dependent on the loading rates as well, i.e., they increase as the loading rate increases up to 0.1 mN/s and then hold constant, which is independent of creep time (≤100 s). These loading-rate-dependent behaviors are interpreted as the result of viscoelastic effect rather than free volume kinetics.

A generalized polytetrahedral cluster approach to partial coordination numbers in binary metallic glasses

Partial coordination numbers (CNs) play a substantial role in description of hetero-coordinated local structure and related short-range order in metallic glasses. By defining a polytetrahedral aggregation for solute–solvent type atomic clusters, which retains cluster sphericity, high solute–solvent and solvent–solvent CNs, a preliminary model is developed to estimate the CN of a solute atom within a non-isolated cluster embedded in the glassy environment. Employing the result, a generalized quasi-hard sphere model is constructed by defining intra- and inter-cluster correlations, which can yield significantly close values to experimentally derived partial CNs in a great many metallic glass systems, such as Fe–B, Ni–B, Ni–P, Co–P, Pd–Si, Al–Y, Co–Zr, Co–Ti, Ni–Ti, Zr–Ni, Zr–Pd, Zr–Pt and Cu–Zr. The approach allows evaluation of a complete set of partial CNs in a given binary system as a function of atomic radii and composition.

Incorporation of defects into the central atoms model of a metallic glass

The central atoms model (CAM) of a metallic glass is extended to incorporate thermodynamically stable defects, similar to vacancies in a crystalline solid, within the amorphous structure. A bond deficiency (BD), which is the proposed defect present in all metallic glasses, is introduced into the CAM equations. Like vacancies in a crystalline solid, BDs are thermodynamically stable entities because of the increase in entropy associated with their creation, and there is an equilibrium concentration present in the glassy phase. When applied to CuZr and NiZr binary metallic glasses, the concentration of thermally induced BDs surrounding Zr atoms reaches a relatively constant value at the glass transition temperature, regardless of composition within a given glass system. Using this “critical” defect concentration, the predicted temperatures at which the glass transition is expected to occur are in good agreement with the experimentally determined glass transition temperatures for both alloy systems.

Favored composition region for metallic glass formation and atomic configurations in the ternary Ni–Zr–Ti system derived from n-body potential through molecular dynamics simulations

Abstract