Binary Bulk Metallic Glasses Research Articles

Composition optimization of Zr55Cu30Al10Ni5 bulk metallic glass using cluster formula approach

Compositions of typical binary and ternary bulk metallic glasses have previously been interpreted via a cluster formula approach, i.e., a good glass former is formulated by a nearest-neighbor cluster matched with one or three glue atoms, with a free electron number per unit formula approaching 24. In this study, a classical quaternary glass composition Zr55Cu30Al10Ni5, representative of bulk metallic glasses of complex chemistry and dual-glassy structure, is interpreted and optimized using a dual-cluster formula approach, i.e., a chemical formula representing two single-cluster formulas. First, the number of atoms in the dual-cluster formula is estimated from combinations of all possible single-cluster formulas of the major devitrification phase CuZr2, which ranges from 28 to 34. Second, the free electron number of each element is estimated using the Cu-Zr single-cluster formulas. Third, the reference composition Zr55Cu30Al10Ni5 is approximated into integer forms of 27 to 36 atoms and their free electron numbers are calculated using the assigned free electron numbers of the elements. The so-formulated compositions are examined for glass forming abilities via copper-mold arc melting. The atomic densities, the critical temperatures, the HV hardness, the volume fractions of the glassy phase in the ingots, and the activation energies of crystallization all point to a 32-atom chemical formula Zr17Cu10Al3Ni2 (Zr53.13Cu31.25Al9.38Ni6.25) as the optimal glass former, with its electron number per unit formula falling slightly below 48. This work confirms the 24-electron rule for single-cluster formulas and provides an easy route towards understanding the complex chemistries of bulk metallic glasses via multiple single-cluster formulas.

Icosahedral clusters in Cu100-xZrx (x=32,34,36,38.2,40 at.%) metallic glasses near the peak of glass-forming ability (x=36): A balance between population and encaging strength

Understanding the origin of glass-forming ability (GFA) is important for the discovery or development of new metallic glasses (MGs). It is well known that the GFA in Cu–Zr binary MGs is sensitive to the composition and possesses a peak around Cu64Zr36 (at.%). The reason for this, however, is still not fully understood due to the complexity of the physics behind glass formation. Here, we use molecular dynamics simulations and statistics to study the structure and particularly the atomic energy state of the supercooled liquid and glass phases in the Cu100-xZrx (x = 32, 34, 36, 38.2, 40 at.%) alloy series. We show that the shell atoms of icosahedral clusters (the most dominant cluster type) in all these alloys have significantly lower (by > 1 eV/atom) potential energy than the core atoms. With rising Zr-content, the core–shell energy difference of the icosahedral clusters increases, making these clusters more strongly encaged and harder to break before or during crystallization. This effect counteracts the decrease in the population of the icosahedral clusters that has been noticed in previous studies. The net result of the interplay between these two factors can be represented by the product of the fraction of icosahedral shell-atoms and the core-shell energy difference, which exhibits a maximum at 36% Zr where the highest GFA has been observed. These findings provide new insights into the origin and peaking behavior of the GFA in this important binary bulk MG system, which could facilitate the search for the best glass-forming compositions in other alloy systems.

Enhancement of plasticity and toughness of 3D printed binary Zr50Cu50 bulk metallic glass composite by deformation-induced martensitic transformation

3D printing based on selective laser melting (SLM) provides a new route to fabricate bulk metallic glass (BMG) components with desirable geometries. However, repeated laser scanning induced partial crystallization in heat affected zones (HAZs) in the 3D printing process leads to a deterioration in plasticity and fracture toughness of the printed BMGs. Here, we applied the SLM technique to fabricate a Zr50Cu50 bulk metallic glass composite reinforced with in situ-generated B2 austenite in HAZs, where the B2 phase could transform to B19’ martensite during deformation and enhance the ductility due to the transformation-induced plasticity (TRIP) effect. In addition, the B2 phase in HAZs and the amorphous phase in molten pools form a so-called “brick-and-mortar” structure, which effectively blocks crack propagation, thus improving the fracture toughness. The present work provides a new avenue for the fabrication of large-sized bulk metallic glass composites by SLM with outstanding mechanical properties.

Deformation-enhanced hierarchical multiscale structure heterogeneity in a Pd-Si bulk metallic glass

The multiscale structures in a Pd82Si18 binary bulk metallic glass before and after deformation were studied using electron microscopies, high-energy synchrotron X-ray diffraction, and small-angle scattering techniques. The experimental results revealed an enhancement of hierarchical structure heterogeneities on multiple length scales after deformation. Hierarchical multiple shear bands of high number density were observed after bending, introducing complex but periodically distributed residual strain. Pair distribution function analysis revealed that the connectivity of the short-range clusters on the medium-range scale determines the packing density difference between the tension side and the compression side in the sample after bending. In-situ synchrotron X-ray diffraction study also revealed a transformation of connection modes among short-range clusters under uniaxial tension and compression, which is consistent with those of triaxial tension/compression parts upon bending in Pd82Si18 glassy alloys. The nanoscale heterogeneities for metallic glasses after deformation observed by small-angle scattering and transmission electron microscopy may be attributed to the nanoscale amorphous phase separation and interacting multiple shear bands enhanced by plastic deformation. Our findings suggested that the enhancement of hierarchical heterogeneous structure on multiple length scales may explain the excellent plasticity of Pd-Si glassy alloys, deepening the understanding of structure-property relation during plastic deformation in metallic glasses.

Deformation-Enhanced Hierarchical Multiscale Structure Heterogeneity in a Pd-Si Bulk Metallic Glass

The multiscale structures in a Pd82Si18 binary bulk metallic glass before and after bending were studied using electron microscopy, high-energy synchrotron X-ray diffraction, and small-angle X-ray scattering. The experimental results revealed the enhancement of hierarchical structure heterogeneities on multiple length scales after deformation. Hierarchical multiple shear bands of high number density can be observed after bending, introducing complex but periodically distributed residual strain. Pair distribution function analysis revealed that the connectivity of the short-range cluster on the medium-range scale determines the packing density difference between the tension side and the compression side in the sample after bending. The electron density fluctuations on the nanoscale observed in the deformed part of the sample are indicative of a nanoscale amorphous phase separation induced by plastic deformation. Our findings will deepen the understanding of the plastic deformation mechanism of metallic glasses and shed light on designing glassy alloys of excellent plasticity and toughness through hierarchical multiscale structure heterogeneities.

Dynamic characterization of Cu–Zr binary bulk metallic glasses: A molecular dynamics study

In the present study, a molecular dynamics simulation employing embedded atom method potential is performed to investigate the formation and characterization of CuZr bulk metallic glasses (BMGs). To elucidate the effect of component concentration of three samples of BMGs including Cu25Zr75, Cu50Zr50, and Cu75Zr25 that are formed by melt quenching. The local structure of BMGs is analyzed by means of radial distribution function and local atomic number density, ρ. The mechanical behavior of three compositions is investigated using uniaxial compressive loading at a constant strain rate. It is revealed from the results that yield strength increases with increasing Cu concentration. Thermal expansion of CuZr BMGs is examined and variation in length and volume is measured. The analysis revealed that Cu25Zr75, and Cu50Zr50 exhibited the typical expansion behavior while Cu75Zr25 showed an anomalous behavior.

Enhanced mechanical properties of Ni62Nb38 bulk metallic glasses by Ta substitution

By substituting Ta for Nb in a binary Ni62Nb38 bulk metallic glass (BMG), ternary Ni62Nb38−xTax (x=5–35) BMGs were fabricated by copper mold suction casting. The present NiNbTa BMGs not only have good glass forming ability (GFA), but also have combinations of both high strength and relatively-large plastic strains, which are superior to the binary Ni62Ni38 BMG and some other Ni-Nb-based BMGs. The Ni62Nb18Ta20 BMG has fracture strength of about 3.2GPa and a plastic strain of about 1.36%. The enhancement of the mechanical properties of the NiNbTa BMGs is related to the change of the liquid behavior and glass forming ability by Ta-substitution.

A criterion for the glass-forming ability of binary bulk metallic glasses

We proposed a parameter ϕ=(ΔHmc−RΔTemix)/ΔHmc as a criterion for glass-forming ability (GFA) of binary alloys near eutectic composition using the thermodynamic data of the equilibrium phase diagram by considering the relative thermodynamic stability of the liquid phase. The larger φ is, the larger GFA is in different binary alloy systems. A linear fit of Rc=7.73×1023 exp(−65.26φ) is obtained, where Rc denotes the critical cooling rate to form glass from liquid. Since φ is determined directly from thermodynamic data of elements and well known equilibrium phase diagram, GFA of binary alloys can be easily estimated without any thermal measurement after fabrication of BMGs.

Composition interpretation procedures of bulk metallic glasses via example of Cu64Zr36

In the present report, the composition interpretation procedures based on the cluster-plus-glue-atom model are explicitly elucidated to simplify the understanding of the complicated chemistries of metallic glasses, via example of a well-known binary bulk metallic glass (BMG) Cu64Zr36. The key step of deriving the right cluster that enters into the cluster formula, termed the principal cluster, is emphasized, including defining the nearest-neighbor cluster and determining the principal cluster that is most representative for the phase structure. Clusters in the Cu8Zr3 phase structure, devitrified from glassy Cu64Zr36, are precisely identified according to a newly developed criterion based on Friedel oscillation. The principal cluster is then determined as [Cu-Cu7Zr5] for its fairly large cluster separation, as well as for the pronounced spherical periodicity order in the radial atomic density distribution profile. After matched with one Cu as glue atom, a 24-electron chemical unit [Cu-Cu7Zr5]Cu=Cu64.3Zr35.7 is formulated that explains perfectly the experimental Cu64Zr36 BMG. Via this specific example, detailed steps towards composition design of BMGs are established.

Composition, Elastic Property and Packing Efficiency Predictions for Bulk Metallic Glasses in Binary, Ternary and Quaternary Systems

The results of database based on the efficiency packed model for metallic glasses. The database contains the atomic radii information as well as elastic properties of the most commonly used alloying elements, permitting composition, packing efficiency and elastic property predictions to be made for binary, ternary and quaternary bulk metallic glasses. Twenty different alloys per system (binary, ternary and quaternary) experimentally reported in the literature were compared with those estimated by the database. Comparison charts and diagrams showed good agreement between the composition predictions and those reported from the experimentally processed metallic glasses. The elastic properties predictions could be used to elaborate Blackman diagrams in order to know, in advance, the intrinsic toughness that the investigated alloys might present. The database is intended for designing bulk metallic glasses. Finally, some quaternary alloys were experimentally produced based on the prediction obtained with the database, showing a glassy phase. The microhardness values obtained experimentally of the Zr57.52Ag10.62Al10.62Co21.24, Zr57.19Al10.7Ni10.7Cu21.41 and Hf60.22Al9.95Cu9.95Ni19.89alloys, are 3.8, 4.0 and 3.6 GPa, respectively. The Young´s modulus calculated using microhardness values (E/Hv = 20) are closed to the values obtained by the mixing rules.

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.

24 electron cluster formulas as the ‘molecular’ units of ideal metallic glasses

It is known that ideal metallic glasses fully complying with the Hume-Rothery stabilization mechanism can be expressed by a universal cluster formula of the form [cluster](glue atom)1 or 3. In the present work, it is shown, after a re-examination of the cluster-resonance model, that the number of electrons per unit cluster formula, e/u, is universally 24. The cluster formulas are then the atomic as well as the electronic structural units, mimicking the ‘molecular’ formulas for chemical substances. The origin of different electron number per atom ratios e/a is related to the total number of atoms Z in unit cluster formula, e/a = 24/Z. The 24 electron formulas are well confirmed in typical binary and ternary bulk metallic glasses.

Formation and Mechanical Properties of Pd-Si Binary Bulk Metallic Glasses

Glassy spherical samples in the diameters up to 10 mm were produced in a binary Pd-Si alloy system. These Pd-Si bulk metallic glasses (BMGs) combine high strength of about 1600 MPa and superplasticity of over 70% together. In addition to abundant micrometer-scale shear bands, 10–20 nanometer-sized shear bands were also observed on the side surface of the deformed sample. The excellent ductility shown by the Pd-Si BMGs is suggested to arise from the nanoscale structural inhomogeneity.

Glass Formation in Ni-Zr-(Al) Alloy Systems

Structural and thermal properties of binary Ni100-xZrx (30<x<75) alloys obtained by melt spinning and copper mold casting methods were investigated. The fully amorphous samples in a bulk form cannot be obtained in the binary Ni-Zr alloys over a wide composition range, though they have Tg/Tl and γ values close to or even higher than those of the binary Cu-Zr bulk metallic glasses (BMGs). The low thermal stability of the supercooled liquid against crystallization and the formation of the equilibrium crystalline phases with a high growth rate are responsible for their low glass-forming abilities (GFAs). Relatively low thermal conductivities of Ni-based alloys are also considered to be another factor to limit their GFAs. The GFA of the binary Ni65.5Zr34.5 alloy alloyed with 4% or 5% Al was enhanced, and a fully glassy rod with a diameter of 0.5 mm was formed.

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 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.

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]

Binary Ni-Ta Bulk Metallic Glasses Designed by Using a Cluster-Plus-Glue-Atom Model

With the aid of the atomic-cluster-plus-glue-atom model (ACPGA model) proposed by Dong et al [1] for bulk metallic glasses (BMGs), the formation and characteristic of Ni-Ta binary BMGs were investigated in this work. Binary glass-forming compositions containing 56.3–62.5 at.%-Ni were obtained by a composition formula [M-Ni6Ta6]Ni3based on the ACPGA model. It was found that Ni-Ta BMGs with a diameter of 2 mm was obtained over a composition range of 59 ~ 62 at.%-Ni by copper mold casting method, which are in good agreement with our model prediction. Newly-developed Ni–Ta BMGs are a kind of extreme materials, which exhibit superior thermal stability (Tg= 993K) and a ultrahigh fracture strength of about 3.5 GPa.

An assessment of binary metallic glasses: correlations between structure, glass forming ability and stability

<title/>This manuscript explores the influence of atomic structure on glass forming ability and thermal stability in binary metallic glasses. A critical assessment gives literature data for 628 alloys from 175 binary glass systems. The atomic structure is quantified for each alloy using the efficient cluster packing model. Comparison of atomic structure with amorphous thickness and thermal stability gives the following major results. Binary glasses show a strong preference for discrete solute to solvent atomic radius ratios R*, which give efficient local atomic packing. Of 15 possible R* values, only five are common and only four represent the most stable glasses. The most stable binary glasses are also typically solute rich, with enough solute atoms to fill all the solute sites and roughly one-third of the solvent sites. This suggests that antisite defects, where solutes occupy solvent atom sites, are important in the glass forming ability of the most stable glasses. This stabilising effect results from an increase in the number of more stable solute-solvent bonds in solute rich glasses. Solute rich glasses also enable efficient global atomic packing. Together, these structural constraints represent only a narrow range of topologies and thus give a useful predictive tool for the exploration and discovery of new binary bulk metallic glasses (BMGs).