Amorphous Cu-Zr Research Articles

How can a minor element added to a binary amorphous alloy simultaneously improve the plasticity and glass-forming ability?

Minor elements, when added to binary amorphous alloys in small percentages, can often lead to significant improvements in both the plasticity and glass-forming ability (GFA) of the alloys. Considering that plasticity and GFA are two contrasting properties dependent on short-range orders (SROs) of differing degrees, this experimental observation at first seems paradoxical when considered from an SRO viewpoint. In this study, comparative studies on amorphous alloys Cu50Zr50 and Cu47.5Zr47.5Al5 were performed using experiments and simulations to elucidate how these two apparently mutually exclusive properties can be realized at the same time. Using molecular dynamics simulations, we resolved the local structures of Cu50Zr50 and Cu47.5Zr47.5Al5 in terms of icosahedral medium-range orders. In addition, the role of the minor element (Al) on the formation of the icosahedra and their medium-range structures during cooling, as well as their disordering behavior during subsequent plastic relaxation, was clarified.

Improving the ductility of thin film metallic glasses via nano-twinning

To reduce the brittle problem of thin film metallic glasses (TFMGs), the MgZrCu TFMGs, with a positive mixing heat between Mg and Zr, are fabricated via co-sputtering in an attempt to separate the pure Mg nano-rods (with or without nano-twins) from the amorphous ZrCu matrix. The nanocrystalline Mg nano-rods are expected to hinder the propagation of shear bands in TFMGs. When the Mg contents in the TFMGs vary from about 48 to 73 at%, discontinuous Mg nano-rods are dispersed in the amorphous matrix. Microcompression results obtained from the micropillars reveal that the TFMGs with Mg contents greater than 65 at% exhibit smooth and ductile behavior. The optimum Mg65 micropillar deforms by multiple shear bands. Due to the separated Mg nano-rods with nano-twins, the Mg-based TFMG composite micropillars possess rather high modulus ~80GPa, yield stress ~1.7GPa and lateral strain ~52%, favorable for future applications in micro-electro-mechanical systems or biomedical devices.

Improved mechanical properties of sputtered and evaporated Zn films deposited on a flexible substrate with an adhesive layer of amorphous ZrCu film

Zn films with or without a 15 nm amorphous ZrCu adhesive layer are coated onto flexible polyimide substrate by sputtering and evaporation. Their tensile behavior is evaluated. The sputtered Zn films possess a nanogranular morphology measuring 30 ± 10 nm in diameter, while the evaporated Zn films show a hexagonal plate-like structure measuring 300 ± 100 nm in width. The amorphous ZrCu membrane functions as a good adhesive layer in reducing the microcrack occurrence during tensile deformation, especially for evaporated Zn films.

Tensile behavior of amorphous/nanocrystalline ZrCu/Cu multilayered films with graded interfaces

The microstructure and tensile response of amorphous ZrCu and nanocrystalline Cu multilayered thin films, with sharp or graded interfaces, are examined and analyzed. The extracted tensile properties of the multilayered films can be compared with the predicted values based on the iso-strain Rule of Mixture model. The multilayered films with graded interfaces, each about 50 nm thick, consistently exhibit higher tensile strength and elongation. This can be rationalized by the reduced stress and strain incompatibility along the interfaces.

Superplastic-like deformation in metallic amorphous/crystalline nanolayered micropillars

In this study, the ZrCu thin film metallic glasses (TFMGs) are incorporated with soft Cu thin film layers with optimum film layer thickness. Such multilayered amorphous/crystalline samples exhibit superplastic-like homogeneous deformation at room temperature. It is found that the deformability of the resultant micropillars depends on the thickness of Cu layers. Microstructural observations and theoretical analysis suggest that the superplastic-like deformation mode is attributed to homogeneous co-deformation of amorphous ZrCu and nanocrystalline Cu layers because the 100-nm-thick Cu layers can provide compatible flow stress and “plastic zone” size well matched with those of ZrCu amorphous layers.

Is the compression of tapered micro- and nanopillar samples a legitimate technique for the identification of deformation mode change in metallic glasses?

In situ transmission electron microscopy compression was conducted on amorphous ZrCu nanopillars, 140 nm in diameter and with a taper angle. The aspect ratio of nanopillars is over 6. Experimental observations and simulations both demonstrate that the deformation is localized near the top of the tapered metallic glass pillar, in sharp contrast to pillars of classic Mises plastic solids, which exhibit typical barreling behavior in compression. Localized shear bands appear to be absent, but they form inside the tapered samples.

Relationship between activation energy for hydrogen permeation and hydrogen permeation properties of amorphous Cu50Zr50 and Cu65Zr35 membranes

We investigate the role of hydrogen solubility and diffusivity on the activation energy for hydrogen permeability and the relationship between the hydrogen permeability and resistance to hydrogen embrittlement in amorphous membranes, Cu50Zr50 and Cu65Zr35. The membrane with the lower activation energy for permeation showed higher hydrogen permeability, but it showed lower durability under hydrogen atmosphere. This relationship between the two properties is discussed on the basis of the nature of the short-range ordered (SRO) structures constituting amorphous membranes under hydrogen ambient.

Tension behavior of free-standing amorphous film and amorphous–crystalline nanolaminates in submicron scale

Free-standing amorphous ZrCu film and amorphous–crystalline ZrCu/Cu nanolaminates are subjected to membrane deflection experiments to investigate their tensile deformation. The ZrCu specimen exhibits poor deformability with brittle fracture. In comparison, moderate plasticity occurs for the nanolaminates, and the plasticity increases as the thickness of the individual layers decreases from 100 down to 25 nm. The ZrCu/Cu nanolaminates with a layer thickness of 25 nm exhibit a fracture angle close to 45° and a dimpled fracture surface, implying moderate tensile ductile fracture.

Dissolved-Oxygen-Induced Intensive Pitting Corrosion of Amorphous ZrCu Alloys in Thin NaCl Solutions

Intensive pitting corrosion of amorphous (a-) Zr55Cu45 and a-Zr50Cu50 were observed by immersion in 0.2–200 mM NaCl solutions open to air at room temperature. The decreased corrosion rate in Ar-purged NaCl solution and no corrosion in ultrapure water open to air suggested that dissolved O2 and Cl− played an important role on the corrosion of the ZrCu-base amorphous alloys. The open-circuit potential showed a negative shift with increasing the NaCl concentration from 0.2 to 200 mM like that of pure Cu. These observations suggest that the corrosion of the ZrCu-base amorphous alloys in the NaCl solutions was triggered by Cu dissolution which was induced by the reduction of dissolved O2. The dissolution of Cu and Zr self-catalytically progressed by the local concentration of Cl− and hence the corrosion pits and holes were intensively formed even at the thinner NaCl concentrations.

High-Accuracy X-Ray Diffraction Analysis of Phase Evolution Sequence During Devitrification of Cu50Zr50 Metallic Glass

Real-time high-energy X-ray diffraction (HEXRD) was used to investigate the crystallization kinetics and phase selection sequence for constant-heating-rate devitrification of fully amorphous Cu50Zr50, using heating rates from 10 K/min to 60 K/min (10 °C/min to 60 °C/min). In situ HEXRD patterns were obtained by the constant-rate heating of melt-spun ribbons under synchrotron radiation. High-accuracy phase identification and quantitative assessment of phase fraction evolution though the duration of the observed transformations were performed using a Rietveld refinement method. Results for 10 K/min (10 °C/min) heating show the apparent simultaneous formation of three phases, orthorhombic Cu10Zr7, tetragonal CuZr2 (C11b), and cubic CuZr (B2), at 706 K (433 °C), followed immediately by the dissolution of the CuZr (B2) phase upon continued heating to 789 K (516 °C). Continued heating results in reprecipitation of the CuZr (B2) phase at 1002 K (729 °C), with the material transforming completely to CuZr (B2) by 1045 K (772 °C). The Cu5Zr8 phase, previously reported to be a devitrification product in C50Zr50, was not observed in the present study.

A first-principle study of the structural and electronic properties of amorphous Cu-Zr alloys

The atomic and electronic structures of amorphous Cu x Zr100−x (x=36, 46, 50, 56, 64) alloys were simulated using first-principle calculations within a 400-atom supercell. The pair correlation function, coordination numbers, local cluster structures and electronic density of states were analyzed. Reasonable agreements between the theory and the experiments were obtained. The amorphous alloys exhibit different local cluster structures and can all be explained with cluster formulas [cluster](glue)1,3, where the clusters are derived from known Cu-Zr compounds. There is always a pseudogap in the density of state at the Fermi level.

On the deposition mechanisms and the formation of glassy Cu–Zr thin films

We report on molecular dynamics (MD) simulations and physical vapor deposition experimental results concerning the development of glassy and nanocrystalline Cu–Zr thin films. MD has revealed that when Cu and Zr are deposited sequentially, a thin film overlayer is formed that consists of nanocrystalline a-Zr and t-Zr2Cu, while if Cu and Zr are simultaneously deposited, amorphous CuZr thin film emerges, due to the formation of icosahedral-like clusters that impede nucleation. Thin films grown by pulsed laser deposition and magnetron sputtering techniques were analyzed by x-ray diffraction and high-resolution transmission electron microscopy and yielded unequivocal evidence that validates our MD predictions. These findings may indicate an alternative pathway for the growth of metallic nanocomposites or glassy films.

Computer simulation and experimental study of elastic properties of amorphous Cu-Zr alloys

Molecular-dynamics simulations were performed to determine the elastic constants of CuxZr100−x (33.3≤x≤64.5) metallic glasses at room temperature. The accuracy of the interatomic potentials used to obtain the model glass structures was tested by comparing to the total structure factors obtained from high-energy synchrotron x-ray diffraction and, more importantly, to acoustic velocities measured from melt spun ribbons. Both the simulated and measured acoustic velocities increased at comparable rates with increasing Cu concentration, but the former underestimated the latter by about 10%. Young’s moduli of the simulated models were determined by combining the ultrasonic data with densities that were obtained from simulations. In addition, the compositional dependence of Poisson’s ratio, shear modulus, and bulk modulus for this series of simulated metallic glasses was determined. Examination of partial-pair correlations deduced from simulated glass structures shows a correlation between higher bulk moduli in Cu-rich compositions and concomitant changes in Zr-Zr nearest neighbors, which exhibit a stronger sensitivity to an imposed hydrostatic stress than do Cu-Cu or Cu-Zr nearest-neighbor distances.

Cold-rolling induced amorphization in Cu–Zr, Cu–Ti–Zr and Cu–Ti–Zr–Ni multilayers

Amorphous Cu60Zr40, Cu55Ti35Zr10 and Cu47Ti35Zr10Ni8 alloys were synthesized at ambient temperature by repeated cold rolling with intermediate folding starting from a layered array of individual elemental sheets. A comparison was made of three kinds of glass-forming systems in terms of their glass-forming abilities during solid state processing. The microstructural development during the mechanical alloying process was investigated by X-ray diffraction, scanning and transmission electron microscopy. The progress of solid state reaction in different glass-forming systems was studied in detail at different deformation levels for a better understanding of the early stages of the crystal-to-amorphous transition during intense mechanical deformation. At the beginning of the solid state reaction i.e., at low deformation levels, the amorphous phase initially forms at the interlayer interface of the multilayers. The amorphous layer thickness increases with increasing deformation level and in the final stage of rolling, all samples transform almost completely to the amorphous state. The main key to the synthesis of a fully amorphous phase from pure elemental foils is the application of large deformation levels to overexceed the limit of critical solutal supersaturation for amorphization.

Atomic scale characterization of shear bands in an amorphous metal

The shear bands in a model amorphous CuZr metallic glass are examined using atomistic simulations. The localized shear zones are characterized at atomic scales by the nearest-neighbor atomic bond rotation angle, the nearest-neighbor atomic bond length, the atomic volume, and the potential energy. It is shown that the shear band is indeed a region with a liquidlike structure and different thermomechanical properties. Shear band formation could be a result from both local heating and free volume generation. The variation of shear band properties with temperature and strain rate is also briefly reported.

Cu-based amorphous alloy electrodes for fuel cells

The present work describes the characteristics of Cu–Zr and Cu–Ti amorphous alloys as catalysts for oxidation of methanol and formaldehyde in alkaline solutions. Two Cu-based amorphous alloys, Cu60Zr40 and Cu60Ti40 both prepared by melt spinning were investigated. Two types of electrode were used: as-quenched amorphous alloys and surface-activated amorphous alloys. The surface-activation treatment consisted in the immersion of the ribbons in 1 M HF solution for 30 s. The determination of the oxidation activity of methanol and formaldehyde was carried out by galvanostatic and by steady-state polarization measurements and cyclic voltammetry experiments in 1 M NaOH solutions containing CH3OH or CH2O, deoxygenated by nitrogen bubbling for 30 min at 30 °C. The HF-treated amorphous alloys exhibit catalytic activity only for formaldehyde oxidation; Zr-based alloys show higher current densities than pure crystalline copper and Ti-based alloys.

Experimental investigations of the enthalpies of formation of Zr‐based metallic amorphous binary and ternary alloys

AbstractThe enthalpies of formation of amorphous Zr‐Cu, Zr‐Ni, Zr‐Cu‐Al, Zr‐Ni‐Al and Zr‐Ni‐Ti alloys from pure crystalline elements at 298 K were measured in an isoperibolic Calvet‐type solution calorimeter which is operating in the temperature range 298–1300 K. The alloys of the following compositions were investigated:Zr1‐xCux (x = 0.30, 0.40, 0.50, 0.60, 0.65, 0.70)Zr1‐xNix (x = 0.33, 0.40, 0.67)(Zr0.65Cu0.35)1‐x (Zr0.65Cu0.35)x (x = 0, 0.10, 0.20, 0.40, 0.50, 0.60, 0.70, 0.80)(Zr0.60Ni0.40)1‐x (Zr0.60Ni0.40)x (x = 0, 0.125, 0.250, 0.374, 0.500)(Zr0.33Ni0.67)1‐x (Ti0.33Ni0.67)x (x = 0, 0.15, 0.30, 0.45)The results are discussed in comparison with thermodynamic data about enthalpies of formation of liquid alloys and intermetallic compounds available in the literature. In all investigated systems the enthalpies of formation show great negative values and a strong chemical interaction between the atoms.

Hot-Consolidation of Mechanically Milled Amorphous Zr-Cu Powders

Sinterability of mechanically milled amorphous powders has been examined in the Zr-Cu system The focus of the study is to see the effect of pressure and additive elements on sintering, microstructural evolution and grain growth during annealing. The results of sintering experiments showed that ball-milled powders exhibited a poor sinterability in spite of high surface and grain boundary energy produced by mechanical milling. The sinterability could be improved by the external applied pressure. Soft metals were used as binder phases or additive elements to enhance the ability of consolidation or to produce composite structures. The results showed that high pressure and the use of additives can assist in consolidating the mechanically milled metastable powders.

The composition dependence of the electronic properties of CuHf and CuZr amorphous metals

The nature of the electron state band in the amorphous metal alloys CuHf and CuZr has been studied using synchrotron radiation as a source in photoemission experiments. In the photon energy range - 45 eV the valence band undergoes resonant photoemission which emphasizes Hf 5d-like states in CuHf and Zr 4d-like states in CuZr, while at higher photon energies contributions from these states are strongly de-emphasized because of Cooper minima in their cross-sections. The ratio of the intensities of the valence band as compared with other peaks in the spectrum characteristic of Cu or Hf (Zr) shows that the near-Fermi-level region is not wholly Hf (Zr) d-like in character, but that the degree of hybridization of these states is less than in alloys such as FeZr where both elements have partially filled shells.

Recent advances in the catalytic properties of metastable materials

New catalysts and electrocatalysts which are amorphous alloys themselves or prepared from amorphous and other metastable alloys are reviewed in this paper. The most important characteristic of metastable materials including amorphous and nanocrystalline alloys from the chemical point of view is the homogeneous single phase nature consisting of a variety of elements, whose concentrations sometimes exceed the solubility limits at equilibrium. For the enhancement of the activity various treatments are carried out before catalytic reaction, such as oxidation-reduction and selective dissolution of alloy constituents. For example, after immersion in HF, amorphous Cu-Zr alloys show higher activity than amorphous Cu-Ti alloys for dehydrogenation of 2-propanol and hydration of acrylonitrile. This is due to dissolution of smaller amounts of titanium from Cu-Ti alloys than dissolution of zirconium from Cu-Zr alloys in HF, since the corrosion resistance of titanium in HF is higher than zirconium. The catalysts prepared from some amorphous nickel alloys are most effective for methanation of CO 2 at atmospheric pressure. Amorphous nickel-refractory metal alloys are the best cathode materials for electrolytic hydrogen evolution. The catalyst for CO 2 methanation and the electrode for electrolytic hydrogen evolution are used for building a CO 2 recycling plant to avoid global warming and to supply abundant energy.