Zr Amorphous Alloys Research Articles

The relation between density, thermal expansion and glass-forming ability in polycrystalline Cu–Zr alloys

In contrast to the generally accepted practice for searching metallic glassformers by trial-and-error quenching, we propose using density and coefficient of thermal expansion of as-cast polycrystalline alloys as indicators of the glass-forming ability. We study experimentally these characteristics for Cu100−xZrx alloys over the whole concentration range and a wide temperature range from room temperature up to 1120 K. In the concentration range of 35 < x < 55 at% corresponding to high glass-forming ability, strongly non-monotonous behavior of both quantities is observed. We reveal that location of the extrema on concentration dependencies of both the coefficient of thermal expansion and the density deviation from the additive law correlates with those for critical thickness of Cu–Zr amorphous alloys. We argue that correlation between glass-forming ability and anomalous behavior of thermal expansion is because as-cast polycrystalline samples of glass forming alloys possess highly distorted structure, which cause phonon anharmonicity. Our findings propose new express method to search metallic alloys with high glass-forming ability.

Local atomic structures of amorphous Pd80Si20 alloys and their configuration heredity in the rapid solidification

ABSTRACTThe local atomic structures of amorphous Pd80Si20 alloys and their configuration heredity in the rapid solidification are investigated by a molecular dynamics simulation with the help of cluster-type index method based on Honeycutt–Anderson bond-type index and an inversely tracking technique of atomic trajectories. Their short-range orders are found to be various Kasper clusters as well as their distorted configurations, and among which (10 2/1441 8/1551) bi-capped square Archimedean anti-prism (BSAP) clusters are dominated, e.g. Si-centred Pd10Si1 clusters. These Kasper clusters mainly exist in the form of isolated basic clusters. Few medium-range orders can be detected, especially for Si-centred Kasper clusters. Similarly to icosahedrons of Cu–Zr amorphous alloys, their sustainable configuration heredity also occurs firstly in the super-cooled liquid region, and BSAP clusters have higher onset temperature Tonset and bigger descendible fraction F than other Kasper clusters in the rapid solidification of Pd80Si20 alloys.

Effect of Sub-T g Annealing on the Corrosion Resistance of the Cu–Zr Amorphous Alloys

Despite the economy of material cost and excellent toughness of Cu-based amorphous alloys, especially Cu50Zr50, their poor corrosion resistance to a chloride medium limits their widespread applications. In this study, corrosion tests were performed on the Cu50Zr50 amorphous alloy with different degrees of short-range order, which were prepared by annealing below the glass transition temperature (T g). It was found that the corrosion resistance of amorphous alloys is improved to a significant level when the alloys were heated below T g. Calorimetric studies showed that thermally activated relaxation process of created disorder, which occurs during sub-T g annealing, is responsible for the improvement in the corrosion resistance. Molecular dynamics simulations performed on the Cu–Zr amorphous alloys demonstrated that the relaxation process of the alloys is associated with the formation of energetically stable icosahedra and icosahedron-like structures. Our study highlights the effects of sub-T g annealing on the improvement in the corrosion resistance of the amorphous alloys from the viewpoint the relaxation process of the short-range orders.

Skeletal Au prepared from Au–Zr amorphous alloys with controlled atomic compositions and arrangement for active oxidation of benzyl alcohol

Residual ZrO2 in skeletal Au prepared from Au–Zr amorphous alloys contributed to an enhancement of the catalytic activity.

Hydrogenation of 1-octene over skeletal Pd catalysts prepared from Pd–Zr amorphous alloys and the effect of Ni addition

The skeletal Pd catalysts were prepared from Pd–Zr alloys which possess amorphous or crystalline state. The hydrogenation of 1-octene to octane over skeletal Pd catalysts proceeded efficiently while Pd–Zr alloy was inactive. The skeletal Pd prepared from amorphous alloy showed higher catalytic activities despite the smaller surface area compared to that prepared from the corresponding crystalline alloy. These catalytic activities were attributed to the number of surface-exposed Pd of skeletal Pd which was strongly affected by the atomic arrangement of the precursor. Moreover, we investigated the effect of Ni addition to the P–Zr alloy as the third metal. Skeletal Pd with 16at% Ni showed an excellent selectivity while keeping as high catalytic activity as skeletal Pd.

Active skeletal Ni catalysts prepared from Ni–Zr amorphous alloys by oxygen treatment

Skeletal Ni catalysts were prepared from an amorphous Ni40Zr60 alloy (a-NiZr) by the selective extraction of Zr moieties by the HF treatment, followed by heating at various temperatures under vacuum, oxygen and hydrogen atmosphere. The HF treatment selectively extracted Zr moieties from a-NiZr, which resulted in the formation of high-surface-area and skeletal Ni metals. The oxygen treatment at 423K and following hydrogen treatment at 473K enhanced the catalytic activities of the skeletal Ni catalyst in the hydrogenation reaction of 1-octene with molecular H2. The temperatures and atmosphere of the pretreatment strongly affected to the catalytic performances of the skeletal Ni catalysts.

Formation and structure of V–Zr amorphous alloy thin films

Although the equilibrium phase diagram predicts that alloys in the central part of the V–Zr system should consist of V2Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V–Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic-scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to the first and second nearest neighbours in the system.

Hydrogen-induced hardening and softening of Ni–Nb–Zr amorphous alloys: Dependence on the Zr content

The influence of absorbed hydrogen on the mechanical behavior of a series of Ni–Nb–Zr amorphous metallic ribbons was investigated through nanoindentation experiments. It was revealed that the influence is significantly dependent on Zr content, that is, hydrogen induced softening in relatively low-Zr alloys, whereas hydrogen induced hardening in high-Zr alloys. The results are discussed in terms of the different roles of mobile and immobile hydrogen in the plastic deformation.

Elastic and plastic characteristics of a model Cu–Zr amorphous alloy

Athermal quasistatic simulation of shear deformation has been conducted for a realistic model Cu–Zr amorphous alloy to investigate characteristic features of elasticity and plasticity of the material. Significant reduction of the shear modulus by nonaffine atomic displacements and appreciable nonlinearity of elasticity have been observed. The fourth-order elastic constant in shear deformation and the ideal shear strength have been evaluated. Plastic deformation has been observed to start with isolated local shear transformations (LSTs) followed by collective LSTs leading to the formation of a shear band. Participation-ratio analysis (PRA) has demonstrated how the nonaffine displacement field converges as the system approaches the critical point of losing structural stability. PRA has also evaluated quantitatively the numbers of atoms participating in LSTs – the average number is about 30. Spatially anisotropic development of nascent shear band on a plane has been shown, attributable to anisotropic internal stress field induced by an LST. The evaluated stresses for the shear-band nucleation and for its propagation have indicated that the yielding in real materials is controlled by the shear-band propagation, as previously pointed out.

Atomic modeling to design favored compositions for the ternary Ni–Nb–Zr metallic glass formation

An interatomic potential is constructed for the Ni–Nb–Zr ternary metal system under the newly proposed long-range empirical formulism, which has been verified to be applicable for face-centered cubic, body-centered cubic and hexagonal close-packed transition metals and their alloys. Applying the constructed potential, Monte Carlo simulations were performed to study the alloy compositions favored for Ni–Nb–Zr metallic glass formation. Simulations reveal that the underlying physics of metallic glass formation is the collapse of the crystalline lattice when the solute concentration exceeds a critical value. An intrinsic glass formation region for the Ni–Nb–Zr system can then be predicted, reflecting the possible composition region energetically favored for metallic glass formation. Furthermore, an optimal composition sub-region was pinpointed within which the driving forces for the crystalline-to-amorphous transition are larger than those outside the sub-region. The Ni–Nb–Zr amorphous alloys with the optimal compositions are expected to be more stable or easier to be produced in practice. Moreover, Voronoi tessellation analysis reveals that the atomic structure of the Ni–Nb–Zr ternary metallic glass is obviously influenced by the alloy composition, the size difference and chemical interactions of constituent metals. Ni- (or Nb-centered) icosahedral clusters are inferred as the basic local structural feature in the Ni–Nb–Zr metallic glass.

Preparation of the Catalyst for Methanol Steam Reforming from Cr–Zr Amorphous Alloys

Amorphous Cu–Zr metallic alloys were studied as precursors to catalysts for methanol steam reforming to produce hydrogen. The alloy ribbons were prepared by single-roll rapid solidification, and the optimum pretreatment method and conditions were determined. The activity of the catalysts prepared by the improved pretreatment method was greater than that of previously prepared catalysts by two degrees of magnitude. The addition of a very small amount of Pt enhanced the activity without increasing CO formation.

Formation and properties of high Fe-content Fe-(B-Si)-Zr bulk amorphous alloys

The present work is devoted to the development of Fe-(B-Si)-Zr amorphous alloys with high glass-forming ability and good magnetic properties. Using the cluster-plus-glue-atom model proposed for ideal amorphous structures, [FeFe11B3Si](Fe1−xZrx) was determined as the cluster formula of Fe-(B-Si)-Zr alloys. The glass formation and thermal stability of the serial alloys, namely, [FeFe11B3Si](Fe1−xZrx) (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.75, and 1.0), were studied by the combination of copper mold casting, X-ray diffraction, and differential thermal analysis techniques. The maxima of glass-forming ability and thermal stability were found to occur at the compositions of [FeFe11B3Si](Fe0.6Zr0.4) and [FeFe11B3Si](Fe0.5Zr0.5). The alloys can be cast into amorphous rods with 1.5 mm diameter, and upon reheating, the amorphous alloys exhibit a large undercooled liquid span of 37 K. The saturation magnetization of the [FeFe11B3Si](Fe0.5Zr0.5) amorphous alloy was measured to be 1.4 T.

Increasing Ti–6Al–4V brazed joint strength equal to the base metal by Ti and Zr amorphous filler alloys

Microstructural features developed along with mechanical properties in furnace brazing of Ti–6Al–4V alloy using STEMET 1228 (Ti–26.8Zr–13Ni–13.9Cu, wt.%) and STEMET 1406 (Zr–9.7Ti–12.4Ni–11.2Cu, wt.%) amorphous filler alloys. Brazing temperatures employed were 900–950 °C for the titanium-based filler and 900–990 °C for the zirconium-based filler alloys, respectively. The brazing time durations were 600, 1200 and 1800 s. The brazed joints were evaluated by ultrasonic test, and their microstructures and phase constitutions analyzed by metallography, scanning electron microscopy and X-ray diffraction analysis. Since microstructural evolution across the furnace brazed joints primarily depends on their alloying elements such as Cu, Ni and Zr along the joint. Accordingly, existence of Zr2Cu, Ti2Cu and (Ti,Zr)2Ni intermetallic compounds was identified in the brazed joints. The chemical composition of segregation region in the center of brazed joints was identical to virgin filler alloy content which greatly deteriorated the shear strength of the joints. Adequate brazing time (1800 s) and/or temperature (950 °C for Ti-based and 990 °C for Zr-based) resulted in an acicular Widmanstätten microstructure throughout the entire joint section due to eutectoid reaction. This microstructure increased the shear strength of the brazed joints up to the Ti–6Al–4V tensile strength level. Consequently, Ti–6Al–4V can be furnace brazed by Ti and Zr base foils produced excellent joint strengths.

Hydrogen permeability, thermal stability and hydrogen embrittlement of Ni–Nb–Zr and Ni–Nb–Ta–Zr amorphous alloy membranes

Amorphous alloys are a promising alternative to Pd alloy membranes for hydrogen separation because of their lower cost and comparable hydrogen permeability. A series of amorphous alloy membranes consisting of Ni 60Nb 20Zr 20 (at%), (Ni 0.6Nb 0.4) 100− x Zr x and (Ni 0.6Nb 0.3Ta 0.1) 100− x Zr x (where x = 0, 10, 20 or 30) were prepared by melt spinning and then coating the foil surfaces with a thin (500 nm) layer of Pd using physical vapor deposition (PVD). A (Ni 0.6Nb 0.4) 70Zr 30 membrane exhibited the highest hydrogen permeability (1.4 × 10 −8 mol m −1 s −1 Pa −0.5) of any of the materials, measured in pure hydrogen at 450 °C. Membrane permeability increased with Zr content, but membranes higher in Zr were more susceptible to brittle failure and were more thermally unstable. Decreases in hydrogen permeability were almost always observed during long-term permeability tests at 400 and 450 °C. The addition of Ta slightly increased the thermal stability, but moderately lowered the hydrogen permeability. An AES depth profile of the membrane surface showed that metallic interdiffusion had taken place between the Pd coating and the bulk membrane, which probably accounts for the reduction in hydrogen permeability over time at 400–450 °C.

Effect of nano-crystallization on the plasticity in Cu–Zr amorphous binary alloys

Two types of stress–strain behaviors were observed during tensile deformation of Cu–Zr amorphous ribbons at room temperature and at strain rates varying between 10 −3 s −1 and 10 −5 s −1. Experimental observations carried out using HRTEM and XRD on the extent of nano-crystallization during the deformation revealed that the extent of nano-crystallization determines the nature of stress–strain behavior. Molecular dynamics simulations (MD) were carried out to support the inference.

Hydrogen induced phase transitions in amorphous alloys

Hydrogen induced structure transformations in amorphous alloys ZrPd, ZrRh and ZrRhBe were studied by neutron and X-ray-diffraction technique under gas pressure up to 2 kbar. It was shown that the amorphous alloy Zr 0.7Pd 0.3 which was hydrogenated at temperature below the temperature of crystallization decomposes into metastable hyper-stoichiometric Pd-hydride with bcc lattice and amorphous hypo-stoichiometric Zr-hydride. The polyamorphous transition (transition between the amorphous phases) was reversible while the formation of metastable phase was not.

Evolution of microstructure in Zr 55Cu 30Al 10Ni 5 bulk amorphous alloy by high power pulsed Nd:YAG laser

In this work surface modification of Zr-based bulk amorphous alloy Zr 55Cu 30Al 10Ni 5 was carried out with pulsed Nd:YAG laser. The as-cast and modified surface of amorphous alloys at different conditions were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques. Phases like Cu 10Zr 7, CuZr 2 and NiZr 2 were identified which result in the enhancement of hardness of the modified alloys surface. It was observed that low laser power and prolonged irradiation time provides large nucleation frequency and limited growth rate which intern produces small spherical particle dispersion homogeneously distributed in an amorphous matrix resulting significant increase in surface hardness.

Ni-based amorphous alloy membranes for hydrogen separation at 400 °C

Amorphous alloy membranes composed primarily of Ni and early transition metals (ETMs) are an inexpensive alternative to Pd-based alloy membranes, and these materials are therefore of particular interest for the large-scale production of hydrogen from carbon-based fuels. Catalytic membrane reactors can produce hydrogen directly from coal-derived synthesis gas at 400 °C, by combining a commercial water–gas-shift (WGS) catalyst with a hydrogen-selective membrane. In order to explore the suitability of Ni-based amorphous alloys for this application, the thermal stability and hydrogen permeation characteristics of Ni–ETM amorphous alloy membranes has been examined. A fundamental limitation of these materials is that hydrogen permeability is inversely proportional to the thermal stability of the alloy. Alloy design is therefore a compromise between hydrogen production rate and durability. Amorphous Ni 60Nb 40− X Zr X membranes have been tested at 400 °C in pure hydrogen, and in simulated coal-derived gas streams with high steam, CO and CO 2 levels, without severe degradation or corrosion-induced failure. Ni–Nb–Zr amorphous alloys are therefore prospective materials for use in a catalytic membrane reactor for coal-derived syngas.

Atomic packing density and its influence on the properties of Cu–Zr amorphous alloys

This study examined the structural factors that determine the properties of amorphous alloys . The properties of amorphous alloys were found to be intimately related to the atomic packing density, characterized by short-range ordered (SRO) atomic clusters and free volume. With increasing packing density, the modulus and strength increased, while the plasticity decreased. We discuss the fundamental microstructural aspects that influence several representative properties of amorphous alloys in terms of the packing state characterized by fractions of SRO atomic clusters.

Evolution of microstructure and non-equilibrium phases in electron beam treated Zr 55Cu 30Al 10Ni 5 bulk amorphous alloy

Electron beam (EB) is becoming very popular for the modification of the surfaces as it involves localized melting and fast cooling which helps in achieving the non-equilibrium phases as well as fine microstructure. Surface modification of Zr-based amorphous alloy Zr 55Cu 30Al 10Ni 5 has been carried out by EB melting. Differential scanning calorimetry (DSC) was employed to determine the supercooled liquid region and activation energy of crystallization. The as-cast and modified surfaces of amorphous alloy at different beam conditions were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques. Various phases like NiZr 2, CuZr 2 and Cu 10Zr 7 were identified which resulted in the enhancement of hardness of the modified alloy surface.