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There is a strong surge in interest for hydrogen and direct alcohol fuel cells as zero-emission energy conversion and storage devices due to their high energy density compared to that for batteries...

Metallic glasses exhibit excellent properties such as ultrahigh strength and excellent wear and corrosion resistance, but there is limited understanding on the relationship between their atomic structure and mechanical properties as a function of their structural state. In this paper, we bridge the processing-structure-property gap by utilizing molecular dynamics simulation for a model binary metallic glass, namely Ni80P20. The structural statistics including the fraction of Voronoi index, the distribution of Voronoi volume, and medium-range ordering are calculated to explain the observed changes in mechanical behavior and strain localization upon relaxation and rejuvenation. Our findings demonstrate that the evolution of mechanical properties can be linked to the atomic structure change in terms of short- and medium-range ordering. With the help of structural statistics, the mechanical properties are determined based on simple Voronoi analysis.

The effect of ion irradiation on the hardness and yield strength of Zr57Nb5Cu15.4Ni12.6Al10 bulk metallic glass has been studied using nanoindentation and micropillar compression tests. The results for the amorphous alloy were compared with those for 304 stainless steel. After Ni2+ ion irradiation, the metallic glass was found on an average to have 16% lower hardness and 13% lower yield strength. In contrast, 304 stainless steel showed significant hardening (~ 50% hardness and strength increase) after irradiation under identical conditions. The irradiation-induced hardening of the steel was attributed to hindrance of dislocation movement from the defects generated, while the softening behavior of the metallic glass was attributed to structural change in the irradiated region while retaining a fully amorphous state. The present work paves the way for better understanding of the irradiation response of metallic glasses to develop degradation-resistant alloys for next-generation nuclear applications.

Metallic glasses are a unique class of materials combining ultrahigh strength together with plastic-like processing ability. However, the currently used melt quenching route to obtain amorphous alloys has a high cost basis in terms of manufacturing and expensive constituent elements often necessary to achieve the glassy state, thus hindering widespread adoption. In contrast, multimaterial electrodeposition offers a low-cost and versatile alternative to obtain amorphous alloys. Here, we demonstrate multiscale manufacturing of a model binary amorphous system by a facile and scalable pulsed electrodeposition approach. The structural and mechanical characteristics of electrodeposited Ni-P metallic glasses are investigated by a combination of experiments and molecular dynamics simulations. The property dependence on slight change in alloy chemistry is explained by the fraction of short-range-order clusters and geometrically unfavorable motifs. Bicapped square antiprism polyhedra clusters with two-atom connections result in more homogeneous deformation for Ni

There is limited understanding of time-dependent plastic deformation behavior of amorphous alloys as a function of their structural state. Here, the creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass was investigated in its as-cast and relaxed states using nanoindentation technique with applied load in the range of 500–1500 mN and temperature in the range of room temperature to 573 K. The creep displacement increased with increasing load and temperature since the creep process is thermally activated and diffusion rate is enhanced at elevated temperature and higher load. The creep strain rate sensitivity, which is a measure of the creep mechanism, increased with increase in temperature and decrease in applied load. This was attributed to the transition from localized to more homogeneous creep. Reduction in free volume for the relaxed alloy resulted in lower creep displacement and larger strain rate sensitivity compared to its as-cast counterpart. The results suggest that diffusion-based deformation dominate at higher temperature in contrast to shear transformation mediated plasticity at room temperature. The volume of shear transformation zone for the metallic glass was calculated using cooperative shearing model and correlated with the structural state for fundamental insights into the deformation process as a function of temperature and load.

Cryogenic thermal cycling was done for the rejuvenation of Cu46Zr47Al7 and Cu45Zr47Al7Ti1 amorphous alloys. The effect of rejuvenation on the tribological behavior of these two amorphous alloys was investigated against Si3N4 ball by reciprocating sliding wear test. A significant improvement in the tribological properties was seen for the rejuvenated Cu46Zr47Al7 alloy, which showed the lowest COF and wear rate among the studied alloys. Rejuvenation resulted in an increase in H/E and H3/E2 ratios, where H = hardness and E = elastic modulus. The Cu45Zr47Al7Ti1 amorphous alloy showed relatively smaller increase in relaxation enthalpy after rejuvenation. The wear mechanisms were determined to be mainly adhesive, fatigue, and oxidative wear. Our results indicate that rejuvenation may be an effective way to improve the tribological performance of metallic glasses both in the form of bulk alloys as well as coatings.

Amorphous metallic alloys are widely used in bulk form and as coatings for their desirable corrosion and wear behavior. Nevertheless, the effects of heat treatment and thermal cycling on these surface properties are not well understood. In this study, the corrosion and wear behavior of two Zr-based bulk metallic glasses were evaluated in as-cast and thermally relaxed states. Significant improvement in wear rate, friction coefficient, and corrosion penetration rate was seen for both alloys after thermal relaxation. A fully amorphous structure was retained with thermal relaxation below the glass transition. There was an increase in surface hardness and elastic modulus for both alloys after relaxation. The improvement in surface properties was explained based on annihilation of free volume.

Metallic glass nanocomposite thin films were synthesized for an immiscible Ag-Cu alloy system by magnetron sputtering. The structure of the films was unique, consisting of homogeneously dispersed nanocrystallites in an amorphous matrix. The size and volume fraction of the nanocrystallites increased with increasing film thickness resulting in increased elastic modulus and hardness. The high electrical conductivity of the nanocomposite films was examined by a valence-band study, which showed that exchange interaction between Ag and Cu in the nanocomposite structure resulted in enhanced charge carrier concentration. The inverse correlation between electrical conductivity and film thickness was explained by surface and interface scattering of electrons with increasing volume fraction of nanocrystallites. The small temperature dependence of conductivity was attributed to the distorted Fermi surface of the nanocomposite films resulting in a greater contribution from structure scattering, which is temperature-independent.

Electro-catalyst design with superior performance and reduced precious metal content (compared to state-of-the-art Pt/C) has been a challenge in proton exchange membrane fuel cells, preventing their widespread adoption. Metallic glasses have recently shown promising performance and large electrochemical surface area in catalytic reactions. The electro-catalytic behavior of recently developed Pt-, Pd-, and Pt/Pd-based metallic glasses was evaluated in this study using scanning electrochemical microscopy. The influence of chemistry and electronic structure on catalytic behavior was studied using scanning kelvin probe technique. The work function for the metallic glasses was lower by 75 mV to 175 mV compared to pure Pt. This resulted in higher catalytic activity for the amorphous alloys, which was attributed to the ease of charge transfer on the surface. The binding energy for the metallic glasses, measured using X-ray photoelectron spectroscopy, was higher by 0.2 eV to 0.4 eV. This explained easier removal of adsorbed species from the surface of amorphous alloys. The synergistic effect of Pt and Pd in alloys containing both the noble metals was demonstrated towards hydrogen oxidation reaction.

The electrodeposition route to obtain amorphous alloys offers a facile, low-cost, and versatile alternative to conventional melt quenching. However, there are significant knowledge gaps in tribological properties and wear mechanisms for electrodeposited metallic glasses (EMGs). Here, the wear behavior and the scratch response of a model binary amorphous alloy system were investigated. Electrodeposited Ni–P metallic glasses were systematically studied as a function of composition, with amorphous alloy formation over the narrow range of 10 at% to 20 at% phosphorus. The electrodeposited metallic glasses showed hardness values in the range of 6.6–7.4 GPa, modulus in the range of 155–163 GPa, and friction coefficient around 0.50. Among the studied alloys, electrodeposited Ni80P20 showed the lowest wear rate, which was two orders of magnitude lower than electrodeposited pure Ni. The wear mechanism was determined to be extensive plastic deformation along with mild ploughing, micro tears, and formation of discontinuous lubricious oxide patches. Scratch tests showed an increase in critical load for damage initiation with the increase in phosphorus content among the amorphous alloys following the trend: Ni80P20 > Ni85P15 > Ni90P10. The overall wear rate for the electrodeposited metallic glasses was found to be lower than most reported bulk metallic glasses (BMGs). This represents a fundamental study on structure-property correlations in electrodeposited metallic glasses and demonstrates the versatility of electrodeposition in tuning the surface properties of amorphous alloys.

High temperature mechanics of nanomoulded amorphous metals was investigated by in situ nanomechanical testing. Nanopillars of Pd43Cu27Ni10P20 metallic glass were synthesized by thermoplastic forming and their stress–strain response was obtained concurrent with direct observation of their deformation behaviour. This allowed the measurement of mechanical behaviour from nanopillars and the corresponding bulk substrate with identical thermal history. A rise in elastic modulus was seen with increase in temperature for both the nanopillars and substrate, which was explained by diffusive rearrangement of atomic-scale viscoelastic units. The results provide fundamental insights into structural rearrangement in metallic glasses.

The mechanical behavior of nano-scale metallic glasses was investigated by in situ compression tests in a scanning electron microscope. Platinum-based metallic glass nano-pillars were fabricated by thermoplastic forming. The nano-pillars and corresponding bulk substrate were tested in compression over the range of room temperature to glass transition. Stress–strain curves of the nano-pillars were obtained along with in situ observation of their deformation behavior. The bulk substrate as well as nano-pillars showed an increase in elastic modulus with temperature which is explained by diffusive rearrangement of atomic-scale viscoelastic units.

The dry sliding wear behavior and mechanisms in a bulk metallic glass composite, Ti 48 Zr 20 V 12 Cu 5 Be 15 , composed of in situ crystalline dendrites in an amorphous matrix, was studied in reciprocating mode against a WC counterface loaded at 5 N and 10 N. The composite showed higher wear rates but lower coefficient of friction compared to a monolithic fully amorphous glass. Nanomechanical characterization was done to map the hardness and modulus of the crystalline and amorphous constituents of the composite. Nano-scratch test was done on each phase to evaluate the coefficient of friction. The observed hardness values scale according to Archard’s relationship for sliding wear behavior. No tribolayer formation was seen for the composite in sharp contrast to that of the monolithic metallic glass.

Bulk metallic glass matrix composites represent a unique microstructural design strategy for overcoming the strength/ductility trade-off in structural alloys. Site-specific mechanical behavior of a Ti-based bulk metallic glass composite was evaluated at microstructural length-scale. The micro-pillars on the amorphous matrix showed an average yield point of 1.9 GPa followed by serrated plastic deformation characteristic of shear banding. In contrast, micro-pillars on the crystalline dendritic phase showed a much lower yield strength of 0.72 GPa on average for the four different crystallographic orientations chosen, smooth plastic flow with no recognizable load burst, and stable strain-hardening after yield point. A number of micro-pillars were made at the interface between the glassy matrix and the crystalline dendrite. The mixed micro-pillars showed homogeneous deformation and greater plasticity which was attributed to their smaller stored elastic energy. Shear bands initiated in the amorphous matrix were arrested by the crystalline dendrite, which accommodated plasticity through slip bands and dislocations pile-ups. The interface remained intact after plastic deformation, with no observable signs of devitrification in the amorphous phase.

Noble-metal based metallic glasses have recently shown excellent performance towards a range of catalytic reactions, which is attributed to their disordered atomic structure and high density of active sites on the surface. An in-depth understanding of the mechanisms that promote the catalytic performance of amorphous alloys would facilitate the design and development of highly efficient catalysts with significantly lower precious metal loading. In the present study, the electro-catalytic behavior of amorphous Pt- and Pd-based metallic glasses was studied towards hydrogen oxidation reaction by cyclic voltammetry and scanning electrochemical microscopy. The electrochemically active surface area for the metallic glasses was found to be several folds higher compared to pure Pt and Pd. Density functional theory calculations showed that hydrogen oxidation on the surface of the amorphous alloys preferentially followed the reverse Heyrovsky-Volmer pathway. The minimum over-potential for the amorphous alloys was calculated to be significantly lower compared to pure Pt and Pd, consistent with the experimental trends. The enhanced catalytic activity for the amorphous alloys was attributed to the reduced chemisorption of hydrogen on the metallic glass surface, particularly for the alloys containing both Pt and Pd.

Laser-material interaction for a Zirconium based bulk metallic glass was investigated. The change in glass transition, crystallization temperature and enthalpies along with microstructure of the amorphous alloy was explained by the non-isothermal history experienced during processing. Varying volume fractions of amorphous and crystalline phases were found through the cross-section. A finite element based multi-physics modeling approach was employed to predict the temperature history and associated cooling rates during laser processing. Maximum devitrification occurred in sub-surface regions that experienced suitable combination of thermodynamic and kinetic conditions during laser treatment. It was demonstrated that the location and length scale of crystallized zone can be controlled by tailoring the laser processing parameters.

Noble-metal-based amorphous alloys show exceptional electrocatalytic activity and durability, and are promising for next generation electrochemical devices for energy conversion and storage. Here, we report on the electro-mechanical behavior of pulsed laser deposited Pt-based metallic glass thin film of composition Pt57.5Cu14.7Ni5.3P22.5. Pulsed laser deposition is ideally suited for the synthesis of thin films with complex compositions since the congruent evaporation process ensures that the composition of the ablation target is replicated in the film being grown. High resolution transmission electron microscopy and X-ray diffraction show that the deposited film is fully amorphous. X-ray photoelectron spectroscopy shows that the constituent elements in the thin film are in their elemental state, forming an isotropic amorphous structure. Nano-indentation determined elastic modulus and hardness values of 52.8 and 2.98 GPa, respectively. In addition, the thin-films exhibited unusually high ductility and low coefficient of friction. The sheet resistance of the as-deposited films was found to be around 76 mΩ sq−1. By current measurement during indentation, we demonstrate current drop during shear band nucleation, which is attributed to increased electron scattering from localized heating.

Evolution of atomic distribution during devitrification of a zirconium-based bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10.0Be22.5, was investigated using three-dimensional atom probe microscopy. Atom probe analysis showed uniform distribution of constituent elements for the as-cast alloy, with no phase separation. The metallic glass was annealed at different temperatures to study the crystallization pathways and the length scale, distribution, and morphology of the phases formed. Devitrification was found to proceed via the formation of a metastable icosahedral phase. The devitrification pathway is explained based on “strong-liquid” behavior of this alloy.

Metallic glasses show a unique combination of high strength, excellent corrosion, and wear resistances because of their amorphous structure having a short-range order. In spite of excellent properties, the application of metallic glasses is restricted because of their inherent limitations in the bulk form, including limited tensile ductility. Using metallic glasses as the coatings for structural applications is an attractive way of taking advantage of their superior properties. Additionally, metallic glass-based composites having crystalline phases embedded in a amorphous matrix have also shown improved properties. Thus, metallic glasses can be synthesized as the coatings or subjected to surface modification to provide functionally superior surfaces. This article is a review of metallic glass-based coatings and surface modification of metallic glasses to achieve functionally superior surfaces for structural applications. Essential theoretical concepts were discussed which influence the processing. Common ...

The corrosion behavior of a Zirconium-based bulk metallic glass subjected to friction stir processing was evaluated. Processing was done at the tool rotational speeds of 300 rpm, 500 rpm and 900 rpm. Potentiodynamic polarization and electrochemical impedance spectroscopy studies were carried out in 0.1 M NaCl solution. A strong correlation was found between the corrosion rate and enthalpy of structural relaxation, which is a measure of free volume. Electrochemical impedance spectroscopy measurements showed lower polarization resistance for processed samples as compared to as-cast metallic glass which is consistent with the DC polarization results. Shear bands in processed specimens were found to be preferred nucleation sites for the formation of corrosion pits. In contrast, pitting was more stochastic for the as-cast metallic glass.

Temperature-induced variation in magnetic properties for cobalt-based metallic glass was investigated. The formation of metastable nanocrystalline phases prior to complete devitrification and their effect on magnetic properties for Co72B19.2Si4.8Cr4 metallic glass was studied. The nature, shape, and distribution of the intermediate nanocrystalline phases were characterized using transmission electron microscopy and x-ray diffraction. A drastic change in magnetic properties was found in going from a fully amorphous state to different stages of nanocrystallization. The coercivity changes from amorphous soft magnetic state (H c ~ 0.12 Oe) to a nanocrystalline-dispersed hard magnetic state (H c ~ 187 Oe), with no significant change in saturation magnetization. This suggests potential use in futuristic magnetic switches, fluxgate sensors, and electromagnetic shielding devices.

Understanding the electrical and mechanical behaviour of nanoporous materials is critical for their use in energy applications. A palladium-rich nanoporous film, 500 nm thick with pore size ranging from 10 to 50 nm, was obtained by electrochemical dealloying of a Ni–Pd–P–B metallic glass. Nanomechanical and electrical properties were measured simultaneously, as a function of depth, for the nanoporous structure as well as the unaltered metallic glass substrate. The elastic modulus for the nanoporous structure was found to be 22 GPa compared to 131 GPa for the metallic glass substrate. The ratio of moduli scales with the square of the relative density in agreement with linear elasticity models for cellular materials. The electrical resistivity of the nanoporous layer was found to be 2.2 times higher compared to the metallic glass substrate, which was attributed to the tortuosity of current path in cellular structures.

Metallic glass composites represent a unique alloy design strategy comprising of in situ crystalline dendrites in an amorphous matrix to achieve damage tolerance unseen in conventional structural materials. They are promising for a range of advanced applications including spacecraft gears, high-performance sporting goods and bio-implants, all of which demand high surface degradation resistance. Here, we evaluated the phase-specific electrochemical and friction characteristics of a Zr-based metallic glass composite, Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5, which comprised roughly of 40% by volume crystalline dendrites in an amorphous matrix. The amorphous matrix showed higher hardness and friction coefficient compared to the crystalline dendrites. But sliding reciprocating tests for the composite revealed inter-phase delamination rather than preferred wearing of one phase. Pitting during potentiodynamic polarization in NaCl solution was prevalent at the inter-phase boundary, confirming that galvanic coupling was the predominant corrosion mechanism. Scanning vibration electrode technique demonstrated that the amorphous matrix corroded much faster than the crystalline dendrites due to its unfavorable chemistry. Relative work function values measured using scanning kelvin probe showed the amorphous matrix to be more electropositive, which explain its preferred corrosion over the crystalline dendrites as well as its characteristic friction behavior. This study paves the way for careful partitioning of elements between the two phases in a metallic glass composite to tune its surface degradation behavior for a range of advanced applications.

The crystallization behavior and viscosity of an in situ metallic glass composite was investigated using a non-contact electrostatic levitation technique. The width of the supercooled liquid region for the amorphous matrix was found to be 74 K, indicating its excellent stability against crystallization. The viscosity data follows the Vogel–Fulcher–Tammann relation, which was used to determine the fragility of the composite melt. The viscosity of the composite is compared to monolithic metallic glasses in the context of “strong-fragile” liquid behavior.

Bulk metallic glasses are fully amorphous multi-component alloys with homogeneous and isotropic structure down to the atomic scale. Some attractive attributes of bulk metallic glasses include high strength and hardness as well as excellent corrosion and wear resistance. However, there are few reports and limited understanding of their mechanical properties at elevated temperatures. We used a nondestructive sonic resonance method to measure the Young’s modulus and Shear modulus of a bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10Be22.5, at elevated temperatures. The measurement system was designed using a laser displacement sensor to detect the sonic vibration produced by a speaker on the specimen in high-temperature furnace. The OMICRON Bode-100 Vector Network Analyzer was used to sweep the frequency and its output was connected to the speaker which vibrated the material in its flexural mode and torsional modes. A Polytec OFV-505 laser vibrometer sensor was used to capture the vibration of the material at various frequencies. The flexural and torsional mode frequency shift due to the temperature variation was used to determine the Young’s modulus and Shear modulus. The temperature range of measurement was from 50°C to 350°C. The Young’s modulus was found to reduce from 100GPa to 94GPa for the 300°C temperature span. Similarly, the Shear modulus decreased from 38.5GPa at 50°C to 36GPa at 350°C.

Refinement of crystalline dendrites in a metallic glass composite, Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5, was demonstrated by friction stir processing. The surface hardness of the amorphous matrix as well as the crystalline dendritic phase was found to increase by nearly a factor of two for the processed specimens. Higher hardness for the amorphous matrix was explained by the interaction of shear bands, while that for the crystalline dendrite was explained by grain refinement.

Structural changes in a bulk metallic glass subjected to high-strain plastic deformation was investigated. A zirconium-based bulk metallic glass was friction stir processed at different tool rotational speeds. The alloy retained its fully amorphous structure at lower speed. At higher tool rotational speed there was partial nano-crystallization with nearly three times increase in surface hardness. Changes in the glass transition temperature, relaxation and crystallization enthalpies were analyzed to explain the physics of high-strain deformation in amorphous metals.

An in situ composite coating on pure aluminium substrate was prepared by novel microwave processing of iron based metallic glass powder precursor. The microstructure obtained after processing comprised of a molybdenum rich harder phase uniformly distributed in iron rich softer phase. The hardness and modulus of each phase were obtained by nanoindentation. Scratch test across the substrate coating indicated good interface strength and strong adhesion of the coating with the substrate.

The liquid impingement erosion behavior of a zirconium-based bulk metallic glass (BMG), Zr44Ti11Cu10Ni10Be25, was evaluated in this study. For comparison, commonly used hydroturbine steel was evaluated under the same test conditions. BMG demonstrated more than four times higher resistance against cavitation erosion compared with hydroturbine steel. The unusually high erosion resistance for BMG is attributed to its uniform amorphous structure with no grain boundaries, higher hardness, and ability to accommodate strain through localized shear bands.

The wettability of different nanotextured metallic glass surfaces is investigated. Wettability is quantified by the sessile drop technique using a distilled water droplet. It is demonstrated that hydrophilic–hydrophobic nature of the metallic glass surface can be controlled through nanotopography. The contact angle was found to increase from 70° for the flat metallic glass surface to 110° for a nanorod patterned surface. The difference in contact angle is explained in terms of the surface topography/roughness measured using atomic force microscopy.

The slurry erosion behavior of a zirconium based bulk metallic glass, Zr 44 Ti 11 Cu 10 Ni 10 Be 25 , was evaluated in this study. Slurry erosion tests were carried out using a non-circulating type test rig at impingement angles of 30°, 60°, and 90°. Commonly used hydroturbine steel was evaluated under the same test conditions. At 30° impingement angle, the metallic glass demonstrated nearly 2.6 times higher erosion resistance compared to the steel. At normal impingement, the metallic glass was marginally better. Lower erosion rates of the bulk metallic glass at oblique impingement angles was attributed to its significantly higher hardness and deformation induced partial devitrification. For normal impingement, material was removed in the form of fragments from highly strained platelets due to limited plasticity of bulk metallic glasses. The metallic glass demonstrated a brittle mode of erosion with higher erosion rate at higher angle of impingement. In contrast, hydroturbine steel showed ductile mode of erosion under the same test conditions.

Synthesizing metallic nanostructures with control over morphology, surface chemistry, and length-scale is important for a wide range of applications. Nanostructures having large surface area paired with suitable chemistry are particularly desirable in catalytic applications to facilitate the reaction kinetics. However, the techniques used for nanostructure synthesis are often lengthy, difficult, require expensive precursors/stabilizers, and limit the control over nanostructure morphology/chemistry. Here tuning metallic-glass nanostructures to a wide range of morphologies, where the surface is enriched with catalytic noble metal, is reported. By combining thermoplastic nanofabrication together with electrochemical processing, hierarchical metallic nanostructures with large electrochemical surface area and high catalytic activity are synthesized. Due to the versatility in processing and independent control over multiple length-scales, the approach may serve as a tool-box for fabricating complex hierarchical nanostructures for wide ranging applications.

High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe-Co metallic glass system of composition [(Co1-x Fe x )0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent 'Stoner type' magnetization for the amorphous alloys in contrast to 'Heisenberg type' in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study.

Dendritic palladium nanostructures are obtained from Ni 60 Pd 20 P 17 B 3 metallic glass by cyclic voltammetry in sulfuric acid. The length-scale and growth kinetics of the dendrites are dictated by the self-assembling template formed during electro-dissolution of the alloy constituents, but can alternatively be manipulated using the metallic glass in the form of a suitable nano-pattern template. The morphology evolution, surface chemistry, and electro-catalytic behavior of the dendrites are discussed. These branched palladium-rich dendrites show high activity for carbon monoxide and methanol oxidation in an alkaline medium, suggesting their potential use as high-performance catalysts in direct alcohol fuel cells.

Bulk metallic glasses (BMGs) are fully amorphous multi-component alloys with homogeneous and isotropic structure down to the atomic scale. Some attractive attributes of BMGs include high strength and hardness as well as excellent corrosion and wear resistance. The research goal of this paper is to determine the mechanical properties at elevated temperatures. To accomplish this goal, we have used two methods in this paper to determine the Young's modulus of a BMGs, Zr41.2Ti13.8Cu12.5Ni10Be22.5 at elevated temperatures: sonic resonance method and nanoindentation. In the sonic resonance method, the system was designed using a laser displacement sensor to detect the sonic vibration produced by a speaker on the specimen in high-temperature furnace. The Young'ss modulus was found to reduce from 100 GPa (350°C) to 94 GPa (50°C). In the nanoindentation method, modulus was determined from the unloading curve and found to be in the same range as measurements from sonic resonance technique.

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

We report on the slurry erosion and cavitation behavior of a zirconium based bulk metallic glass (BMG), Zr44Ti11Cu10Ni10Be25. Slurry erosion and cavitation tests were carried out using a non-circulating type test rig at different impingement angles. For comparative analysis, commonly used hydroturbine steel, CA6NM (13Cr4Ni), was also evaluated under similar test conditions. For low impingement angles, BMG demonstrated nearly 3 times higher erosion resistance compared to CA6NM. However, under normal impingement condition, BMG showed marginally better erosion performance. The cavitation resistance for BMG was four times higher compared to hydroturbine steel. The unusually high erosion resistance for BMG is attributed to its uniform amorphous structure with no grain boundaries, higher hardness, and ability to accommodate strain through localized shear bands.

The trends in glass formation among bulk metallic glass-forming alloys are investigated within the framework of viscosity and specific volume measurements. This investigation was carried out using four alloys (Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, Zr52.5Cu17.9Ni14.6Al10Ti5 and Ni59.5Nb40.5) that have widely different glass forming abilities (GFAs). This study shows that the viscosity at the melting temperature is correlated with volume change upon crystallization in accordance with Cohen–Grest free-volume theory. The viscosity of the best glass former (Zr41.2Ti13.8Cu12.5Ni10Be22.5) is found to be an order of magnitude larger compared to the worst glass former (Ni59.5Nb40.5) at their respective liquidus temperatures. The other two alloys have intermediate values. The specific volume results also support the trend in GFA with the best glass former showing the smallest volume change upon crystallization and worst glass former showing the largest change. This study suggests that high viscosity and correspondingly small free volume of the liquid at the melting temperature contribute significantly to improve the GFA.

The containerless high-temperature high vacuum electrostatic levitation (ESL) technique was used for the determination of thermo-physical properties of the binary eutectic alloy, Ni 59.5 Nb 40.5 , and the ternary alloy Ni 60 Nb 34.8 Sn 5.2 . The thermo-physical properties measured were density, specific heat over hemispherical total emissivity, surface tension and viscosity. The density for both the melts was found to be higher than that predicted by the rule of mixtures. This was attributed to strong attractive interaction between the constituents. The temperature dependent specific heat capacity of the binary alloy was determined by fitting with TEMPUS results. The hemispherical total emissivity was estimated to be 0.27. Using the specific heat data and the hemispherical total emissivity, the enthalpy and entropy of fusion were calculated for the binary alloy. The viscosities for both the alloys were fit with the Vogel–Fulcher–Tammann equation to obtain the fragility parameter. The surface tension for the binary alloy showed an anomalous positive gradient while that of the ternary alloy showed a small negative gradient with temperature.

The atomic distribution in catalytically active metallic glass alloys, Pd43Cu27Ni10P20and Pt57.5Cu14.7Ni5.3P22.5, was investigated using three-dimensional atom probe microscopy. Atom probe analysis showed uniform distribution of constituent elements for both the starting amorphous alloys, with no phase separation. Both the crystallized alloys showed eutectic microstructure with a very sharp interface (~0.5 nm as determined from atom probe). The atomic distribution in the devitrified state is explained based on the “fragile liquid” behavior for these noble-metal glassy alloys.

We demonstrate the refinement and uniform distribution of the crystalline dendritic phase by friction stir processing (FSP) of titanium based in situ ductile-phase reinforced metallic glass composite. The average size of the dendrites was reduced by almost a factor of five (from 24 μm to 5 μm) for the highest tool rotational speed of 900 rpm. The large inter-connected dendrites become more fragmented with increased circularity after processing. The changes in thermal characteristics were measured by differential scanning calorimetry. The reduction in crystallization enthalpy after processing suggests partial devitrification due to the high strain plastic deformation. FSP resulted in increased hardness and modulus for both the amorphous matrix and the crystalline phase. This is explained by interaction of shear bands in amorphous matrix with the strain-hardened dendritic phase. Our approach offers a new strategy for microstructural design in metallic glass composites.

Different surface patterns with nanometer length-scales were obtained by electrochemical processing of metallic glasses. The wetting behaviour of these surface patterns was studied using sessile drop technique with distilled water droplet. It is demonstrated that the hydrophilic-hydrophobic nature of the metallic glass surface can be controlled through nano topography. The contact angle was found to increase from 70° for the flat metallic glass surface to 112° for the nano-dendritic structure. Atomic force microscopy was utilized to explain the difference in contact angle in term of the surface roughness for different nano-textures.

A pronounced effect of overheating is observed on the crystallization behavior for the three zirconium-based bulk metallic glasses: Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, and Zr52.5Cu17.9Ni14.6Al10Ti5. A threshold overheating temperature is found for each of the three alloys, above which there is a drastic increase in the undercooling level and the crystallization times. Time–temperature-transformation (TTT) diagrams were measured for the three alloys by overheating above their respective threshold temperatures. The TTT curves for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5 are very similar in shape and scale with their respective glass transition temperatures, suggesting that system-specific properties do not play a crucial role in defining crystallization kinetics in these alloys. The critical cooling rates to vitrify the alloys as determined from the TTT curves are about 2 K/s for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and 10 K/s for Zr57Cu15.4Ni12.6Al10Nb5. The measurements were conducted in a high-vacuum electrostatic levitator.

Micro fuel cells (MFC) have been identifi ed as promising alternative power sources for portable electronics. Using noncorrosive electrolytes, they offer high theoretical power densities at low operating temperatures, with the potential for stable long-term operation. [ 1 ] Although these attributes make MFCs attractive for many portable device applications, [ 2 ] the primary design challenge is to identify the most effective lowcost materials and fabrication methods. [ 3 ] Here, we present a micro fuel cell in which the catalyst layer, gas diffusion layer, and fl ow fi elds are fabricated from bulk metallic glass (BMG) using thermoplastic forming (TPF). We show that TPF is a scalable and economical technique, for the fabrication of multi-scale BMG components of a MFC. BMGs have high electrical conductivity [ 4 ] and corrosion resistance, [ 5 ] and we demonstrate that end-plates with serpentine fl ow fi elds can be embossed into Zr 35 Ti 30 Cu 8.25 Be 26.75 (Zr-BMG) through a TPFbased process. The BMG fuel cell embodies the processing advantage of TPF into hierarchical structures involving length scales ranging from nanometers to centimeters, [ 6 ] and signifi es the fabrication of fuel cell components from a single material. We show that a hierarchical architecture fabricated through TPF-based embossing of Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 (Pt-BMG) can function as a high-surface area catalyst as well as a porous gas diffusion layer, which allows us to demonstrate the concept of a metallic glass MFC. The ability to create structures over a wide range of length scales combined with remarkable electrochemical properties, suggests applications beyond MFCs, including sensors, lab-on-a-chip platforms, micro-reactors, and heterogeneous catalysis. [ 7 ]

The time-temperature-transformation curves for three zirconium-based bulk amorphous alloys are measured to identify the primary factors influencing their glass-forming ability. The melt viscosity is found to have the most pronounced influence on the glass-forming ability compared to other thermodynamic factors. Surprisingly, it is found that the better glass former has a lower crystal-melt interfacial tension. This contradictory finding is explained by the icosahedral short-range order of the undercooled liquid, which on one hand reduces the interfacial tension, while on the other hand increases its viscosity.

We explore finite size effects in the crystallization of a bulk metallic glass with nm-scale dimensions. Nanorods of Pt57.5Cu14.7Ni5.3P22.5 are produced by thermoplastic extrusion of supercooled liquid through a nanoporous template. The nanorods exhibit remarkable differences in their crystallization behavior above the glass transition. Crystallization for 100 and 200 nm diameter nanorods occurred at 6 and 24 °C lower, respectively, than the nominal crystallization temperature for bulk material while the glass transition temperatures were unchanged from the bulk value. Size dependent crystallization kinetics is discussed within a framework of classical nucleation theory, as well as possible shear and surface-induced effects.

High-temperature surface tension and viscosities for five bulk metallic glass-forming alloys with widely different glass-forming abilities are measured. The measurements are carried out in a high-vacuum electrostatic levitator using the drop oscillation technique. The surface tension follows proportional mathematical addition of pure components' surface tension except when some of the constituent elements have much lower surface tension. In such cases, there is surface segregation of the low surface tension elements. These alloys are found to have orders of magnitude higher viscosity at their melting points compared to the constituent metals. Among the bulk glass-forming alloys, the better glass former has a higher melting-temperature viscosity, which demonstrates that high-temperature viscosity has a pronounced influence on glass-forming ability. Correlations between surface tension and viscosity are also investigated.