Pd-based Metallic Glass Research Articles

Effect of Cu addition on the properties of Pd-based metallic glasses by experimentation and molecular dynamics simulations

The effect of trace Cu element additions on the properties of Pd-based amorphous alloys was investigated by a combination of experimental and molecular dynamics (MD) simulations. Thin ribbons of Pd40Ni40P20 and containing different proportions of Cu (1 %, 3 %, and 5 %) were prepared, analyzed by X-ray diffraction (XRD) to determine the phase structure, tested for their thermal properties by differential scanning calorimetry (DSC), and nanoindentation to test their mechanical properties. Meanwhile, the amorphization process and compression behavior of these alloys were simulated by MD simulation, and the microstructures of the alloys were resolved by using the radial distribution function (RDF) and the coordination number distribution function (PDF), and the glass transition temperatures (Tg) were deduced from the energy variations. The Voronoi polyhedral analyses showed that the addition of trace amounts of Cu altered the content of the specific clusters, which affected the macroscopic mechanical properties. Both nanoindentation experimental and simulation results confirmed that the addition of trace Cu reduces the Tg and improved the mechanical properties of the Pd40Ni40P20 alloy while maintaining the amorphous structure.

Unveiling the Structural Origins of Dynamic Diversity in Pd-Based Metallic Glasses.

The β-relaxation is one of the major dynamic behaviors in metallic glasses (MGs) and exhibits diverse features. Despite decades of efforts, the understanding of its structural origin and contribution to the overall dynamics of MG systems is still unclear. Here two palladium-based Pd─Cu─P and Pd─Ni─P MGs are reported with distinct different β-relaxation behaviors and reveal the structural origins for the difference using the advanced X-ray photon correlation spectroscopy and absorption fine structure techniques together with the first-principles calculations. The pronounced β-relaxation and fast atomic dynamics in the Pd─Cu─P MG mainly come from the strong mobility of Cu atoms and their locally favored structures. In contrast, the motion of Ni atoms is constrained by P atoms in the Pd─Ni─P MG, leading to the weakened β-relaxation peak and sluggish dynamics. The correlation of atomic dynamics with microscopic structures provides a way to understand the structural origins of different dynamic behaviors as well as the nature of aging in disordered materials.

Atomic Structures and Glass-Forming Ability of Pd-Based Bulk Metallic Glasses

To understand the relation of the glass-forming ability (GFA) to the atomic configurations of Pd42.5Ni7.5Cu30P20 (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained anomalous x-ray scattering, x-ray and neutron diffraction and applying reverse Monte Carlo modeling. By comparing the results of PNCP with Pd40Ni40P20 and Pd40Cu40P20 having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures around the secondary metals of Ni and Cu, such as the concentration inhomogeneity, the fraction of pure and slightly distorted icosahedra, and the existence of large persistent homology rings. The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible mixture of covalent-like Pd-P configurations and icosahedral clusters around the secondary metals.

Hidden shear bands of diversified structures in a bent heterogeneous metallic glass

It is highly disputed about the microstructure and evolution of shear bands (SBs) in metallic glasses (MGs) which affect their yielding and plasticity. We reconstruct the nanostructure of SBs in a bent heterogeneous Pd-based MG fiber with synchrotron X-ray nano-computed tomography and perform finite element analysis on their formation and evolution. It is found that apart from plain SBs (PSBs) under small shear offsets, a large amount of hidden SBs (HSBs) of density fluctuations appear inside the MG, which do not induce shear offsets. The average densities of the HSBs are ∼5%–∼25% smaller than the matrix around, and their thicknesses range from ∼100 nm to ∼530 nm. The diversified structures of the HSBs closely relate to the MG's multiscale structural heterogeneity and local strain rate. The HSBs can develop into PSBs that are densified near surface caused by structural relaxation under high temperatures, or be smeared by free volume redistribution in the low-density central area. The results could further the understanding of the formation and evolution of SBs' microstructure in heterogeneous glassy materials.

Comparison of structural heterogeneity in Zr- and Pd-based metallic glasses

Metallic glasses (MGs) are known to exhibit a nanoscale heterogeneous structure that consists of a weakly bonded region (WBR) and a strongly bonded region (SBR), but its characterization is not yet fully understood. This study investigates the structural heterogeneity of Zr- and Pd-based MGs using a dynamic viscoelastic model. Volume fraction (V) and elastic modulus (Ec) are independently derived from the model to characterize each region. In addition, the structural heterogeneity is evaluated using χ = VEc,WBR/VEc,total. As a result, the strong Zr-based MG is estimated to own approximately 28% of VWBR, approximately 87% of Ec,WBR/Ec,SBR, and χ ∼ 0.241. On the other hand, the fragile Pd-based MG is estimated to own approximately 26% of VWBR, approximately 70% of Ec,WBR/Ec,SBR, and χ ∼ 0.208. Furthermore, a comparison of χ, fragility, and Poisson's ratio shows that as χ increases, fragility decreases and Poisson's ratio increases. Notably, the diversification of constituent elements is inversely related to χ, fragility, and Poisson's ratio in the Zr- and Pd-based MGs. Thus, the existing understanding of the structural heterogeneity inherent in MGs is updated using the constructed dynamic viscoelastic model in this study.

Synergistic enhancement of hydrogen interactions in palladium–silicon–gold metallic glass with multilayered graphene

Presence of only a few layers of graphene boosts hydrogen intake of Pd-based metallic glass thin films by 2.6 times with 4.5 times higher electrocatalytic hydrogen evolution reaction activity, a tremendous improvement in metal–hydrogen interactions.

Structural evolution in Au- and Pd-based metallic glass forming liquids and the case for improved molecular dynamics force fields.

The results of a combined experimental and computational investigation of the structural evolution of Au81Si19, Pd82Si18, and Pd77Cu6Si17 metallic glass forming liquids are presented. Electrostatically levitated metallic liquids are prepared, and synchrotron x-ray scattering studies are combined with embedded atom method molecular dynamics simulations to probe the distribution of relevant structural units. Metal-metalloid based metallic glass forming systems are an extremely important class of materials with varied glass forming ability and mechanical processibility. High quality experimental x-ray scattering data are in poor agreement with the data from the molecular dynamics simulations, demonstrating the need for improved interatomic potentials. The first peak in the x-ray static structure factor in Pd77Cu6Si17 displays evidence for a Curie-Weiss type behavior but also a peak in the effective Curie temperature. A proposed order parameter distinguishing glass forming ability, 1/ST,q1-1, shows a peak in the effective Curie temperature near a crossover temperature established by the behavior of the viscosity, TA.

Effect of composition and thermal history on deformation behavior and cluster connections in model bulk metallic glasses

The compositional dependence and influence of relaxation state on the deformation behavior of a Pt–Pd-based bulk metallic glasses model system was investigated, where platinum is systematically replaced by topologically equivalent palladium atoms. The hardness and modulus increased with rising Pd content as well as by annealing below the glass transition temperature. Decreasing strain-rate sensitivity and increasing serration length are observed in nano indentation with increase in Pd content as well as thermal relaxation. Micro-pillar compression for alloys with different Pt/Pd ratios validated the greater tendency for shear localization and brittle behavior of the Pd-rich alloys. Based on total scattering experiments with synchrotron X-ray radiation, a correlation between the increase in stiffer 3-atom cluster connections and reduction in strain-rate sensitivity, as a measure of ductility, with Pd content and thermal history is suggested.

Softening in an ultrasonic-vibrated Pd-based metallic glass

The mechanical behavior of the Pd40Cu30P20Ni10 metallic glass after ultrasonic vibration was systematically investigated by nanoindentation. A notable softening after ultrasonic treatment was demonstrated by about 25% and 40% reduction of hardness and elastic modulus, respectively. The Maxwell-Voigt model was utilized to characterize the structure evolution during ultrasonic-assistant plastic deformation. Combined with differential scanning calorimeter experiments, it is found that the flow defects with shorter characteristic relaxation time activated under ultrasonic-frequency cycling loading benefits fast atomic diffusion with a low energy barrier, which eventually leads to the pronounced creep displacement, and thereby the large moldability at ambient temperature. This work might enlighten the structural origin for the plastic flow of metallic glasses under stress-assisted molding, imprinting as well as cold joining, which may help us in deeply understanding the defect activation mechanism in disordered systems.

Correlation symmetry analysis of electron nanodiffraction from amorphous materials

Angular symmetry in diffraction reflects rotational symmetry in the sample. We introduce the angular symmetry coefficient as a method to extract local symmetry information from electron nanodiffraction patterns of amorphous materials. Symmetry coefficients are the average of the angular autocorrelation function at the characteristic angles of a particular rotational symmetry. The symmetry coefficients avoid non-structural features arising from Fourier transformation and Friedel symmetry breakdown that affect the angular power spectrum approach to determining angular symmetries in amorphous nanodiffraction. Both methods require thin samples to avoid overlapping diffraction from clusters of atoms separated in the thickness of the sample, but symmetry coefficients are more forgiving. Electron nanodiffraction experiments on a Pd-based metallic glass sample demonstrate both potentially misleading information in angular power spectrum and the utility of symmetry coefficients.

Enhancement of Interfacial Hydrogen Interactions with Nanoporous Gold-Containing Metallic Glass.

Contrary to the electrochemical energy storage in Pd nanofilms challenged by diffusion limitations, extensive metal-hydrogen interactions in Pd-based metallic glasses result from their grain-free structure and presence of free volume. This contribution investigates the kinetics of hydrogen-metal interactions in gold-containing Pd-based metallic glass (MG) and crystalline Pd nanofilms for two different pore architectures and nonporous substrates. Fully amorphous MGs obtained by physical vapor deposition (PVD) co-sputtering are electrochemically hydrogenated by chronoamperometry. High-resolution (scanning) transmission electron microscopy and corresponding energy-dispersive X-ray analysis after hydrogenation corroborate the existence of several nanometer-sized crystals homogeneously dispersed throughout the matrix. These nanocrystals are induced by PdHx formation, which was confirmed by depth-resolved X-ray photoelectron spectroscopy, indicating an oxide-free inner layer of the nanofilm. With a larger pore diameter and spacing in the substrate (Pore40), the MG attains a frequency-independent impedance at low frequencies (∼500 Hz) with very high Bode magnitude stability accounting for enhanced ionic diffusion. On the contrary, on a substrate with a smaller pore diameter and spacing (Pore25), the MG shows a larger low-frequency (0.1 Hz) capacitance, linked to enhanced ionic transfer in the near-DC region. Hence, the nanoporosity of amorphous and crystalline metallic materials can be systematically adjusted depending on AC- and DC-type applications.

Porosity and thickness effect of Pd–Cu–Si metallic glasses on electrocatalytic hydrogen production and storage

This contribution places emphasis on tuning pore architecture and film thickness of mesoporous Pd–Cu–Si thin films sputtered on Si/SiO2 substrates for enhanced electrocatalytic and hydrogen sorption/desorption activity and their comparison with the state-of-the-art thin film electrocatalysts. Small Tafel slope of 43 mV dec–1 for 1250 nm thick coating on 2 µm diameter pores with 4.2 µm interspacing electrocatalyst with comparable hydrogen overpotentials to the literature suggests its use for standard fuel cells. The largest hydrogen sorption has been attained for the 250 nm thick electrocatalyst on 5 µm pore diameter with 12 µm interspacing (2189 µC cm−2 per CV cycle), making it possible for rapid storage systems. Moreover, the charge transfer resistance described by an equivalent circuit model has an excellent correlation with Tafel slopes. Along with its very low Tafel slope of 42 mV dec–1 10 nm thick electrocatalyst on 2 µm diameter pores with 4.2 µm interspacing has the highest capacitive response of ∼ 0.001 S sn cm−2 and is promising to be used as a nano-charger and hydrogen sensor. The findings of Si/SiO2 supported mesoporous Pd-based metallic glass (MG) assemblies suggest a similar design applicability for crystalline systems and other MG types.

Hyper-ordered structures and glass-forming abilities of Pd-based metallic glasses

Hyper-ordered structures and glass-forming abilities of Pd-based metallic glasses

Severe plastic deformed Pd-based metallic glass for superior hydrogen evolution in both acidic and alkaline media

The electrocatalytic hydrogen evolution reaction (HER) on a Pd40Cu30Ni10P20 metallic glass (MG) in both acidic and alkaline media is enhanced through severe plastic deformation using high-pressure torsion (HPT). The severe plastic deformation increased the energy state of the sample by approximately 21.7J/g without causing crystallization. The overpotentials at 10 mA cm−2 of the HPT-treated Pd40Cu30Ni10P20 MG are 76 mV and 209 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively, which are much smaller than those of 179 mV and 379 mV for the melt-spun Pd40Cu30Ni10P20 MG at the same conditions. The improved HER performances should be mainly attributed to the significantly increased density of flow units in the amorphous matrix by severe plastic deformation.Severe plastic deformation by the high-pressure torsion (HPT) on a Pd40Cu30Ni10P20 metallic glass (MG) would generate more flow units upon the amorphous matrix and significantly improve the electrocatalytic hydrogen evolution reaction (HER) performances in both acidic and alkaline media.

Size effect of the heat transfer in palladium-based metallic glass nanofilms

Metallic glasses have special mechanical and electromagnetic properties because of their disordered atomic arrangements and are widely used in various fields including soft magnetic materials and micromachining. Metallic glass nanofilms are used as wear- and corrosion-resistant coatings, smart skins and thermoelectric materials. Research on the heat transfer in metallic glass nanofilms is crucial because local overheating during processing or in applications can lead to structural failures. There have been few reports on the heat transfer characteristics of metallic glass nanofilms. The present study measured the thermal conductivities of Pd80Si20 metallic glass nanofilms with thicknesses of 4.0–194.1 nm at room temperature using time-domain thermoreflectance. The thermal conductivity increases linearly with thickness in sufficiently thin nanofilms and the increment gradually decreases in thicker nanofilms. A model was proposed to predict the thermal conductivity of the Pd-based metallic glass nanofilms based on non-equilibrium molecular dynamics, surface scattering and ballistic transport theories. The predictions showed that electrons rather than phonons primarily dominate the heat transfer, while the size effect primarily depends on ballistic transport in nanofilms thinner than the electron mean free path, and on surface scattering in thicker nanofilms. The comparison with literature shows that when thicker than the electron mean free path, the metallic nanofilms have as good heat transfer performance as some bulk metallic glass.

Relation of the fragility and heat capacity jump in the supercooled liquid region with the shear modulus relaxation in metallic glasses

Fragility constitutes a major parameter of supercooled liquids. The phenomenological definition of this quantity is related to the rate of a change of the shear viscosity η at the glass transition temperature. Although a large number of correlations of the fragility with different properties of metallic glasses were reported, an adequate understanding of its physical nature is still lacking. Attempting to uncover this nature, we performed the calculation of the fragility within the framework of the interstitialcy theory (IT) combined with the elastic shoving model. We derived an analytical expression for the fragility, which shows its relation with the high-frequency shear modulus G in the supercooled liquid state. To verify this result, specially designed measurements of η and G were performed on seven Zr-, Cu- and Pd-based metallic glasses. It was found that the fragility calculated from shear modulus relaxation data is in excellent agreement with the fragility derived directly from shear viscosity measurements. We also calculated the heat capacity jump ΔC sql at the glass transition and showed that it is related to the fragility and, consequently, to shear modulus relaxation. The ΔC sql-value thus derived is in a good agreement with experimental data. It is concluded that the fragility and heat capacity jump in the supercooled liquid state can be determined by the evolution of the system of interstitial-type defects frozen-in from the melt upon glass production, as suggested by the IT. This connection is mediated by the high-frequency shear modulus.

Hydrogen Evolution Activity of Mesoporous Pd-Based Metallic Glass Assemblies with Tunable Pore Architecture and Film Thickness

The necessity for efficient green energy production compels scientists to develop advanced nanostructured metallic alloys with tunable properties. This contribution places emphasis on tuning pore architecture and film thickness of mesoporous Pd – Cu – Si thin films sputtered on Si substrates for enhanced electrocatalytic and hydrogen sorption/desorption activity and their comparison with the state-of-the-art electrocatalysts. The small Tafel slope of 44 mV dec–1 with a rapid rise in the cathodic current rendering low hydrogen overpotentials of 24 mV at 10 mA cm–2, and the largest sorption/desorption peaks for the T4H2 electrode suggest its use for a fuel cell electrode. Moreover, the kinetic parameters described by the equivalent circuit model have a very good correlation with Tafel slopes and sample mass. The 10 nm T1H1 sample has one of the highest capacitive responses where the hydrogen-metal interaction takes place and is promising as a nano-charger and sensing of hydrogen.

Detailed structural analysis of amorphous Pd40Cu40P20: Comparison with the metallic glass Pd40Ni40P20 from the viewpoint of glass forming ability

To clarify the relationship between the glass-forming ability (GFA) and local atomic structure of Pd-based metallic glasses, the structures of an amorphous Pd40Cu40P20 (PCP) were investigated using the same methods as for Pd40Ni40P20 (PNP) metallic glass, i.e., by a combination of anomalous X-ray scattering, X-ray and neutron diffraction, and reverse Monte Carlo modeling. From the comparison between the results of PNP with an excellent GFA and the PCP with a worse GFA, important features characteristic to the GFA are found in the hyper-ordered structures. Firstly, the compositional inhomogeneity of Cu in PCP is larger than Ni in PNP. Secondly, a Voronoi tessellation reveals that icosahedral arrangements are found with considerable probability around the Ni atoms in PNP, while a less existence around the Cu atoms in PCP. Thirdly, a persistent homology (PH) analysis shows smaller intermediate-size Cu PH rings in PCP than Ni PH rings in PNP.

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd40Ni40P20) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO diversity seems to enable easier shear banding and hence enhance the deformability.

Effect of loading rate on crystallization of metallic glass supercooled liquids

Metallic glasses exhibit unique thermoplastic processing capability, which is enabled by their metastable supercooled liquid state below the crystallization temperature. The thermoplastic processing critically depends on the crystallization time (processing time window), temperature (viscosity), applied load, and strain-rate. Among these parameters, the effects of crystallization time and processing temperature have been extensively studied. However, the effects of load and loading rate have not been thoroughly investigated. In this work, we performed a systematic study of load on the supercooled liquid state of three metallic glass formers: Pt-based, Zr-based, and Pd-based. The results show that the load-response of a metallic glass supercooled liquids is strongly composition dependent. The onset temperature of crystallization decreases with increasing load in Pt-based metallic glass whereas for Zr-based and Pd-based metallic glasses the onset temperature remains unchanged. The crystallization peak time is reduced for all three metallic glasses after thermoplastic forming. The results are discussed in terms of nucleation and growth of crystallites in metallic glasses.