Pd40Ni40P20 Bulk Metallic Glass Research Articles

Correlations between shadow glass transition, enthalpy recovery and medium range order in a Pd40Ni40P20 bulk metallic glass

Two distinct enthalpic states were induced in a Pd40Ni40P20 bulk metallic glass through cyclic annealing at 473 K (0.81Tg) and 558 K (0.96Tg), targeting individual β or α relaxation modes, here referred to as the so-called shadow glass transition (SGT) and enthalpy recovery, respectively. Fluctuation electron microscopy (FEM) was employed to systematically investigate the medium-range order (MRO) within the amorphous structure in the post-annealed state and subsequent quenching after the SGT. Comparing the results with literature data from the same glass quenched from SCL samples, a significant increase in MRO was observed after annealing at 0.81Tg. Conversely, samples quenched directly after the SGT exhibit a reduced MRO, similar to those quenched from the supercooled liquid (SCL). These findings suggest a correlation between the SGT and a decrease in MRO within the glass, a phenomenon discussed in the context of underlying structural rearrangements associated with β relaxation, encompassing the SGT. Additionally, prolonged annealing at 0.96Tg led to glass separation in the material, with remixing observed as an endothermic transformation in the SCL. The origin of this transformation is discussed as either a potential liquid–liquid or glass–liquid transition and their connection to modifications in medium-range order.

Differences in structure and dynamics of ternary Pd–Ni-based bulk metallic glasses containing sulfur or phosphorous

The composition Pd31Ni42S27 has been shown to be the best glass former in the family of recently discovered glass forming PdNiS alloys. In this study, this sample system was systematically investigated using fluctuation- and correlation electron microscopy of which the results are compared to a Pd40Ni40P20 bulk metallic glass that serves as a model system for metallic glasses. Strong differences in the local atomic correlations beyond the short-range order were observed, which are assumed to be a reason for their discrepancy in thermal stability. The relaxation dynamics at room temperature revealed faster dynamics in the sulfur-containing Pd31Ni42S27 glass.

Impact of Gd minor alloying on the crystallization kinetics of Pd40Ni40P20 bulk metallic glass

Minor alloying of metallic glasses is understood as a possible way to tune its properties and provides a study framework to the origin of their effects. Here, crystallization kinetics of Pd40Ni40P20 and minor-alloyed samples (Pd40Ni40P20)99Gd1 were studied using isothermal and isochronal calorimetry and the results were analyzed using the Johnson-Mehl-Avrami-Kolmogorov model. The addition of 1 at. % Gd into the primary alloy results in a substantial alteration of the crystallization kinetics. The Avrami exponent obtained directly from the calorimetric measurements varies from 2.5 for the ternary alloy to almost 4 for the minor-alloyed sample, indicating a change of the crystallization mechanism from a diffusion-controlled crystallization process with a constant nucleation rate and a gradually decreasing three-dimensional growth rate, to a transformation with a constant nucleation rate and a constant three-dimensional growth rate. TEM (transmission electron microscopy) microstructural analysis of the crystallized state indicates the formation of Pd-Gd-P areas, which might accelerate crystallization kinetics.

Evidence for strain and a structural reset in Pd40Ni40P20 bulk metallic glass

Individual heat treatments on bulk Pd40Ni40P20 metallic glass samples were performed in a differential scanning calorimeter to either activate mainly β or both α and β relaxation processes as well as heating into the supercooled liquid. The medium-range order of the final amorphous structure was investigated by fluctuation electron microscopy and compared with that of the as-cast state. The results indicate that first, a defined structural reset was achieved by heating into the supercooled liquid. Second, the annealed states exhibit a lower volume fraction of medium-range order, and third, the strain is increased due to the reduction of excess volume after annealing.

Decoupled alpha and beta relaxation kinetics in a thermally cycled bulk Pd40Ni40P20 glass

Reproducible thermodynamic sample states of a Pd40Ni40P20 bulk metallic glass are realized via differential scanning calorimetry by repeated quenching from the supercooled liquid state to temperatures well below the glass transition. Annealing treatments at 0.81 Tg and 0.96 Tg are embedded in the calorimetric method, changing the energetical state of the system. Varying the annealing times, a detailed and reproducible picture of the reversible relaxation dynamics with separated α- and β-relaxation is obtained. An endothermic signature before Tg can either be provoked or depressed depending on the annealing temperature. The activation energy related to this process is obtained via Kissinger analyses yielding about 30 RTg. A large number of annealing cycles at 0.96 Tg irreversibly alters the response of the α-relaxation, while the mechanism of β-relaxation is interestingly not influenced by this alternation. In order to extend the calorimetric response of the relaxation spectra to spatial resolution, the sample states were additionally analyzed using electron correlation microscopy providing information on the glass dynamics on an atomistic scale. The thus obtained kinetic parameters of local dynamics do not show an alteration of room temperature dynamics for different levels of α-relaxation, which is consistent with the results obtained via kinetic analyses of calorimetric data.

Impact of cryogenic cycling on tracer diffusion in plastically deformed Pd40 Ni40 P20 bulk metallic glass

The impact of thermo-mechanical treatment, in particular of room temperature high-pressure torsion (HPT), cold rolling and cryogenic thermo-cycling on shear band diffusion in a model Pd40Ni40P20 bulk metallic glass is investigated. Tracer diffusion measurements reveal the existence of fast short-circuit diffusion in deformed glasses and the diffusion enhancement was found to depend on the processing mode. The HPT processing imposes an extremely large shear deformation under quasi-hydrostatic conditions whereas cold rolling induces individual well-separated shear bands, though shear band diffusion is found to be faster after cold rolling. The cryogenic thermo-cycling enhances the shear band diffusion further and a conceptual explanation for the processes leading to kinetic rejuvenation is proposed.

Influence of plastic deformation by high-pressure torsion on the crystallization kinetics of a Pd40Ni40P20 bulk metallic glass

The influence of high-pressure torsion on the crystallization behavior of the pre-annealed Pd40Ni40P20 bulk metallic glass was investigated by differential scanning calorimetry and fluctuation electron microscopy. Calorimetry shows that deformation splits the single main crystallization peak into a two-step crystallization process including a primary and a main peak. This is in line with fluctuation electron microscopy, which shows a reduction of ordering upon deformation. The overall activation energy and the local activation energy of the crystallization were analyzed by the Kissinger and Kissinger-Akahira-Sunose methods, respectively. The primary crystallization peak occurs at a lower temperature but shows a higher activation energy due to the drastically accelerated atomic mobility. Deformation can accelerate the crystallization process by increasing the nucleation rate.

The impact of micro-alloying on relaxation dynamics in Pd40Ni40P20 bulk metallic glass

The impact of Co micro-alloying on low temperature specific heat capacity and the relaxation processes in a model Pd40Ni40P20 bulk metallic glass in both as-cast and cold-rolled states was investigated. In addition to an excess (Boson) peak at low temperatures of about 12 to 14 K, a magnetic heat capacity anomaly in the Co alloyed glass below 10 K was observed. The measured Boson peak heights and the fictive temperatures both advocate a much faster β relaxation in the micro-alloyed glass. Yet, the micro-alloying does not measurably affect the enthalpy associated with α-relaxation. The impact of shear bands on the thermodynamic properties of the micro-alloyed glass is discussed.

Effects of strain and strain rate on the evolution of shear bands for room temperature rolled Pd40Ni40P20 bulk metallic glass

Room-temperature rolling experiments were performed on Pd40Ni40P20 bulk metallic glass at two different strain rates. It is found that at each strain rate the shear band density increases with increasing strain for the deformed samples. However, for samples endured the same deformation strain, more amounts of shear bands (i.e., a higher shear band density) was introduced into the sample deformed at the higher strain rate. In addition, the higher strain rate also leads to the generation of more amount of free volume for samples endured a same deformation strain. The hardness change behavior investigation result suggests that the introduction of multiple intersected shear bands leads to hardening of Pd40Ni40P20 bulk metallic glass.

Phonon Excitations in Pd<sub>40</sub>Ni<sub>40</sub>P<sub>20</sub> Bulk Metallic Glass by Inelastic X-Ray Scattering

To clarify microscopic elastic properties of a Pd40Ni40P20 bulk metallic glass, inelastic X-ray scattering (IXS) experiments were carried out using high-resolution IXS spectrometer installed at BL35XU/SPring-8. Clear longitudinal acoustic (LA) excitation modes are observed in the whole momentum transfer Q range up to ~22 nm-1. The microscopic sound velocity of the LA modes obtained from the Q → 0 limit of the dispersion relation is about 12% faster than the macroscopic value, suggesting the microscopic heterogeneity in the elastic properties of this BMG. In addition, transverse acoustic (TA) modes can be deduced from a detailed analysis of the IXS spectra. The microscopic Poisson’s ratio obtained from the ratio of the excitation energies of the TA and LA modes is 0.47 ± 0.04, larger than the macroscopic value of 0.40. Thus, the fragile nature of this BMG already found in the macroscopic sense is much enhanced in the microscopic view.

Applications and limitations of electron correlation microscopy to study relaxation dynamics in supercooled liquids

Electron correlation microscopy (ECM) is a way to measure structural relaxation times, τ, of liquids with nanometer-scale spatial resolution using coherent electron scattering equivalent of photon correlation spectroscopy. We have applied ECM with a 3.5nm diameter probe to Pt57.5Cu14.7Ni5.3P22.5 amorphous nanorods and Pd40Ni40P20 bulk metallic glass (BMG) heated inside the STEM into the supercooled liquid region. These data demonstrate that the ECM technique is limited by the characteristics of the time series, which must be at least 40τ to obtain a well-converged correlation function g2(t), and the time per frame, which must be less than 0.1τ to obtain sufficient sampling. A high-speed direct electron camera enables fast acquisition and affords reliable g2(t) data even with low signal per frame.

Shear band relaxation in a deformed bulk metallic glass

Relaxation of shear bands in a Pd40Ni40P20 bulk metallic glass was investigated by radiotracer diffusion allowing to determine for the first time the effective activation enthalpy of diffusion along shear bands in a deformed glass. The shear bands relax during annealing below the glass transition temperature and the diffusion enhancement reveals unexpectedly a non-monotonous, cross-over behavior. The development of shear bands and the subsequent relaxation of stresses after the shear had been switched off are characterized on microscopic to mesoscopic length scales by molecular dynamics simulation subjecting a model glass to a constant strain rate. Mean-squared displacements as well as strain maps indicate that the heterogeneity, as manifested by shear bands in the systems under shear, persist after the shear was switched off. We observe a continued relaxation of residual stresses that remain localized in regions where the shear band has been present before, although the system is – different from the macroscopic experiment – homogeneous with respect to the local density. These results indicate that even on a local scale one may expect strong dynamic heterogeneity in deformed glassy solids due to shear banding that correlates with the existence of short-circuit type diffusion on a macroscale. The results thus suggest that plastically deformed metallic glasses present poly-amorphous systems that necessitate descriptions that are analogous to multiphase materials including the presence of heterophase interfaces.

Impact of micro-alloying on the plasticity of Pd-based bulk metallic glasses

Micro-alloying was performed using additions of Co and Fe to monolithic Pd40Ni40P20 bulk metallic glass. Compression tests showed a plastic strain of 13% for the Co addition (1at.%), whereas the Fe addition (0.6at.%) led to immediate failure after reaching the elastic limit. Plasticity is not reflected by the high Poisson’s ratio of 0.4 since it remained unaffected by minor alloying. Applying the fictive temperature concept to analyze the impact of minor alloying suggests that the total amount of free volume frozen-in during vitrification is less important for the mechanical properties of bulk metallic glasses than its local distribution.

Evolution of shear bands, free volume, and structure in room temperature rolled Pd40Ni40P20 bulk metallic glass

The evolution of the shear band, free volume, and structure in room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass was investigated. It was found that the average shear band density increases monotonously with increasing strain. For the room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass with a strain of 99%, a high density of shear bands with an average spacing of 31 nm was observed. The absolute free volume content was determined based on the free volume model and found to increase monotonously with increasing strain. The free volume content in the room temperature rolled Pd 40 Ni 40 P 20 bulk metallic glass with a strain of 99% is 34% higher than its as-cast counterpart. Neither phase separation nor crystallization occurs in all the deformed samples. The coordination number of the first coordination shell decreases and the degree of disorder of atomic arrangement increases with increasing strain.

Air-oxidation of a Pd40Ni40P20 bulk glassy alloy at 250–420°C

Abstract The oxidation behavior of a Pd40Ni40P20 bulk-metallic glass (Pd3-BMG) was studied over the temperature range of 250–420 °C in dry air. The Pd3-BMG revealed nearly no oxidation at T

Replication of nano/micro-scale features using bulk metallic glass mold prepared by femtosecond laser and imprint processes

This study describes the replication of nano/micro-scale features using a Pd40Ni40P20 bulk metallic glass (BMG) mold prepared using a femtosecond laser and nanoimprinting process. The use of the beam shaper feature of the femtosecond laser enabled the rapid fabrication of periodic nanostripes over an area of ∼5 × 4 mm2 on the BMG mold following a single pulse of irradiation. The ablation pitch of the nanostructure irradiated with 100 mW of femtosecond laser power was determined to be 175.8 nm. The imprinting results demonstrate the applicability of Pd-based BMG in the replication of mold features ranging from 100 µm to 90 nm. Additionally, Pd-based BMG can itself be used as a mold to transfer features onto Au-based BMG and polydimethylsiloxane, where the results could be used to ascertain the workability of BMG for molding in a nano/micro-imprint process.

Structure of shear bands in Pd40Ni40P20bulk metallic glass

The atomic structure of shear bands in Pd40Ni40P20bulk metallic glass has been compared to an undeformed matrix phase using pair distribution functions (PDFs) derived from energy filtered nanobeam electron diffraction. Shear bands do not show any characteristic contrast in transmission electron microscopy (TEM) images when specimens are prepared with uniform thickness. PDFs from a shear band exhibit a slight decrease in the first peak, indicating a slight difference in packing density and short range order compared to the undeformed matrix.

Density and Enthalpy Relaxation Behavior in a Bulk Pd<sub>40</sub>Ni<sub>40</sub>P<sub>20</sub> Metallic Glass

The kinetics of structural relaxation in a Pd40Ni40P20 bulk metallic glass was investigated by the volume relaxation due to density experiments and the enthalpy relaxation due to specific heat experiments. A two-step relaxation process was found in the volume relaxation, while the enthalpy relaxation seemed to be one-step relaxation process with a spectrum of relaxation times. First-step volume relaxation only in as-quenched glass was the process with a spread of relaxation times at lower relaxing temperature, while a Debye-type relaxation behavior was observed at higher temperature near Tg and in pre-annealed glass. The comparison of the kinetics of volume and enthalpy relaxations at the same temperature showed a disagreement with the prediction of free volume theory.

Nanoimprint of gratings on a bulk metallic glass

The authors demonstrate that optical gratings with 600 and 1500nm periods on a Pd40Ni40P20 bulk metallic glass (BMG) can be faithfully imprinted in air from Si dies. Results of scanning electron microscopy, atomic force microscopy, and optical diffraction analysis show the fine line feature of ∼150nm. The gratings have smooth and uniform surface profiles with comparable optical properties as the original Si dies. The BMG gratings can be further used to imprint the second-generation replicas on polymethylmethacrylate. Thereby, BMG is a suitable material not only for imprinting nanostructured parts such as gratings, but also as a good die material for nanoimprints.

Analysis of Nano-Scale Phase Separation in a Pd40Ni40P20 Bulk Metallic Glass Deformed in the Supercooled Liquid Region by HREM and Nano-Diffraction

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.