Ce-based Metallic Glasses Research Articles

Ultrastable metallic glass by room temperature aging

Glasses have markedly different stability around their glass transition temperature (Tg), and metallic glasses (MGs) are conventionally regarded as metastable compared to other glasses such as silicate glass or amber. Here, we show an aging experiment on a Ce-based MG around its Tg (~0.85Tg) for more than 17 years. We find that the MG with strong fragility could transform into kinetic and thermodynamic hyperstable state after the long-term room temperature aging and exhibits strong resistance against crystallization. The achieved hyperstable state is closer to the ideal glass state compared with that of other MGs and similar to that of the million-year-aged amber, which is attributed to its strong fragility and strong resistance against nucleation. It is also observed through the asymmetrical approaching experiment that the hyperaged Ce-based MG can reach equilibrium liquid state below Tg without crystallization, which supports the idea that nucleation only occurs after the completion of enthalpy relaxation.

Quantifying a partial polyamorphic transition in a cerium-based metallic glass during cooling

Cerium-based metallic glasses are prototype polyamorphous systems with pressure-induced polyamorphic transitions extensively reported. Cooling typically has a similar effect on materials as compression with regard to reducing volume. However, previous studies show dramatically different behavior of Ce-based metallic glasses between cooling and compression, whose origin remains unclear. Here, using in situ low-temperature synchrotron high-energy x-ray diffraction, the structural evolution of a Ce68Al10Cu20Co2 metallic glass is accurately determined and analyzed by a structure factor and a reduced pair distribution function (PDF) during cooling from 298 to 83 K. An unusually large linear thermal expansion coefficient is revealed, which is associated with both continuous but inconsistent structural changes between the two subpeaks of the first atomic shell in terms of average bond lengths and coordination numbers. These phenomena are suggested to be attributed to a gradual 4f electron delocalization of only a minimal amount (∼2.6% at 83 K) of Ce atoms by quantitative analysis of the PDF data. However, a previously expected global polymorphic transition from a low-density amorphous state to a high-density amorphous state with an abrupt volume collapse is not observed. Moreover, electrical resistivity also shows a continuous increase during cooling without any sharp change. It is clarified that cryogenic temperatures could facilitate but are not powerful enough alone to trigger a global polymorphic transition in the Ce68Al10Cu20Co2 metallic glass, suggesting a wide distribution of its local atomic environment.

The rejuvenation and relaxation around the glass transition of a Ce-based metallic glass controlled by annealing, quenching and cryogenic treatments

Ce69Al10Cu20Co1 bulk metallic glass (BMG) samples were annealed at different temperatures ranging from sub-Tg to super-Tg. Some of the samples were cooled to room temperature in air to obtain the relaxed ones, while the others were directly quenched in liquid nitrogen. Then the quenched samples were either withdrawn directly from the liquid nitrogen or subjected to cryogenic treatments (CTs) for different time intervals. The glass transition behaviors and heat flow of the samples in different states, including the as-cast, relaxed, quenched and Quenching+CT states, were investigated by using the differential scanning calorimeter (DSC). The work aims to study the tendency of the rejuvenation and relaxation behaviors of MG around the glass transition that were caused by the annealing, quenching and subsequent cryogenic treatments. The results show that the annealing temperature, cooling rate and cryogenic time play key roles in the rejuvenation and relaxation of MG around the glass transition.

Relaxation-to-rejuvenation transition of a Ce-based metallic glass by quenching/cryogenic treatment performed at sub-Tg

Ce69Al10Cu20Co1 metallic glass was fully annealed to its metastable equilibrium supercooled liquid (MESL) state at a temperature below the calorimetric glass transition, i.e., sub-Tg, and quenching/cryogenic combined treatment (Quenching + CT) was subsequently manipulated in liquid nitrogen. The evolution of glass transition, specific heat capacity (Cp) and enthalpy change (ΔH) were investigated by differential scanning calorimetry (DSC). Rejuvenation phenomenon on the perspective of energy level was found compared with the relaxed state of the sample. The evolution of the structural states, including the free volume, internal stress and atomic clusters, that caused by the quenching process and subsequent cryogenic treatment is discussed and acted as the physical interpretation to the relaxation-to-rejuvenation transition.

Specific heat characteristics of Ce70Ga8.5Cu18.5Ni3 metallic glass at low temperatures

Specific heat behaviors have been studied in Ce70Ga8.5Cu18.5Ni3 bulk metallic glass (BMG) from 2 K to 50 K. The low-temperature specific heat of the Ce-based metallic glass is a combined action of the Fermi liquids term, Debye oscillator term, and Einstein oscillator term as well as excess term. We also observed an intense boson peak around 15 K and attributed it to a harmonic localized Einstein mode influenced by the dense-packed atomic cluster structure. It is also demonstrated that Ce70Ga8.5Cu18.5Ni3 BMG belongs to the strongly correlated heavy-fermion system with a great electron specific heat coefficient and a high Wilson ratio. It exhibits a typical Fermi-Liquid feature when the temperature is above 10 K, while it exhibits a Non-Fermi-Liquid feature when the temperature is below 3.5 K.

Slip avalanche in nanoscratching of metallic glasses

Slip avalanches, similar to discrete earthquake events, of Zr-, Co-, and Ce-based metallic glasses during nanoscratching were investigated. Differing from the conventional continuum approach, mean-field theory, which is an inherently-discrete model, was applied to analytically compute intermittent slip avalanches. Mean-field theory was first connected with the potential energy barrier and concentration of free volume in order to study the stick-slip behavior. The results suggest that the motion behavior of free volume affects the critical slip avalanche size.

Unusual energy state evolution in Ce-based metallic glass under high pressure

Metallic Glasses (MGs) are always aging toward the lower energy state, which leads to higher density, modulus, and hardness. We find that high pressure (HP) could lead to similar densification and hardening while the energy is increased. The comparison between two processes under HP and ambient pressure shows that densification happens in denser regions in MGs under HP, while it happens in looser regions under ambient pressure, which leads to the opposite energy changes in the two conditions. This result breaks the common wisdom about the relationship between the free volume and enthalpy and displays different structural and energy evolutions in MGs.

Elastic Anomaly and Polyamorphic Transition in (La, Ce)-based Bulk Metallic Glass under Pressure

Pressure-induced polyamorphism in Ce-based metallic glass has attracted significant interest in condensed matter physics. In this paper, we discover that in association with the polyamorphism of La32Ce32Al16Ni5Cu15 bulk metallic glass, the acoustic velocities, measured up to 12.3 GPa using ultrasonic interferometry, exhibit velocity minima at 1.8 GPa for P wave and 3.2 GPa for S wave. The low and high density amorphous states are distinguished by their distinct pressure derivatives of the bulk and shear moduli. The elasticity, permanent densification, and polyamorphic transition are interpreted by the topological rearrangement of solute-centered clusters in medium-range order (MRO) mediated by the 4f electron delocalization of Ce under pressure. The precisely measured acoustic wave travel times which were used to derive the velocities and densities provided unprecedented data to document the evolution of the bulk and shear elastic moduli associated with a polyamorphic transition in La32Ce32Al16Ni5Cu15 bulk metallic glass and can shed new light on the mechanisms of polyamorphism and structural evolution in metallic glasses under pressure.

The effect of composition on pressure-induced polyamorphism in metallic glasses

Polymaorphism was recently revealed in metallic glasses (MGs) under high-pressure. The electronic structural evolution of solvent component was confirmed to be essentially responsible for the polyamorphic transition in MGs. In this paper, structural evolutions of Ce68−xLaxAl10Cu20Co2 (x=0, 34, and 68) MGs were investigated by in situ high pressure X-ray diffraction measurements. The effect of composition on pressure-induced polyamorphism in MGs was discussed. The results indicated that polyamorphous in Ce-based MGs is strongly related to the Ce concentration. These results provide new insight for searching for polyamorphous and glass structure in MGs.

Pressure-induced polyamorphism by quantitative structure factor and pair distribution function analysis in two Ce-based metallic glasses

We utilized the pair distribution function method to characterize the pressure-induced polyamorphic transition in Ce60Al20Cu20 and Ce55Al45 metallic glass at room temperature. Using synchrotron high-energy x-ray diffraction we collected scattering information from a large Q-space coverage, which in turn gave a high resolution g(r) that provided accurate local structure information. We observed a sudden change in compressibility and the nearest neighbor distance at 3.50–6.32 GPa for Ce60Al20Cu20 and 2.20–6.89 GPa for Ce55Al45. The origin of the volume collapse seemed to be pressure-induced qualitative changes in bond shortening that corresponded to different coordination spheres. The polyamorphic transitions in these two systems from low-density glass (LDG) to high-density glass (HDG) are associated with local atomic rearrangements.

Reversible polyamorphic transitions in Ce65.5Al10Cu22.5Co2 metallic glass

We report here on a reversible polyamorphic transition in Ce65.5Al10Cu22.5Co2 metallic glasses under high pressure investigated by in situ angle-dispersive x-ray diffraction. The alloy is found to display a strong hysteresis in polyamorphic-phase transition upon application and subsequent release of pressure. We demonstrate that, the hysteresis behavior was caused by the densification of Ce-based metallic glasses under high pressure, which was resulted by the irreversible annihilation of free volume and elastic volume change. The transition is strongly correlated 4f-electron delocalization of Ce metal. These results provide a guidance for understanding mechanism of polyamorphic-phase transition in metallic glasses.

Hierarchical densification and negative thermal expansion in Ce-based metallic glass under high pressure.

The polyamorphsim in amorphous materials is one of the most fascinating topics in condensed matter physics. In amorphous metals, the nature of polyamorphic transformation is poorly understood. Here we investigate the structural evolution of a Ce-based metallic glass (MG) with pressure at room temperature (RT) and near the glass transition temperature by synchrotron X-ray diffraction, uncovering novel behaviours. The MG shows hierarchical densification processes at both temperatures, arising from the hierarchy of interatomic interactions. In contrast with a continuous and smooth process for the low- to medium-density amorphous state transformation at RT, a relatively abrupt and discontinuous transformation around 5.5 GPa is observed at 390 K, suggesting a possible weak first-order nature. Furthermore, both positive and abnormal-negative thermal expansion behaviours on medium-range order are observed in different pressure windows, which could be related to the low-energy vibrational motions and relaxation of the weakly linked solute-centred clusters.

Non-crystalline samples under high-pressure conditions

Taking advantage of synchrotron x-ray diffraction, PDF and tomographic techniques, the P-V curve of non-crystalline samples were studied under high-pressure conditions. Two element and several metallic glass cases were performed. The procedure of crystallization of amorphous Se upon compression at room temperature, which was studied in diamond anvil cell combined synchrotron x-ray PDF and 3D imaging techniques; the melting and solidification procedure of Ga in large volume press at room and high temperature; and complicated crystallization, re-rystalization, melting behavior of Ce-based metallic glass, will be presented to show the capability of revealing structure and dynamics behaviors in P-V-T-t domains using these advanced techniques.

Pressure-induced amorphous-to-amorphous reversible transformation in Pr75Al25

A pressure-induced amorphous-to-amorphous reversible transformation was revealed in Pr75Al25 metallic glass (MG) using in situ high-pressure synchrotron x-ray diffraction technique. The transition began at about 21 GPa with a ∼ 5% volume collapse and ended at about 35 GPa. This transition is reversible with hysteresis. Based on the high-pressure behaviors of Ce-based metallic glasses and Pr metal here, we suggest that the pressure-induced polyamorphic transition in Pr75Al25 MG stems from 4f-electron delocalization of Pr metal which leads to abrupt change in bond shortening. These results obtained here provide new insights into the underlying mechanism of the amorphous-to-amorphous phase transition in metallic glasses and will trigger more theoretical and experimental investigations for such transition.

Polyamorphism in cerium based bulk metallic glasses: Electronic and structural properties under pressure and temperature by x-ray absorption techniques

High pressure and high temperature x-ray absorption near edge spectroscopy experiments have been carried out on Ce60Al20Cu20 bulk metallic glass showing an electronic delocalization of the 4f-electron of cerium under pressure. In parallel, high pressure extended x-ray absorption fine structure spectroscopy reveals large structural modifications of the cerium local environment. This study provides experimental evidence that an electronic driven structural transformation occurs in cerium based bulk metallic glasses (Ce-BMGs). The effect of temperature on the hysteresis of this amorphous-amorphous phase transition is also discussed, suggesting the existence of a critical point in the phase diagram of Ce-BMGs. This work will encourage further investigations on Ce-based metallic glasses phase diagrams in order to support, or refute, the actual theoretical understanding of polyamorphism.

Inelastic X-ray scattering in metallic glasses

The behavior of acoustic modes in solids can yield information on the glass dynamics at different length and frequency scales. Inelastic X-ray scattering (IXS) using Synchrotron radiation allows us to obtain detailed information on the sound speed behavior at different length scales as well as approaching the macroscopic limit. This gives an insight to the microscopic mechanisms responsible for the mechanical properties in the THz frequency domain. IXS also provides a method to investigate the fragility of glass-forming liquids via the non-ergodicity factor of the corresponding glasses. Moreover, some questions arise about how phenomena such as the polyamorphism, observed, e.g. in Ce55Al44 upon application of pressure, affect the mechanical properties of a metallic glass at a microscopic level. In this article we reveal a change in the high frequency response at the mesoscopic length scale with respect to the ultrasounds limit in metallic glasses. We will also review further applications of IXS on Pd and Ce-based metallic glasses to determine elastic constants, changes in sound speed due to polyamorphism and to investigate their fragility.

Facile creation of bio-inspired superhydrophobic Ce-based metallic glass surfaces

A bio-inspired synthesis strategy was conducted to fabricate superhydrophobic Ce-based bulk metallic glass (BMG) surfaces with self-cleaning properties. Micro-nanoscale hierarchical structures were first constructed on BMG surfaces and then modified with the low surface energy coating. Surface structures, surface chemical compositions, and wettability were characterized by combining scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and contact angle measurements. Research indicated that both surface multiscale structures and the low surface free energy coating result in the final formation of superhydrophobicity.

Polyamorphic transitions in Ce-based metallic glasses by synchrotron radiation

We report here on a polyamorphic-phase transition upon application of pressure on a Ce-based metallic glass, Ce${}_{70}$Al${}_{10}$Ni${}_{10}$Cu${}_{10}$, investigated by x-ray diffraction and inelastic x-ray scattering. This alloy is found to display a strong hysteresis in volume per atom upon application and subsequent release of pressure. The observed structural changes are correlated with changes observed by inelastic x-ray scattering in the elastic constants, acoustic mode frequencies, and sound speed. The results reported here point toward three different amorphous phases for this alloy existing in the 0--25 GPa pressure region: low- and high-density states and an intermediate mixed state that displays a hysteresis behavior. Finally, we discuss the impact of Ce concentration on the polyamorphic transition for a series of Ce-based metallic glass alloys and link it to the phase transformation between $\ensuremath{\gamma}$-Ce and $\ensuremath{\alpha}$-Ce under pressure.

On calorimetric study of the fragility in bulk metallic glasses with low glass transition temperature: (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10) and Zn38Mg12Ca32Yb18

Compared with conventional bulk metallic glasses, Ce-based and Zn-based bulk metallic glasses have received considerable attention because of their possible application as structural and functional materials. Kinetic fragility parameter m in amorphous material presents degree of deviations from the Arrhenius law above the glass transition temperature (Tg) of the material. Kinetic fragility parameter (m) and Kauzmann temperature (TK) in (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10) and Zn38Mg12Ca32Yb18 bulk metallic glasses have been determined by differential scanning calorimetry (DSC). Results show that Zn38Mg12Ca32Yb18 presents a higher m than (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10). The activation energies Eg for glass transition are 1.51 eV (x = 0), 1.59 eV (x = 5) and 1.83 eV (x = 10) in (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10), and 3.59 eV in Zn38Mg12Ca32Yb18, respectively. The values of Eg increase with increasing the Fe content in (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10) bulk metallic glasses. Kinetic fragility parameter m of bulk metallic glasses increases with the glass transition temperature Tg of bulk metallic glasses, in agreement with previous investigations.

Valence fluctuation and electron–phonon coupling in La68−xCexAl10Cu20Co2 (x=, 34, and 68) metallic glasses

We study the anomalous acoustic softening behavior in La68−xCexAl10Cu20Co2 (x=0, 34, and 68) metallic glasses by magnetic susceptibility, thermal expansion, and specific heat at low temperatures. The Ce-based metallic glass exhibits valence fluctuation phenomenon, anomalous thermal expansion and acoustic Grüneisen parameter at low temperatures. And we provide experimental evidence of the existence of strong electron–phonon coupling in the Ce-based metallic glasses by specific heat. The anomalous acoustic and thermal properties are attributed to the valence instability in the metallic glasses.