A Diagram for Glass Transition and Plastic Deformation in Model Metallic Glasses

In this paper, we report a strain rate related glass transition in model SrCaYbMg（Li）Zn（Cu） metallic glasses at room temperature. A critical strain rate, equivalent to glass transition temperature, is found for the strain rate induced glassy state to liquid-like viscoplastic state translation. The results show that the observation time, equivalent to temperature and stress, is a key parameter for the transition between the glass and supercooled liquid states. A three-dimension glass transition diagram involved in time, temperature and stress in metallic glasses is established.     

Dynamic Mechanical Relaxation in Bulk Metallic Glasses： A Review

Metallic glasses have aroused considerable interest in the past decades because they exhibit fascinating properties. First, this article briefly outlines the mechanical, thermal properties and application of the metallic glasses. In addition, we focus on the dynamic mechanical relaxation behaviors, i.e. main （α） and secondary （β） relaxations, in metallic glasses. The mechanical relaxation behaviors are connected to the mechanical properties and physical properties in glassy materials. The main relaxation in glassy materials is related to the glass transition phenomenon and viscous flow. On the other hand, the β relaxation is linked to many fundamental issues in metallic glasses. In these materials relaxation processes are directly related to the plastic deformation mechanism. The mechanical relaxations, particularly, the β relaxation provides an excellent opportunity to design metallic glasses with desired physical and mechanical properties. We demonstrate the universal characteristics of main relaxation in metallic glasses. The phenomenological models and the physical theories are introduced to describe the main relaxation in metallic glasses. In parallel, we show the dependence of the α. and β relaxations on the thermal treatments in metallic glasses. Finally, we analyze the correlation between the atomic mobility and the thermo-mechanical treatments in metallic glasses. On the one hand, the atomic mobility in metallic glasses is reduced by physical aging or crystallization. On the other hand, the atomic mobility in metallic glass is enhanced by deformation （i.e. compression and cold rolling）. Importantly, to analyze the atomic mobility in amorphous materials, a physical theory is introduced. This model invokes the concept of quasi-point defects, which correspond to the density fluctuations in the glassy materials.     

Correlation Between Local Atomic Symmetry and Mechanical Properties in Metallic Glasses

The local atomic symmetry was investigated and discussed for understanding mechanical, vibrational and dynamical properties in metallic glasses. Local five-fold symmetry was defined based on the ratio of pentagons to the total number of faces in a Voronoi cluster analyzed by Voronoi tessellation method. It is found that the plastic deformation prefer to be initiated in the regions with less degree of local five-fold symmetry （LFFS） and propagate gradually toward the region with more degree of LFFS. On the other hand, the local structures having less degree of LFFS contribute more to the soft low-frequency modes, and thus the so-called boson peak, while those with more degree of LFFS participate more in moderate- and high- frequency modes in metallic glasses. The relationship between local atomic symmetry and structural heterogeneity, mechanical heterogeneity, and glass transition was also discussed. It is shown that local atomic symmetry could be a general structural indicator in metallic liquids and glasses for better understanding the structure-property relationship in amorphous alloys.     

New para-magnetic(CoFeNi)_(50)(CrMo)_(50-x)(CB)_x(x=20, 25, 30)non-equiatomic high entropy metallic glasses with wide supercooled liquid region and excellent mechanical properties

In this study, high entropy metallic glasses(HEMGs) were developed through a combination of concepts for designing metallic glasses(main element + transition metal + metalloid element) and high-entropy alloys(more than five elements, each element having an atomic concentration between 5 at.% and 35 at.%). The developed metallic glass alloys are composed of Co-Fe-Ni main elements, transition metals(Cr,Mo) and metalloid elements(C, B). Moreover, the present work reports the thermal, mechanical and magnetic properties of (CoFeNi)_(50)(CrMo)_(50-x)(CB)_x alloys with x = 20, 25, 30. The developed as-spun HEMGs exhibit typical paramagnetic properties even for a high amount of ferromagnetic elements(Co, Fe, and Ni) and have high elastic modulus(103–160 GPa) and hardness(14–27 GPa), thus possessing mechanical properties similar to well-known Co-based metallic glasses(Co-Cr-Mo-C-B system). In addition, some of the bulk samples prepared with a diameter of 2 mm form bulk metallic glasses with a high compressive strength around 3.5 GPa. The mechanisms determining the stability of the supercooled liquid, as well as the paramagnetic and mechanical properties for the developed non-equiatomic HEMGs, are discussed.     

Enhancement of Electric Conductivity in Transparent Glasse—Ceramic Nanocomposites of Bi2O3—BaTiO3 Glasses

The frequency and temperature dependent electrical conductivity measurements for heat-treated binary glass system with composition of （lO0-x）Bi203-xBaTi03 （x = 20, 30, 40 and 50, in mol%） were carried out. The glass was prepared by melt quenching technique and their corresponding glass-ceramic nanocomposites were obtained by suitable heat treatment. Nanostructured behavior and electrical properties of these glasses and their corresponding glass-ceramic nanocomposites were studied. X-ray diffraction （XRD） and differential scanning calorimetry confirmed the amorphous nature of the glasses. Moreover, XRD patterns of the samples indicate nanocrystallites embedded in the glass matrix. The Fourier transform infrared spectroscopy （FT-IR） spectral analysis showed that the band positions of glass system are within the wave number range of Bi06, Bi03 and Ti06 structural units. It is observed that the electrical conductivity is enhanced by 102-103 times in the transparent glass-ceramic nanocomposite phase. With further heat treatment, the conductivity decreased considerably in the stage of glass-ceramic nanocomposite phase as compared with the glassy phase sample. Therefore, partially devitrified phase is more suitable as cathode material in secondary batteries compared to its vitreous or fully crystalline counterpart. The conduction mechanism was confirmed to obey the adiabatic small polaron hopping （SPH）. AC conductivity measurements were performed as a function of temperature and frequency, showing a very slow increasing rate at low temperatures and then a fast rate at higher temperatures.     

Critical transitions in the shape morphing of kirigami metallic glass

Kirigami, the ancient Japanese paper cutting technique, has been applied to achieve high stretchability and low energy loss of designed metallic glass. Despite the exploration of the underlying deformation mechanism of kirigami-inspired structures from the energy point of view, the morphable responses of the kirigami patterns and the origin of the kirigami response are yet to be fully understood. This study reveals the mechanical driven-forms of the kirigami structure with the corresponding deformation stages. Based on the beam deflection theory, the elastic buckling behavior of kirigami metallic glass is manifested and a critical force prediction model is developed. Moreover, a force concentration parameter is introduced in the rigid-plastic deformation stage, predicting the nominal ultimate force. The kirigami-inspired facture force is firstly proposed. The findings of these models are in good agreement with the experimental sizedependent kirigami responses, and expected to provide significant insights into the understanding of the deformation behavior and the design of kirigami metallic glasses.     

Glass-forming Ability and Corrosion Resistance of Zr—Cu—Al—Co Bulk Metallic Glass

Investigations were carried out to examine the effects of Co addition on the glass-forming ability(GFA)and corrosion resistance of Zr_(46)Cu_(46)(Al)_8 bulk metallic glass in chloride-containing solution. It is found that the GFA of(Zr_(46)Cu_(46)(Al)_8 )100-x Co x(x = 0, 1, 2, and 4 at.%) alloys reduces with the increase in Co content and correlates well with the parameters, such as the supercooled liquid region width ΔT x, the reduced glass transition temperature T rg and γ. The corrosion resistance is however found to be enhanced with the increase in Co concentration. The addition of Co causes the enrichment of Zr and Al, but depletes Cu in the surface films, which effectively enhances the corrosion potential and lowers the corrosion current densit     

TEM specimen preparation for Al-based amorphous alloys

Transmission electron microscopy （TEM） is usually used to identify the amorphicity. However, some artifacts may be introduced due to improper TEM foil preparation. In this paper, three Al-rich metallic glasses with and without a glass transition were selected for characterizing the effect of the electropolishing condition on the as-quenched structure during TEM specimen preparation. It is shown that the occurrence of the modulated bright-dark structure under TEM observation is closely sensitive to the electropolishing condition, which suggests us being careful about the possible artifacts induced by specimen preparation when examining amorphous alloys under TEM.     

Physical and Thermal Properties of P2O5-Al2O3-BaO-La2O3 Glasses

The physical and thermal properties of P2O5-Al2O3-BaO-La2O3 glasses were investigated. The effects of glass compositions on the transition temperature, thermal expansion coefficient, density, hardness and refractive index of glasses were studied. The highest hardness of the glasses is 4143.891 MPa and the lowest thermal expansion coefficient of the glasses is 71.770×10-7/℃. A phosphate glass with high mechanical strength and good thermal characteristic is obtained.     

Thermal Stability and Transformation-mediated Deformability of Cu-Zr-Al-Ni Bulk Metallic Glass Composite

The glass-forming ability and the thermal stabilityof Cu45Zr45Al5Ni5 metallic glass were systematically investigated by the differential scanning calorimetry.The activation energies for the glass transition and the onset crystallization and the crystallization peak were calculated to be 255.1,308.7 and 311.5 kJ/mol,respectively,while the corresponding liquid fragility was determined to be 23.4.By controlling the casting process,Cu45Zr45Al5Ni5 bulk metallic glass composite with the presence of ductile B2 CuZr phase was fabricated.The composite shows a pronounced plastic strain of 6.8 0.05% with obvious work hardening,which results from the formation of multiple shear bands and the deformation-induced martensitic transformation.     

Free volume theories of the glass transition and the special case of metallic glasses

Theories based on the concepts of free volume and the existence of holes in liquids are briefly reviewed. Available experimental data on the changes in specific heat and thermal expansion at the glass transition temperature and the temperature dependence of viscosity near transition have been utilized to evaluate the hole formation energy and critical hole size in palladium-, platinum- and gold-based metallic glasses. It has been found that in conformity with theoretical predictions, transport in metallic glasses occurs by the movement of highly ionized atoms. A linear relationship exists between the hole formation energy and glass transition temperature of metallic glasses. It is suggested that a high energy of hole formation is a necessary criterion for easy vitrification of metallic melts. The behaviour of vacancies in crystalline metals is compared with the behaviour of holes in metallic glasses.On leave from the Department of Metallurgical Engineering, Banaras Hindu University, Varanasi-5, India     

The elastic properties, elastic models and elastic perspectives of metallic glasses

Bulk metallic glass (BMG) provides plentiful precise knowledge of fundamental parameters of elastic moduli, which offer a benchmark reference point for understanding and applications of the glassy materials. This paper comprehensively reviews the current state of the art of the study of elastic properties, the establishments of correlations between elastic moduli and properties/features, and the elastic models and elastic perspectives of metallic glasses. The goal is to show the key roles of elastic moduli in study, formation, and understanding of metallic glasses, and to present a comprehensive elastic perspectives on the major fundamental issues from processing to structure to properties in the rapidly moving field.A plentiful of data and results involving in acoustic velocities, elastic constants and their response to aging, relaxation, applied press, pressure and temperature of the metallic glasses have been compiled. The thermodynamic and kinetic parameters, stability, mechanical and physical properties of various available metallic glasses especially BMGs have also been collected. A survey based on the plentiful experimental data reveals that the linear elastic constants have striking systematic correlations with the microstructural features, glass transition temperature, melting temperature, relaxation behavior, boson peak, strength, hardness, plastic yielding of the glass, and even rheological properties of the glass forming liquids. The elastic constants of BMGs also show a correlation with a weighted average of the elastic constants of the constituent elements. We show that the elastic moduli correlations can assist in selecting alloying components with suitable elastic moduli for controlling the elastic properties and glass-forming ability of the metallic glasses, and thus the results would enable the design, control and tuning of the formation and properties of metallic glasses.We demonstrate that the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to understand flow in metallic glass and glass-forming supercooled liquid, and the model presents a simple and quantitative mathematic expression for flow activation energy of various glasses. The elastic perspectives, which consider all metallic glasses exhibit universal behavior based on a small number of readily measurable parameters of elastic moduli, are presented for understanding the nature and diverse properties of the metallic glasses.     

Dynamic mechanical relaxation behavior of Zr35Hf17.5Ti5.5Al12.5Co7.5Ni12Cu10 high entropy bulk metallic glass

Non-equiatomic high entropy bulk metallic glasses were reported recently and show unique mechanical and physical properties.Dynamic mechanical relaxation behavior of Zr35Hf17.5Ti5.5Al12.5C07.5Ni12Cu10 high entropy bulk metallic glass was investigated by dynamic mechanical analysis(DMA)and the mechanical spectra could be well described by the quasi-point defects(QPD)theory.Compared to typical metallic glasses,the intensity of the β relaxation of Zr35Hf17.5Ti5.5Al12.5Co7.5Ni12Cu10 high entropy bulk metallic glass is lower due to the sluggish diffusion.At the same time,the correlation factor χ is higher than that of conventional metallic glasses and this is ascribed to the high configuration entropy.In paral-lel,physical aging below the glass transition temperature leads to a decrease of atomic mobility,caused by a decrease of the concentration of defects.     

On the Atomic Anisotropy of Thermal Expansion in Bulk Metallic Glass

Glass transition temperature and plastic yield strength are known to be correlated in metallic glasses. We have observed by in situ synchrotron high energy X‐ray diffraction anisotropy of the thermal expansion behavior in the nearest neighbor and second nearest neighbor atomic distances in the building blocks of Zr? Cu? Ni? Al based bulk metallic glass, leading inevitably to shear. Mechanical yielding of the latter on the atomic scale leads to the glass transition and the increase of the free volume. These experimental results uncover the mechanism, how glass transition and yield strength are linked.     

Reentrant glass transition leading to ultrastable metallic glass

Polyamorphs are often observed in amorphous matters, and a representative example is the reentrant glass transition in colloid systems. For metallic amorphous alloys, however, the cases reported so far are limited to metallic glasses (MGs) that undergo electronic transitions under gigapascal applied pressure, or the presence of two liquids at the same composition. Here we report the first observation of a reentrant glass transition in MGs. This unusual reentrant glass transition transforms an MG from its as-quenched state (Glass I) to an ultrastable state (Glass II), mediated by the supercooled liquid of Glass I. Specifically, upon heating to above its glass transition temperature under ambient pressure, Glass I first transitions into its supercooled liquid, which then transforms into a new Glass II, accompanied by an exothermic peak in calorimetric scan, together with a precipitous drop in volume, electrical resistance and specific heat, as well as clear evidence of local structural ordering on the short-to-medium-range scale revealed via in-situ synchrotron X-ray scattering. Atomistic simulations indicate enhanced ordering of locally favored motifs to establish correlations in the medium range that resemble those in equilibrium crystalline compounds. The resulting lower-energy Glass II has its own glass transition temperature higher than that of Glass I by as much as 50 degrees. This route thus delivers a thermodynamically and kinetically ultrastable MG that can be easily retained to ambient conditions.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

Main α relaxation and slow β relaxation processes in a La30Ce30Al15Co25 metallic glass

Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La₃₀Ce₃₀Al₁₅Co₂₅ bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature T_g leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed.     

Pd-Ni-P metallic glass film fabricated by electroless alloy plating

In the present study a Pd-Ni-P film has been fabricated by electroless alloy plating. The fabricated Pd-Ni-P film was found to be a metallic glass on the basis of two features, namely, an amorphous structure and a glass transition followed by crystallization during heating. The thermal stability of the supercooled liquid region, however, was lower than that of bulk Pd-Ni-P metallic glass. And unlike the conventional metallic glasses, the fabricated Pd-Ni-P film did not have a uniform microstructure. The non-uniform microstructure of this film resulted from the inhomogeneous distribution of the free volume accompanying the electroless alloy plating reaction.     

Atomic level stresses

The birth and subsequent development of the concept of the atomic level stresses are reviewed, and its future is discussed. Initially it was conceived as a means of describing the local structure in metallic glasses. It gradually became evident that its capacity is beyond the initial expectation. It appears that this concept is a powerful tool in understanding diverse phenomena in strongly disordered systems, such as the atomic dynamics in liquids, glass transition, mechanical failure and structural relaxation. This concept also has a potential to bridge distinct fields of glass research beyond metallic glasses, including colloids, molecular liquids, granular matter and other materials.     

Impact of hybridization on metallic-glass formation and design

The formation mechanism of glass at the atomic scale has been under debate over centuries. In this work, we demonstrate that hybridization, as manifested by Mott’s pseudogap, has a strong influence on the bond length as well as atomic packing, which can potentially tailor the formation of metallic glasses at microscopic time and length scales. A p–d orbital hybridization between the post-transition metal Al and the transition metal was shown by the ²⁷Al isotropic shifts and the spin–lattice relaxation time of Zr–Co–Al alloys using nuclear magnetic resonance. These bonds lead to a charge transfer between the specific atomic pairs and the shrinkage of interatomic distances. Such chemical bonding favors the formation of metallic glasses by introducing a string-like structure and further stabilizes metallic glasses via a reduction in the density of states at the Fermi level. Our work has implications for understanding the glass formation mechanism at the electronic level and may open up new possibilities on the design of glass from the perspective of atomic interactions.