Johari-Goldstein relaxation as the origin of the excess wing observed in metallic glasses

Measurements of the shear loss modulus G″ of amorphous Zr₆₅Al_7.5Cu_27.5 at 5.4 kHz were reported by Rösner, Samwer and Lunkenheimer. They observed that the measured G″ are in excess of the contribution from the α-relaxation, indicating the existence of an ‘excess wing’ in amorphous metals like that found in molecular glass-formers. They speculated that the excess wing found in amorphous metal is due to the presence of an unresolved Johari-Goldstein (JG) secondary relaxation. In this work, the coupling model is used to calculate the temperature T_JG at which the JG relaxation frequency coincides with the experimental frequency of the isochronal measurement. The location of T_JG is well within the temperature range where the excess wing of Zr₆₅Al_7.5Cu_27.5 appears at 5.4 kHz. Hence, the result supports the assertion of Rösner et al. that the excess wing found in the metallic glass originates from the intrinsic JG relaxation.     

Effect of strain rates on the fracture morphologies of Zr-based bulk metallic glasses

The effect of strain rates from 1 × 10⁻⁴ s⁻¹ to 2 × 10³ s⁻¹ on tensile fracture morphologies of Zr_52.5Al₁₀Ni₁₀Cu₁₅Be_12.5, Zr₆₅Al₁₀Ni₁₀Cu₁₅, and Zr_52.5Al₁₀Ni_14.6Cu_17.9Ti₅ bulk amorphous alloys was investigated by scanning electron microscopy. The results show that the tensile fracture morphologies of three compositions of bulk amorphous alloys are dependent on strain rate. At low strain rates, the tensile fracture surface morphology of Zr-based bulk metallic glasses presents cleavage veins. However, the morphology will become microvoid-coalescence dimples when the strain rate is high enough.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

High-temperature relaxation mechanisms in inorganic glasses

At temperatures below T_g two relaxation processes are observed in sheet glass (200–500°C) and low-alkali glass (300–600°C): the fast R₁ and the slow R₂ processes which are not connected with the viscous flow, and the structural relaxation occurring R₃ above T_g. The processes R₁ and R₂ proceed at an invariable structure and are characterized by activation energies as high as 5 kcal mol^?1 and 13–15 kcal mol^?1, respectively. The contribution of R₂ amounts to 70–80%. The process R₃, observed near and above T_g, is accompanied with structural variations and, therefore, its activation energy depends on temperature; at T_g it is equal to 60 kcal mol^?1.The processes R₁ and R₂ are due to the mobility and rearrangement of large kinetic units. On the contrary, R₃ is characterized by a low volume of kinetic units. This shows that the ions of silicon and oxygen are involved in this process. The relaxation process R₁ is assumed to be connected with the local fluctuation deformationsof the glass network as in the case of reverse glass deformation under high pressures, and the process R₂ with the mobility of microscopic areas of the glass micro-inhomogeneous structure (structural complexes, microblocks). The continuous spectra corroborate the existence of several high-temperature relaxation processes in silicate glasses.Thus, three relaxation processes are observed in alkali-silicate glasses in the temperature range 200–600°C: the processes R₁ and R₂ are mechanical relaxations, whereas the process R₃ is a structural relaxation determining the viscous flow of glass. The contribution of R₃ to stress relaxation amounts to 5%.There exists a temperature T_k (20–30° below T_g) which is the upper limit of the process R₂. At higher temperatures beginning from T_k the stress relaxation is first determined by the two processes R₁ and R₃, and then by one process R₃. At temperatures below T_k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R₃ becomes negligible and, therefore, in the glass annealing range (below T_k) the mechanical relaxation R₂ and R₁ are mainly responsible; their contribution to the whole relaxation process is as high as 95%.     

Kinetics of ion exchange in glasses

The kinetics of $\text{K}{}^{\mathrm{+}}\text{?}\text{Na}{}^{\mathrm{+}}$ exchange in two glass systems, 20Na₂O·(60?x)B₂O₃· (20 + x)Si₂ (where x = 0, 15, 30 and 45 mol%) and Na₂O·3SiO₂, were studied as a function of glass composition, salt bath composition, exchange temperature and time The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron probe. Stresses in these speciments were measured photoelastically. The interdiffusion coefficient $\text{D}$ for ion exchange was calculated as a function of local composition in the glass using the Boltzmann-Matano method. The strong variation of $\text{D}$ in any particular glass approximated that predicted by a mixed alkali model (as advanced by Lacharme), where the glass in the ion-exchanged region approximates a composite of stacked layers of mixed alkali glasses with a gradually varying alkali ratio. The small discrepancy between the experiment and the mixed alkali model was partly, but not fully, reconciled by considering the strains in the glasses. The observation which remained unexplained was that the calculated stress profiles did not show perfect agreement, both in magnitude and in shape, with the experimentally measured stress profiles. It appeared that the kinetics of ion exchange in the glasses were also influenced by a network relaxation process which may have occurred well below the glass transition temperature.     

Dielectric relaxation in iron-containing borate glasses

The ac and dc conductivities, dielectric constant and dielectric loss of a series of calcium borate glasses containing Fe₂O₃ up to 10 mol% were measured as a function of temperature (300–700 K) and frequency (10²–10⁵ c/s). Glasses melted in platinum crucibles were found to show slightly higher dielectric constants and conductivities than glasses melted in zirconia crucibles under the same melting conditions. Relaxation effects with a distribution of relaxation times were observed for all the glasses of this system. The data show that the pre-exponential factor τ₀ is about 10^?11 s. Peaks in tan δ were found to shift to higher temperatures when the frequency of measurements was increased. Activation energy for dc conduction was found to be equal to that of ac conduction. No polarization effects were observed in these glasses, indicating that conduction is predominantly electronic.     

Dynamic mechanical properties of metallic glasses

Dynamic mechanical properties of two metallic glasses, Fe₄₀Ni₄₀P₁₄B₆ and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆, have been studied at frequencies ranging from 0.1 to 3 kHz and at temperatures between ? 160 and 390°C. Each of the samples exhibits an internal friction maximum at about ?20°C with activation energies of 25 and 34 kcal/mol. A possible mechanism for the low-temperature internal friction peak is suggested.     

Stability of Ni-based bulk metallic glasses

Several ternary (Ni_xNb_ySn_z) refractory alloy glasses (RAGs) were studied at elevated temperatures in order to assess the stability of the amorphous state, i.e. devitrification, and to identify subsequent phase transformations in these materials. differential scanning calorimetry (DSC) experiments indicated a complex phase transformation sequence with several distinct crystallization and melting events being recorded above the glass transition temperature, T_g. Below T_g the RAG samples were studied with an in situ environmental X-ray furnace facility, which allowed step-wise isothermal ramping experiments commencing at a temperature below the reduced temperature of T/T_g ≈ 0.80. Distinct crystalline phases were observed when T/T_g ≈ 0.84 for ternary RAG alloys, while similar experiments on Zr-based Vit 106 glass alloys did not reveal any apparent phase separation until T/T_g ≈ 0.96. The phase separation kinetics followed an Arrhenius type of relationship with Ni₃Sn, and Nb₂O₅ being the principle crystalline precipitates.     

Positron annihilation in thermally treated metallic glasses

The positron lifetime dependence on the annealing temperature has been studied for two transition metal-metalloid glasses (FeNi-based and Co-based). Small variations of the lifetime observed after annealing below crystallization temperature are attributed to elimination of the structural defects inherent in the as-quenched metallic glasses. More abrupt changes at higher temperatures are ascribed to crystallization processes which lead to polycrystalline phases. Differences in positron lifetime between annealed FeNi-based and Co-based metallic glasses are discussed.     

Neutron measurements of magnetic correlations in metallic glasses

Neutron diffraction experiments have been made on FeB and PdSi transition metal alloy glasses prepared by melt-spinning. The magnetic neutron scattering has been analysed to provide information about the magnetic order in the specimens. An Fe₈₃B₁₇ metallic glass has magnetic correlations at room temperature with a range commensurate with the variations in atomic structure. These correlations appear to have a structure based on fcc gamma-iron, and the magnetic disorder is small - given that the specimen was in a field of 1 kG. Magnetic scattering was observed from Pd₇₀Si₂₀Co₁₀ and Pd₆₇Si₂₀Fe₁₃ at room temperature, which indicated a large magnetic moment value for the transition metal atoms. Cooling the PdSiCo sample to 80 K induced an antiparallel moment correlation, while further cooling to 7 K produced little obvious change in the scattering. These latter results are discussed in the light of the considerable disagreement which exists between different investigations of these alloys.     

Dielectric relaxation study of chalcogenide glasses

The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (?′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (?′). The curves: log ?′ versus temperature, ?′ versus log ω, ?″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times.Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys.Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys.It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.     

Dissoluble and degradable CaLi-based metallic glasses

We study the degradable and dissoluble features of a Ca-Li-Mg-Zn bulk metallic glass in pure water at room temperature. A remarkable degradable feature of the metallic glasses is that the degradation is controllable by changing the composition and components. The degradable metallic glasses with superior combined properties of polymer-like thermal plasticity at low temperature (40-70 °C), the ultralow elastic moduli comparable to that of human bones, and ultralow density (< 2 g/cm³) in known metallic glasses to date, and good machinability at a lower temperature in the supercooled liquid region could have potential applications. The metallic glasses also provide a model system to study the corrosion behavior in glasses.     

High mixing entropy bulk metallic glasses

Bulk metallic glasses (BMGs) are usually based on a single principal element such as Zr, Cu, Mg and Fe. In this work, we report the formation of a series of high mixing entropy BMGs based on multiple major elements, which have unique characteristics of excellent glass-forming ability and mechanical properties compared with conventional BMGs. The high mixing entropy BMGs based on multiple major elements might be of significance in scientific studies, potential applications, and providing a novel approach in search for new metallic glass-forming systems.     

Rare earth based bulk metallic glasses

Recently, the rare earth based bulk metallic glasses (REBMGs) have attracted increasing interest due to their unique properties and potential applications as functional glassy materials. These REBMGs display many fascinating properties such as heavy fermion behavior, thermoplastic properties near room temperature, excellent magnetocaloric effect, hard magnetism, and polyamorphism, all of which are of interest not only for basic research but also for metallurgy and technology. These characteristics and properties are ascribed to the unique electronic, magnetic and atomic structures of the REBMGs. In this review paper, the fabrication, glass-forming ability, polyamorphism, elastic, thermal, and physical properties are summarized and discussed. Owing to the unique electronic structure of rare earth elements, the electric and magnetic properties of the REBMGs are especially addressed. The works have implications for seeking novel metallic glasses with controllable properties and for understanding the nature of glass formation. The development of REBMGs as functional materials might promote and extend the commercial applications of metallic glasses.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Study of the kinetics of free volume in Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glasses during isothermal relaxation by enthalpy relaxation experiments

Enthalpy relaxation experiments were conducted to study the kinetics of free volume in Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glasses (BMGs) during isothermal relaxation using differential scanning calorimetry (DSC) at the temperature within the range from 648 to 684 K. Stretched exponential relaxation functions and the ?esták–Berggren SB (m, n) model were employed to analyze the kinetics of free volume. It is found that the relaxation time decreases from 2643.5 to 242.8 s while the Kohlrausch exponents increase from 0.717 to 0.892 in the investigated temperature range. The activation energy fluctuates slightly in the course of the relaxation process and its mean value is determined to be 239.7 kJ/mol. By fitting the first derivative of the conversion degree as a function of annealing time using the ?esták–Berggren SB (m, n) model, it is found that the values of m at all annealing temperatures are very small and can be approximated as zero. The values of the pre-exponential factors Z and the kinetic parameter n are found to be slightly varying with the annealing temperature, indicating the complexity of the kinetics of the isothermal relaxation. Finally, an approximate rate equation describing the kinetics of free volume during isothermal relaxation is proposed.