Irradiation-induced diffusion in tracks of swift ions

Radiation-induced diffusion in tracks of swift ions caused by excitation of electrons and resulting in the formation of dilatations has been considered. The expressions for the longitudinal and radial atomic displacements are derived as well as those for the diagonal components of the strain tensor. The atomic-density distribution in the vicinity of a track is obtained in terms of the parameters of the three-dimensional energy distribution in an excited electron gas. The results obtained theoretically are compared with the known experimental data. The suggested approach provides the evaluation of the time of formation of track dilatations, the mobility of the atoms in the target irradiated with swift ions, and the energy losses for formation of wirelike dilatations in the tracks. It is shown that the ion irradiation of semiconductor crystals can be used for obtaining one-dimensional quantum objects.     

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.     

Kinetics of strain relaxation in metallic glasses

Torque relaxation and strain recovery measurements under linear heating for three Fe-, Ni- and Co-based metallic glasses are presented. Torque relaxation is determined by the pre-annealing temperature, T_a: at T < T_a, the degree of relaxation is small and high-rate torque relaxation starts near T_a. Analogously, high-rate strain recovery starts near the temperature of preliminary homogeneous deformation. The temperatures where high-rate stress relaxation and strain recovery begins are calculated in the framework of an activation energy spectrum model and are consistent with experiment. A homogeneous-flow model is developed which qualitatively explains macroscopically reversible behaviour as a result of directed irreversible microscopic relaxation events with distributed activation energies.     

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.     

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.     

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.     

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.     

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.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Thermal behavior of substituted FeCo-based metallic glasses

Three compositions of metallic glasses, Fe₅₈Co₂₄Nb₃Ta₁Mo₁B₁₃, Fe_61.5Co_20.5Nb₃Ta₁Mo₁B₁₃ and Fe₆₆Co₁₈Si₁B₁₅ were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 °C. The samples were analyzed by X-ray diffraction (XRD) and Mössbauer spectroscopy. All Mössbauer spectra were fitted with a six-line pattern corresponding to the crystalline α-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Mössbauer spectra of annealed Fe₆₆Co₁₈Si₁B₁₅ revealed the presence of a secondary crystalline phase, namely (FeCo)₃(BSi). Moreover, the last Mössbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mössbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mössbauer results.     

Entropy model for flow behavior in metallic glasses

A model based on configurational entropy is proposed to interpret phenomenologically the flow behavior in metallic glasses. From available thermal and viscosity measurements, the effect of temperature, strain rate, pressure and composition on flow stress of metallic glasses have been evaluated. The values calculated agree fairly well with experimental data. The failure of atomic rearrangements to relieve the imposed strain results in stress concentrations and local structural disorder at deformed regions. This leads to inhomogeneous flow and softening of the metallic glasses.     

Extended elastic model for flow in metallic glasses

We report that both shear and bulk moduli, not only shear modulus, are critical parameters involved in both homogeneous and inhomogeneous flows in metallic glass. The flow activation energy (?F) of various glasses when scaled with average molar volume V_m, which is defined as flow activation energy density ρ_E = ?F/V_m, can be expressed as: . The extended elastic model is suggestive for understanding the glass transition and deformation in metallic glasses.     

Distinguish bonding characteristic in metallic glasses by correlations

By statistically analyzing 48 kinds of metallic glasses, we report clear correlations between the dimensionless ratio of glass transition temperature/Debye temperature (T_g/Θ_D) and density (ρ), and between Young's modulus or shear modulus and T_g/ρ, for the glasses consisting of only metal elements, while the metallic glasses alloyed with metalloid elements exhibit distinct deviation from the correlations. It is suggested that the alloying of metalloid elements would show covalent-like bonding characteristics in metallic glass, and the found correlations can be used to distinguish different bonding characteristics in metallic glasses.     

Coring micron- and milli-scale holes in metallic glasses

Metallic glasses (MGs) combined with copper cores possess many advantages in precision casting and coring of alloys. In this study, inner holes with different dimensions and complex sectional shapes were fabricated through direct casting and coring in MGs. The MG castings show good surface finish and the cored inner holes exhibit high aspect ratios larger than 80 while holding a tight dimension tolerance. The dimensional deviation was proved to originate mainly from the contraction disagreement between castings and copper cores by comparing their thermal expansion coefficients and simulating the solidification and cooling processes, while other factors may also contribute to it. The results are expected to provide a convenient and economical way in the near net-shape fabrication of inner holes in metallic devices and in the assembly of MG components.     

Fast ion transport in oxide glasses

The discovery of fast ion transport in solids has in recent years stimulated much interest in the scientific community both with regard to obtaining improved understanding of the phenomenon and applying such materials in advanced battery systems. More recently the phenomenon of fast ion transport has also been observed to occur in increasing numbers of amorphous systems. In this paper we review recent transport data obtained in over 100 glasses which appear to exhibit exceptionally high Ag, Li, Na and F ion conductivities at temperatures far from the melting points. We also include results recently obtained in our laboratory for glasses in the lithium borate system. Common characteristics of these glasses are summarized and are compared with predictions of classical diffusion theory. Relatively low and composition-independent values of σ₀ indicate poor agreement with a simple isolated-ion diffusion model, while composition-dependent activation energies are related to structural changes. Some glasses share similar properties with their crystalline counterparts while others do not. This is discussed in terms of the relative disorder already existing within the crystals. The need for improved characterization of glasses is also discussed.