Bulk metallic glasses based on rare-earth elements in lanthanum series

A series of bulk metallic glasses (BMGs) based on the rare-earth (RE) elements in lanthanum family have been obtained by a copper mold casting method. These chemical comparable RE elements with ‘continuous’ range of atomic size, electron structure and elastic constants may provide a good system for systematically exploring the BMG-forming characteristics. The glass-forming ability, elastic properties, thermal stability and their correlations in the RE-based BMGs have been investigated.     

Lutetium and thulium based rare earth bulk metallic glasses

We report the formation of lutetium and thulium based bulk metallic glasses based on the correlations between the thermodynamic, kinetic, elastic and other properties of metallic glasses. The two novel rare earth based bulk metallic glasses (REBMGs) exhibit excellent glass formation ability, high elastic moduli, considerable smaller Poisson’s ratio, high thermal stability, and even higher mechanical strength than that of typical high strength Zr-based BMGs. The reasons for the properties of the Lu- and Tm-based and other REBMGs are discussed.     

Bulk metallic glasses based on ytterbium and calcium

We report the formation of a family of bulk metallic glasses (BMGs) based on rare earth element of ytterbium and alkaline earth element of calcium. The glass-forming ability, atomic packing density and corrosion behaviors of the BMGs show an extremum around the eutectic point with the change of the concentration of Yb and Ca.     

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.     

Ionic conductivity of binary fluorides of potassium and rare earth elements

The ionic conductivity s of KYF₄ and K₂RF₅ single crystals (R = Gd, Ho, Er) and KNdF₄ and K₂RF₅ ceramic samples (R = Dy, Er) has been studied in the temperature range of 340–500°C. A comparative analysis of the σ values for these objects has been performed. Binary fluorides of potassium and rare earth elements were synthesized by the hydrothermal method (temperature 480°C, pressure 100–150 MPa) in the R₂O₃–KF–H₂O systems. The σ values of tetraf luorides are 3 × 10^–5 S/cm (KYF₄ single crystal) and 3 × 10^–6 S/cm (KNdF₄ ceramics) at 435°C. A K₂ErF₅ single crystal with σ = 1.2 × 10^–4 S/cm at 435°C has the maximum value of ionic conductivity among pentafluorides. The anisotropy of ionic transport was found in K₂HoF₅ single crystals, σ_∥c/σ_⊥c = 2.5, where σ_∥c and σ_⊥c are, respectively, the conductivities along the crystallographic c axis and in the perpendicular direction.     

Crystallization behavior of binary metallic glasses containing Pt

Binary MPt glasses were made by rapid quenching from liquid, where M is Ti, Zr, and Hf. Glass forming compositions were found near eutectics in TiPt, ZrPt and HfPt. Isothermal crystallization for Zr₇₅Pt₂₅ was studied at a temperature range from 780 to 792 K. The non-isothermal transformation was measured by differential scanning calorimetry with heating rates from 5 to 160 K/min. Activation energy for crystallization was obtained from the non-isothermal heating, which is consistent with that from the isothermal annealing.     

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.     

DC conductivity of rare earth ions doped vanado-tellurite glasses

Two novel sets of vanado-tellurite glasses doped with rare earth ions (La₂O and CeO₂) have been investigated for density and dc electrical conductivity in the temperature range 300–500 K. The conductivity data has been analyzed in the light of Mott’s small polaron hopping (SPH), variable range hopping (VRH) and Greave’s VRH models. The electrical conduction was confirmed to be due to non-adiabatic small polaron hopping in nature. High temperature conductivity behavior was found to be in agreement with Mott’s SPH model. The low temperature conductivity behaved as per Mott’s VRH model. Various physical and polaronic parameters, such as radius, bandwidth and coupling constant, have been determined and discussed.     

Diffuse scattering and chemical short range order in binary metallic glasses

X-ray and neutron scattering from binary metallic glasses is discussed in the context of the Bhatia-Thornton equations for the structure factor. For neutrons a generalised form of the Bhatia-Thornton equation must be used when there are different isotopes present. By making a distinction between correlated and uncorrelated scattering centres an expression can be derived which shows clearly the different contributions to the total scattered intensity. Information about the relative visibility of the alloy components can be obtained from this expression and a figure of merit or degree of contrast between the species defined, which is helpful in understanding results of experiments performed to date.     

Molecular dynamics,diffraction and EXAFS of rare earth phosphate glasses compared with predictions based on bond valence

Rare earth (RE) phosphate glasses, (R₂O₃)_x(P₂O₅)_1−x, commonly form with x < 0.3. As previously discussed by Hoppe, in this composition range Q² groups provide 6 O_nb (non-bridging oxygens) per RE, but higher RE coordinations can occur if RE are bonded to O_nb on Q³ groups, or if RE–O_nb–RE configurations occur (where Qⁿ refers to a PO₄ tetrahedra with connectivity of n). The values of N_RO and R_RO from the majority of the previous diffraction and EXAFS studies of RE phosphate glasses have been surveyed. Overall, the experimental results for 0.18 ⩽ x ⩽ 0.28 indicate RE coordination is from 6.5 to 7.0 for large RE, and slightly lower for small RE. For x ⩾ 0.23 this implies the occurrence of RE–O_nb–RE configurations, as observed in recent diffraction studies. The experimental results for x ⩽ 0.15 indicate RE coordination is from 7.5 to 8, which can be attributed to RE bonding to O_nb on Q³ groups. RE coordination of 8 for x ∼ 0.15 requires 2 Q³ groups (per RE) to be connected to Q² groups. Comparison with estimated connectivities between Q² and Q³ groups indicate that chemically ordered Q²–Q³ linkages are required. A recent MD model of a Tb metaphosphate glass has N_RO = 6, but includes Tb bonded to O_nb on Q³ groups and Tb–O_nb–Tb configurations, because it has a broad Qⁿ distribution.     

Structural and magnetic behaviour of lead-bismuthate glasses containing rare earth ions

Lead-bismuthate glasses containing rare earth ions in the xR₂O₃(1 − x)[3Bi₂O₃⋅ PbO] (R = Nd, Eu, Er and Gd), xCeO₂(1 − x)[3Bi₂O₃⋅ PbO] and xTb₄O₇(1 − x)[3Bi₂O₃⋅ PbO] vitreous systems with x up to 0.25 were prepared and investigated by density, IR spectroscopy and magnetic susceptibility measurements. Density data show that the gradual addition of the rare earth ions leads not only to a simple incorporation of the rare earth ions in the host glass matrix but also generates structural changes of the vitreous matrix. FTIR data permitted to determine the Bi³⁺/Bi⁶⁺ ratio of the samples and to follow the compositional evolution of the structural changes for the studied systems. Magnetic susceptibility data show that the magnetic rare earth ions appear as isolated species for low rare earth oxide contents (x ≤ 0.03 ÷ 0.05) and as both isolated and magnetically coupled species for higher contents. All the magnetic rare earth ions present a very accentuated clustering tendency. Some of the studied rare earth ions appear in the host glass matrix in a single valence state (neodymium, gadolinium, and erbium), namely the 3+ one. Other rare earth ions appear in two valence states (i.e., cerium, europium and terbium), but the 3+ one is the most stable.     

Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses

Aluminate glasses containing 45–71.5 mol% alumina, 10–40 mol% rare earth oxide, and 0–30 mol% silica were synthesized from precursor oxides. The glass transition and crystallization temperatures were determined by differential scanning calorimetry; the structural and mechanical properties were investigated by Raman and Brillouin spectroscopy. The range of the supercooled liquid region varies from ∼40 °C to 200 °C, providing a useful working range for compositions with 5–30 mol% silica. Raman scattering showed the presence of isolated SiO₄ species that strengthen the network-forming structure, enhance glass formation, and stabilize the glass even when they are present at fairly low concentrations. Sound velocities were measured by Brillouin scattering. From these and other values, various elastic moduli were calculated. The moduli increased with both aluminum and rare earth content, as did the hardness of the glasses. Young’s modulus was in the range 118–169 GPa, 60–130% larger than that for pure silica glass.     

Evaluation of glass-forming ability for bulk metallic glasses based on characteristic temperatures

A new criterion ω₂, defined as T_g/(2T_x?T_g)?T_g/T_l (wherein T_g is the glass transition temperature, T_x the onset crystallization temperature, and T_l the liquidus temperature), has been proposed to assess the glass-forming ability (GFA) of bulk metallic glasses (BMGs) based on the classical crystallization theory and the crystallization resistance. The analysis indicates that the factors T_g/(2T_x?T_g) and T_g/T_l could reflect the crystallization resistance and liquid phase stability of metallic glasses, respectively. From the available experimental data in literatures, the new criterion ω₂ has a better correlation with the GFA of metallic glasses than all other existing criteria such as T_rg(=T_g/T_l), ΔT_x(=T_x?T_g), γ(=T_x/(T_g+T_l)), ΔT_rg(=(T_x ? T_g)/(T_l ? T_g)), α(=T_x/T_l), β(=T_x/T_g + T_g/T_l), δ(=T_x/(T_l ? T_g)), φ(=T_rg(ΔT_x/T_g)^0.143), γ_m(=(2T_x ? T_g)/T_l), β(=T_x × T_g/(T_l ? T_x)²) and ξ(=ΔT_x/T_x+T_g/T_l). It has also been demonstrated that this ω₂ parameter is a simple and efficient guideline for exploring new BMG formers.     

New fluoride glasses based on 4f and 5f elements

New fluoride glasses have been isolated in the ternary systems ThF₄-LnF₃-BaF₂ with Ln = the lanthanides Yb, Y, Tm. The melt must be quenched to reduce crystallization. They are simply prepared from oxides by an appropriate NH₄F, HF treatment. The IR transmission is remarkable, the optical window lying in the 0.25-10 μm region.     

Novel nontoxic nanopigments based on zirconia and rare earth mixed oxides

This paper embodies the result of the investigations carried out on the synthesis of zirconia‐ rare earth mixed oxide pigments: ZrCe_1‐xY_xO_4‐δ and ZrCe_1‐xPr_xO_4‐δ by the solution combustion method and their characterization by X‐ray powder diffraction, transmission electron microscopy, energy dispersive spectrometric analysis, thermal analysis, reflectance spectral data and surface area measurements. The synthesized nano pigments possess low a and high b and exhibit yellow color when doped with Y and changes from yellow to orange‐red color when doped with Pr, with increase in x values. These pigments show maximum reflectance of 90% when the wavelength is above 550 nm. Due to the homogeneity of the phase formed and resistance to water, acids and alkali, the rare earth mixed oxides developed are found to be promising candidates as nontoxic pigments. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on the formability of Al-based metallic glasses/nanocomposites based on isochronal DSC analysis

The Al₈₆Si_0.5Ni_4.06Co_2.94Y₆Sc_0.5 metallic glass of highly improved glass-forming ability (GFA) has been investigated by isochronal differential scanning calorimetry measurements, as well as the Al₈₅Ni₅Co₂Y₈ for comparison. The experimental results indicate that the Al₈₆Si_0.5Ni_4.06Co_2.94Y₆Sc_0.5 exhibits enlarged temperature interval between the first and second crystallization onsets (termed as primary fcc-Al/glass region), as well as enlarged second activation energy (E_p2) against the nucleation and the growth of intermetallic compounds besides fcc-Al. The variation of the Avrami exponent demonstrates that the primary crystallization process is a rapid two- and three-dimensional diffusion-controlled nucleation and growth mechanism initially, and the whole process is strongly controlled by the growth of fcc-Al crystals. According to the analysis on the basis of atomic mobility of alloy components, it demonstrates that the enlarged primary fcc-Al/glass region obtained through proper coexistence of dissimilar and similar elements would be in favor of preparing Al-based metallic glasses or nanocomposites in greater size.     

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.     

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.     

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.