Thermodynamic prediction of bulk metallic glass forming alloys in ternary Zr–Cu–X (X = Ag,Al, Ti,Ga) systems

Prediction of bulk metallic glass (BMG) forming compositions has always been a challenge due to thermodynamic and kinetic constraints. In the present investigation, a parameter based on the enthalpy of chemical mixing (?H_chem) and the mismatch entropy (?S_σ/k_B) has been used to correlate with glass forming ability in some Zr based BMGs. The new thermodynamic parameter, P_HS = ?H_chem × ?S_σ/k_B, is found to have strong correlation with glass forming ability in the configurational entropy (?S_config/R) range of 0.9–1.0. P_HS has been calculated for compositions in Zr–Cu–Ag, Zr–Cu–Al, Zr–Cu–Ti and Zr–Cu–Ga ternary systems. It is observed that in all the systems studied, the best BMG composition (highest critical diameter (Z_c) of glass formation) is the one that corresponds to the highest negative P_HS value. Present approach using P_HS could be road map to design new BMG forming compositions.     

Synthesis and structure–property relations in xCuO–(1 − x)Bi2O3 (0.5 ⩽ x ⩽ 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses

Synthesis of the xCuO–(1 ? x)Bi₂O₃ (0.5 ? x ? 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses was done via nitrate–citrate gel route. Glassy phase is ascertained by XRD studies. Magnetic susceptibility results in the range 4.2–400 K show weak paramagnetic nature with exchange integrals ～0.024–0.13 eV in the glasses. The electron paramagnetic resonance (EPR) in the range 4.2–363 K shows g ≈ 2.0 and the trend of the g-matrix elements g_|| > g_⊥ > g_e for the glasses C1–C5 at 4.2 K are due to the Cu²⁺ (3d⁹) paramagnetic site in the glasses which is in a tetragonally elongated octahedron [O_1/2–CuO_4/2–O_1/2] having D_4h symmetry. IR spectroscopic results show the presence of octahedron [BiO_6/2]^3? and [CuO_6/2]^4? units and pyramidal [BiO_2/2O]^? unit in the glasses.     

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(3 3 5) surface with a perfect distribution of monoatomic steps separated with (1 1 1) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4 monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (1 1 1) terraces.     

Formation and mechanical behavior of (Fe0.5Ni0.5)80−xMoxB20 (x = 0–8) amorphous alloys

Glass forming ability, thermal stability and mechanical behavior of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ (x = 0, 2, 4, 6, 8) amorphous alloys were studied by XRD, TEM, SEM, DSC, tensile test, microhardness test and tearing test. The effects of Mo addition on glass formation, strength and ductility of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ amorphous alloys were discussed. The substitution of Mo for Fe and Ni simultaneously causes improvement in glass forming ability and thermal stability, and changes the crystallization process. The tensile fracture strength of amorphous alloy depends on both hardness and ductility; the alloy with high hardness and good ductility simultaneously also has a high tensile fracture strength. The (Fe_0.5Ni_0.5)₇₈Mo₂B₂₀ amorphous alloy exhibits good glass forming ability and the highest tensile fracture strength among (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ alloys. Micro-plastic deformation occurred in ductile and brittle amorphous alloys that both show viscous flow characteristics. The mechanical behavior of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ amorphous alloys is related to the average outer shell electron concentration of metal atoms.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Square-wave voltammetry and impedance spectroscopy in tin doped melts with the compositions xNa2O · 15Al2O3 · (85 − x)SiO2 (x = 8.5, 11 and 16)

Glasses with the base compositions xNa₂O · 15Al₂O₃ · (85 ? x)SiO₂ (x = 8.5, 11 and 16) doped with 0.5 mol% SnO₂ were investigated by both square-wave voltammetry and impedance spectroscopy in the temperature range from 1300 to 1600 °C. Each recorded square-wave voltammogram exhibits a well pronounced peak attributed to the Sn²⁺/Sn⁴⁺-redox pair. Impedance spectra were measured in a frequency range from 0.1 to 10⁵ s^?1 as a function of the superimposed dc-potential and were simulated using an equivalent circuit taking into account the resistivity of the melt, the electrochemical double layer, a resistor attributed to a kinetically hindered electron transfer and a Warburg parameter which accounts for the diffusion process of Sn⁴⁺ and Sn²⁺ to and from the electrode. Additionally, two impedance elements, a resistor and a capacitance both attributed to adsorption processes were necessary to fit the impedance spectra.     

Optical and electrical properties of as-prepared and annealed (Se80Te20)100 − xAgx (0 ≤ x ≤ 4) ultra-thin films

Optical and electrical properties of the (Se₈₀Te₂₀)_100 ? xAg_x (0 ≤ x ≤ 4) ultra-thin films have been studied. The ultra-thin films were prepared by thermal evaporation of the bulk samples. Thin films were annealed below glass transition temperature (328 K) and in between glass transition temperature and crystallization temperature (343 K). Thin films annealed at 343 K showed crystallization peaks for Se–Te–Ag phases in the XRD spectra. The transmission and reflection of as-prepared and annealed ultra-thin films were obtained in the 300–1100 nm spectral region. The optical band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical band gap increases, but the refractive index, extinction coefficient, real and imaginary dielectric constant decrease with increase in Ag content. The optical band gap and refractive index show the variation in their values with increase in the annealing temperature. The extinction coefficient increases with increasing annealing temperature. The surface morphology of ultra-thin films has been determined using a scanning electron microscope (SEM). The measured dc conductivity, under a vacuum of 10^? 5 mbar, showed thermally activated conduction with single activation energy in the measured temperature range (288–358 K) and it followed Meyer–Neldel rule. The dc activation energy decreases with increase in Ag content in pristine and annealed films. The results have been analyzed on the bases of thermal annealing effects in the chalcogenide thin films.     

Corrosion behavior of Fe-based ferromagnetic (Fe,Ni)–B–Si–Nb bulk glassy alloys in aqueous electrolytes

Fe-based ferromagnetic [(Fe_1?xNi_x)_0.75B_0.2Si_0.05]₉₆Nb₄ (x = 0, 0.2 and 0.4) bulk glassy alloys (BGAs) with a diameter of 2 mm were prepared by copper mold casting. The corrosion behavior of glassy alloy rods obtained was investigated in 0.5 M NaCl, 0.5 M NaOH and 0.5 M H₂SO₄ aqueous electrolytes, respectively, using weight loss and electrochemical polarization measurements. It was found that the corrosion rates significantly decrease with an increase in Ni content in all examined solutions. The Ni-containing BGAs are spontaneously passivated with wide passive regions and low passive current densities in NaCl and NaOH solutions, but exhibit the active–passive–transpassive behavior in H₂SO₄ solution. The partial substitution of Ni for Fe results in a considerable improvement on the corrosion resistance of [(Fe_1?xNi_x)_0.75B_0.2Si_0.05]₉₆Nb₄ BGAs, because of the structural and chemical homogeneousness of the amorphous phase and the effect of Ni on promoting the formation of a passive film. Besides their high glass-forming ability (GFA), excellent soft-magnetic properties and good mechanical performance, which have been reported before, these FeNi-based BGAs also feature rather high corrosion resistance.     

Crystallization kinetics of a-Se75Te25 − xGax (x = 0, 5, 10 and 15 at wt %) glassy system

Bulk glasses of a-Se₇₅Te_25 ? xGa_x (x = 0, 5, 10 and 15 at wt %) have been prepared by melt quenching technique. These samples were structurally characterized by using X-ray diffraction. Kinetic of crystallization in these glasses was studied under non-isothermal conditions using differential thermal analysis (DTA). DTA is performed at different heating rates of 5, 10, 15, 20 and 30 °C/min. The values of glass transition (T_g) and crystallization peak temperature (T_p) are found to be composition and heating-rate dependent. The obtained results have been analyzed in terms of activation energy of glass transition (E_g) using Kissinger's and Mahadevan et al. relations. Values of E_g obtained by the two relations are in agreement with each other. The results indicate that the crystallization process is a three-dimensional growth.     

Optimization of the Compositions of Solid Electrolytes Cd1 – xRxF2 + x (R = La–Lu,Y) with a Fluorite-Type Structure for Conductivity and Thermal Stability

Crystallography Reports - Optimization of the compositions of Cd1–xRxF2+x nonstoichiometric phases (CaF2 type, R is a rare-earth element) for the ionic conductivity and thermal stability is...     

Magnetic behavior,transport properties and nanocrystallization of melt-spun YxCe50 − xCu42Al8 (0 ≤ x ≤ 50) amorphous system

Amorphous Y_xCe_50 ? xCu₄₂Al₈ (0 ≤ x ≤ 50) ribbons prepared by melt-spinning on the Cu wheel were subjected to X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and to the measurements of magnetization and resistivity in the temperature range from 1.7 to 300 K. Effective activation energies, characteristic crystallization temperatures and enthalpies of as-quenched alloys have been determined. Two stages of crystallization have been observed in most samples (except shallow effects in Ce₅₀Cu₄₂Al₈). The activation energies have been calculated from the Kissinger relation to be 247 ± 18 and 570 ± 56 kJ/mol for Y₂₅Ce₂₅Cu₄₂Al₈ and Y₅₀Cu₄₂Al₈, respectively. The absence of the endothermic effect for x = 50, usually associated with a glass transition, below the primary crystallization event, indicates the presence of dispersed polyamorphous packing with a wide range of local glass transitions. The magnetization versus temperature plot for Y₂₅Ce₂₅Cu₄₂Al₈ points to a magnetic ordering at temperatures considerably below 50 K. This observation has been confirmed by the temperature dependence of resistivity, which exhibits a singularity at the same temperature. Moreover, a negative temperature coefficient of resistivity, characteristic of disordered systems, was observed. The electrical resistivity in the Y₂₅Ce₂₅Cu₄₂Al₈ amorphous alloy is governed by the weak localization of electrons.     

Phase transformations in the La1−xNdxNi3.5Al1.5–H2 (x = 0.1 and 0.2) system

Interactions in the La_1?xNd_xNi_3.5Al_1.5-Н₂ (x = 0.1 and 0.2) system was studied from room temperature up to 950 °C at the initial hydrogen pressure of 5 MPa through differential thermal (DTA) and X-ray phase analyses. Heating two-phase alloys (x = 0.1 and 0.2) in hydrogen results in their disproportionation (at 530 and 560 °С, respectively) and the formation of NiAl and unidentified amorphous products. The single-phase La_0.9Nd_0.1Ni_3.5Al_1.5 alloy decomposes in hydrogen at 900 °С into a hydride of rare-earth metals and an Ni₃Al intermetallic; traces of NiAl and hydride of a phase of the CaCu₅-type structure have also been observed. Heating the disproportionated samples in vacuum to 520–550 °С leads to their recombination into a homogenized phase with a CaCu₅-type structure. In other words, the increase of neodymium content shifts the reaction equilibrium of La_1?xNd_xNi_3.5Al_1.5 alloys with hydrogen towards recombination.     

Growth of Sm1 – ySryF3 – y (0 < y ≤ 0.31) Crystals and Investigation of Their Properties

Crystallography Reports - Sm1&nbsp;–&nbsp;ySryF3&nbsp;–&nbsp;y (0 &lt; y ≤ 0.31) crystals have been grown from melt by directional solidification in a...     

Crystallisation kinetics,glass forming ability and thermal stability in glassy Se100 − xInx chalcogenide alloys

Differential scanning calorimetry (DSC) studies have been done under non-isothermal conditions at different heating rates for glassy Se_100 ? xIn_x (5 ≤ x ≤ 20) alloys. DSC traces with well-defined endothermic and exothermic troughs and peaks at glass transition (T_g), crystallisation (T_c) and melting (T_m) temperatures were observed. The crystallisation kinetics parameters, Avrami index (n), activation energy for crystallisation (E_c) and frequency factor (K_o), have been calculated on the basis of the classical Johnson–Mehl–Avrami (JMA) model and related methods derived by Kissinger, Augis–Bennett and Mahedevan. Activation energy for glass transformation (E_t) has been evaluated on the usual two different non-isothermal methods developed by Moynihan and Kissinger. An extension of the Augis–Bennett method well known for evaluating E_c to calculate E_t has been explored with satisfactory results. Results obtained from these methods are in close agreement with each other. Close correlation between E_t, E_c and heating rate (β) was observed. The glass forming ability (GFA) and thermal stability parameters have been calculated for each glass system. It was found that the proportion of indium additive changed significantly the values of glass/crystal transformation, GFA and thermal stability of the studied system.     

Enhanced 2.0 μm emission in oxyfluoride glass-ceramics containing nanocrystals MF2 (MF3): Ho3+,Tm3+ (M = Ca,Ba, and La)

Transparent glass-ceramics containing MF₂(MF₃):Ho³⁺,Tm³⁺ (M = Ca, Ba, and La) nanocrystals have been prepared by melt quenching and subsequent thermal treatment. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of MF₂ (MF₃) nanocrystals among the glass matrix. Energy-dispersive X-ray spectroscopy results evidenced the incorporation of Tm³⁺ and Ho³⁺ into the MF₂ nanocrystals. Intense 2.0 μm emission originating from the Ho³⁺: ⁵I₇ → ⁵I₈ transition was achieved upon excitation with 808 nm laser diode. A large ratio of the forward Tm³⁺ → Ho³⁺ energy transfer constant to that of the backward process indicated high efficient energy transfer from Tm³⁺ (³F₄) to Ho³⁺ (⁵I₇), and benefited from the reduced ionic distances of Tm³⁺–Tm³⁺ and Tm³⁺–Ho³⁺ pairs and low phonon energy environment with the incorporation of rare earth ions into the precipitated MF₂ nanocrystals. The results indicate that oxyfluoride glass-ceramic is a promising candidate for 2.0 μm laser.     

Optical properties of amorphous (As0.33S0.67)100−xTex (x = 0, 1, 5 and 10) chalcogenide thin films,photodoped step-by-step with silver

We have analysed in detail the effect of silver-content on the optical properties of Ag-photodoped amorphous (As_0.33S_0.67)_100?xTe_x (with x = 0, 1, 5 and 10 at.%) chalcogenide thin films; the chalcogenide host layers were prepared by vacuum thermal evaporation. Films of composition Ag_y[(As_0.33S_0.67)_100?xTe_x]_100?y, with y ? 18 at.%, were successfully obtained by successively photodissolving about 20- or 40-nm-thick layers of silver. The optical constants (n, k) have been accurately determined by an improved envelope method [J.M. González-Leal, R. Prieto-Alcón, J.A. Angel, D.A. Minkov, E. Márquez, Appl. Opt. 41 (2002) 7300], based on the two envelope curves of the optical-transmission spectrum, obtained at normal incidence. The dispersion of the refractive index of the Ag-photodoped chalcogenide films is analysed in terms of the Wemple–DiDomenico single-effective-oscillator model: $n^2(?\omega )=1-E_{\text{o}}{E}_{\text{d}}/(E_{\text{o}}^2-(?\omega {)}^2)$, where E_o is the single-oscillator energy, and E_d the dispersion energy. We found that the refractive index of the Ag-doped samples strongly increases with the Ag-content, whereas the optical band gap, $E_{\text{g}}^{\text{opt}}$, decreases also notably. For instance, in the particular case of x = 10 at.%, the largest Te-content, $E_{\text{g}}^{\text{opt}}$ decreases from 2.17 down to 1.67 eV. It should also be mentioned that, in the case of the undoped samples, when the Te-concentration increases from zero up to 10 at.%, the value of $E_{\text{g}}^{\text{opt}}$ decreases from 2.49 down to 2.17 eV.     

Structure and properties of the 70Li2S · (30 − x)P2S5 · xP2O5 oxysulfide glasses and glass–ceramics

The 70Li₂S · (30 ? x)P₂S₅ · xP₂O₅ (mol%) oxysulfide glasses were prepared by the melt quenching method. The glasses were prepared in the composition range 0 ≦ x ≦ 10. The glass–ceramics were prepared by heating the glasses over crystallization temperatures. The PO_nS_3?n (n = 1–3) oxysulfide units were produced in the glasses and glass–ceramics by partial substituting P₂O₅ for P₂S₅. In particular, the P₂OS₆^4? unit would be produced by substituting a small amount of P₂O₅ for P₂S₅. The oxygen atoms were incorporated into the Li₇P₃S₁₁ crystal structure because the diffraction peaks of the oxysulfide glass–ceramic shifted to the higher angle side. The glass–ceramic with 3 mol% of P₂O₅ exhibited the highest conductivity of 3.0 × 10^?3 S cm^?1 and the lowest activation energy for conduction of 16 kJ mol^?1. The P₂OS₆^4? dimer units in the oxygen-incorporated Li₇P₃S₁₁ crystal would improve conductive behavior of the Li₂S–P₂S₅ glass–ceramics.     

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 14: Concentration Dependence of the Defect Structure of Nonstoichiometric Phases Sr1 – xNdxF2 + x As Grown (x = 0.10, 0.25, 0.40, 0.50)

Crystallography Reports - The defect structure of as grown Sr1&nbsp;–&nbsp;xNdxF2&nbsp;+&nbsp;x (x&nbsp;=&nbsp;0.10–0.50) crystals grown from a melt by...