Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.     

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.     

On the role of liquid phase stability and GFA parameters

Role of liquid phase stability on the glass forming ability (GFA) has been reviewed and the alloy systems have been analyzed by introducing a contribution factor (w_g) to the characteristic temperature T_g, in the γ parameter. The kinetics of glass formation for various alloy systems has been found to vary with liquid phase stability in metastable state. The GFA of a fragile liquid is found to be more responsive to the contribution of metastable stability compared to strong liquid.     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Relaxation of glasses: The Kohlrausch exponent

Relaxation of glasses is analyzed in the framework of the generalized Vogel-Fulcher-Tamann law. In this model the relaxation times of the cooperative and individual motions (α and β) verify the relation log τ_α(T,T′) = 1/n(T,T′) log τ_β(Τ) . In the glass state the equivalent temperature T′ is the temperature of the liquid (at equilibrium) which has the volume of the glass, T′ is then the function of the aging time and of the experiment time. In the liquid state (T = T′), n(T) ~ T − T₀ is the Kohlrausch exponent (T₀ is the Vogel temperature). In the glass state the parameter, n(T,T′), is a function of the aging time and temperature, of the rheological parameters of the liquid (WLF constants C₁ and C₂) and of the expansion coefficients of the liquid and glass states. The Kohlrausch exponents deduced from the properties below Tg (volume, creep, stress relaxation) are deduced from the generalized VFT model.     

Structure of Fe–Ni and Fe–Ni–S molten alloys by neutron diffraction

The static S(Q) liquid structure factors for binary Fe_xNi_100−x, x = 90 and 85 and ternary Fe₈₅Ni₅S₁₀ molten alloys are investigated by means of the concurrent use of neutron diffraction and Reverse Monte Carlo simulations. The measured g(r) radial distributions reveal atomic orderings varying from that present in the Fe₉₀Ni₁₀ alloy, reminiscent of the ideal b.c.c. structure of solid Fe, to a far more open structure found in Fe₈₅Ni₅S₁₀. From data at hand no clear evidence of clustering of sulphur within the Fe–Ni matrix is found but, in contrast, its addition leads to significant changes in structural properties manifested by the loss of the intermediate range order.     

The structural properties of the liquid CdTe based on Reverse Monte Carlo modelling

The Reverse Monte Carlo (RMC) method only provides information on structure, not on dynamics, and so has restricted applicability. We have only attempted to describe the trends in structural terms. The static structure of liquid Cadmium Telluride is performed using RMC modelling technique fitting to neutron diffraction data. In order to retain its tetrahedral environment with the coordination number about four in its liquid state, we have used some plausible physical constraints. In this way, it is possible to derive static structure of liquid CdTe with analysing the experimental data. We have shown the results for the partial static structure factors S_ij(q) and the partial pair distribution functions g_ij(r). We have also determined the distribution of the number of neighbors within the first coordination shell and calculated the bond angle distribution. A good agreement with the experiment is observed for the structural properties of liquid CdTe.     

Microsegregation in liquid Pb-based eutectics

Temperature dependence of electrical conductivity, σ(T), thermoelectric power, S(T), and viscosity, η(T), of Pb-based eutectic systems was studied. Anomalous changes of thermophysical properties in liquid binary Pb_26.1Sn_73.9, Pb₄₄Bi₅₆, Pb₈₃Mg₁₇ and ternary Bi₄₆Pb₂₉Sn₂₅ eutectics occurred well above the liquidus. The temperature range of anomalies reached hundreds of degrees. The obtained results are interpreted assuming that microsegregation areas exist in the eutectic systems.     

Structural study of AsS2–Ag glasses over a wide concentration range

(As_0.33S_0.67)_100-xAg_x (0 ≤ x ≤ 28) bulk glasses showing micro-phase separation in a wide concentration range have been studied by X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure measurements. The AsAgS₂ composition, which forms a homogeneous glass, is modeled with the reverse Monte-Carlo simulation technique. It is established that Ag prefers the environment of S; Ag―As bonding cannot be observed. Similarly to the AsAgS₂ crystalline modifications smithite and trechmannite, the main structural units of the glass are AsS₃ pyramids. The covalent network of As and S atoms becomes fragmented in the glassy AsAgS₂ unlike in the glassy AsS₂. The environment of Ag atoms in the AsAgS₂ glass differs from that in the crystalline state. In average, each Ag atom has four nearest neighbors, three of them being S and one being Ag.     

Structure and properties of glasses in the MI + M2S + (0.1Ga2S3 + 0.9GeS2), M = Li, Na, K and Cs, system

The structure and properties of glasses in the MI + M₂S + (0.1Ga₂S₃ + 0.9GeS₂), M = Li, Na, K and Cs, system were studied using Raman, IR spectroscopy, DSC and density measurements to help better understand the ionic transport in these glasses. The glass forming ranges of these ternary glasses were compared to those of the binary alkali sulfide and germanium sulfide systems. The more extensive glass forming range in the Na₂S system was used to examine the more extensive changes of structure and properties of these glasses as a function of Na₂S content. As expected, non-bridging sulfurs (NBS) form with the addition of alkali sulfide. Unlike their oxide counterparts, however, the alkali sulfide doped glasses appear to support longer-range super-structural units. For example, evidence that the adamantine-like structure exists in the K₂S and Cs₂S modified glasses is found in the Raman spectra of the glasses. The structural role of the alkali iodide addition was also explored since the addition of alkali iodide helps to improve the conductivity. For most of these glasses, as observed in many other oxide glasses, the added MI dissolves interstitially into the glass structure network without changing the alkali sulfide network structure. In 0.6Na₂S + 0.4(0.1Ga₂S₃ + 0.9GeS₂) glasses, however, the added NaI may affect the glass structure as it causes systematic changes in the frequency of the Ge-S network mode as seen in the Raman spectra.     

Relationship between viscous dynamics and the configurational thermal expansion coefficient of glass-forming liquids

We propose a model to describe the relationship between the viscosity of a glass-forming liquid and its configurational contribution to liquid state thermal expansion. The viscosity of the glass-forming liquids is expressed in terms of three standard parameters: the glass transition temperature (T_g), the liquid fragility index (m), and the extrapolated infinite temperature viscosity (η_∞), which are obtained by fitting of the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) expression to measured viscosity data. The model is tested with experimental data for 41 different glass-forming systems. A good correlation is observed between our model viscosity parameter,h(T_g, m, η_∞), and the configurational coefficient of thermal expansion (i.e., the configurational CTE). Within a given class of glass compositions, the model offers the ability to predict trends in configurational CTE with changes in viscosity parameters. Since viscosity is governed by glass network topology, the model also suggests the role of topological constraints in governing changes in configurational CTE.     

Glass formation and switching in the SbSe systems

New glass forming regions in the SbSe system have been observed using splat cooling techniques. The glass region is limited to compositions with less than 50 at % Sb. The glass whose composition corresponds to Sb₂Se₃ exhibits memory and can be switched off only by cooling in liquid nitrogen in the presence of an electric field. The off-state electrical resistance is of the order of 10⁵ times greater than that of the memory on-state.     

Viscosity of germanium sulfide melts

Viscosities of Ge_xS_1−x melts (0.30⩽x⩽0.44) have been measured by penetration viscometry from 10⁷ to 10¹³ Pa s. The temperature dependence of equilibrium viscosities in this range can be expressed approximately by a simple Arrhenius equation. Both the heat capacity change at the glass transition and the activation energy of viscous flow monotonously increase with germanium content as predicted by the Adam–Gibbs theory. Therefore, the connectivity of the germanium–sulfur network is reduced due to decreasing concentration of sulfur which causes increasing fragility of the undercooled liquid. The glass transition temperature exhibits a maximum near the GeS₂ composition where heteropolar bonds are predominantly formed.     

Glass formation and crystallization in highly undercooled Te-Cu alloys

The large undercoolings required for glass formation have been achieved by the slow cooling (10-20°C/min) of liquid Te-Cu alloys in the form of a fine droplet emulsion. Within the region of glass formation, between 19 and 39 at.% Cu, DTA measurements indicate that the glass (T_g) and crystallization (T_c) temperatures during heating exhibit a broad maximum at the eutectic. During slow cooling of Te-rich alloy droplets, the maximum undercooling for nucleation increases from 213°C for pure Te to 264°C for Te-12.5 at.% Cu. An enhanced depression of the nucleation (T_n) temperature compared with the change of the liquidus develops in Te-rich alloys upon approaching the glass forming composition range and can be a useful feature in assessing the glass forming tendency. Thermal cycling experiments indicate that even at an undercooling of 181°C crystallization in an eutectic Te-29 at.% Cu alloy is limited by an inadequate nucleation rate in clean droplet samples. For a eutectic alloy, at undercoolings in excess of 200°C crystal nucleation does develop in the droplet samples, but complete crystallization is hindered by a rapidly rising liquid viscosity with increased undercooling.     

On the entropy difference between the vitreous and the crystalline state

The paper deals with the entropy difference between frozen-in phases and their equilibrium counterparts. First, the nature of data compiled in thermochemical data collections are briefly reviewed, comprising data for non-equilibrium phases. Then, experimental evidence from earlier literature is compiled showing that the conventional entropy of a frozen-in phase at zero Kelvin assumes a non-zero residual value S(0). Based on calorimetric data from multiple sources, the same evidence is elaborated for diopside glass, yielding S_glass(0) = 24.8 ± 3 J/(mol K), a value reproducing a result publishes earlier. The zero Kelvin enthalpy of this glass is H_glass(0) = 81±8 kJ/mol. For S_glass(0), a structural interpretation in terms of silicate chain mixing is proposed, yielding a lower threshold for S_glass(0). From the point of view of statistical mechanics, non-zero residual entropies of frozen-in phases can be derived from ensemble averages, however, not from time averages.     

A combined measurement of thermal and mechanical relaxation

In order to describe relaxation the thermodynamic coefficient can be generalized into a complex frequency-dependent cross response function. We explore theoretically the possibility of measuring for a supercooled liquid near the glass transition. This is done by placing a thermistor in the middle of the liquid which itself is contained in a spherical piezoelectric shell. The piezoelectric voltage response to a thermal power generated in the thermistor is found to be proportional to but factors pertaining to heat diffusion and adiabatic compressibility κ_S(ω) do also intervene. We estimate a measurable piezoelectric voltage of 1 mV to be generated at 1 Hz for a heating power of 0.3 mW. Together with κ_S(ω) and the longitudinal specific heat c_l(ω) which may also be found in the same setup a complete triple of thermoviscoelastic response functions may be determined when supplemented with shear modulus data.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Dispersion of acoustic-like and optic-like vibrational excitations in Ni2B metallic glass

The dynamic structure factor S(Q, E) of Ni₂B metallic glass was studied by inelastic neutron scattering in the ranges of momentum transfers 0.7 < ?Q/Å^?1 < 7, and energy, 2 < E/meV < 70, transfers. The measurements were performed with the IN4 spectrometer (Institut Laue-Langevin (ILL), Grenoble, France) on three samples of the same chemical composition but with different contents of Ni and B isotopes, as a result of which partial contributions were determined. The cuts through S(Q, E) for different values of E and Q demonstrated a characteristic behavior indicating that vibrational excitations, similar to acoustic and optical phonon excitations in crystals, propagate in an amorphous medium in a certain energy and momentum range.     

Structural investigation of an amorphous Si24Nb76 alloy produced by mechanical alloying using reverse Monte Carlo simulations

The atomic structure of an amorphous Si₂₄Nb₇₆ alloy produced by mechanical alloying was investigated by using one X-ray total structure factor S(K) as input data for reverse Monte Carlo (RMC) simulations. The partial S_Si–Si(K), S_Si–Nb(K), S_Nb–Nb(K) structure factors and G_Si–Si(r), G_Si–Nb(r), G_Nb–Nb(r) pair probability functions were obtained from the RMC simulations. The structural parameters (interatomic distances and co-ordination numbers) for the first neighbors were extracted and compared with those found in the Nb₃Si compound. It was observed some resemblance between these phases.