Thermodynamic properties of liquid Ga-Si-Mn alloys

The partial enthalpies of mixing of the constituent components (Δ_mix \(\bar H_i \)) have been determined by high-temperature calorimetry and then used to evaluate the integral enthalpies of mixing (Δ_mix H) of Ga-Si-Mn alloys along three cuts at 1770 ± 5 K. The results demonstrate that, in the composition region studied, the formation of Ga-Si-Mn alloys is an exothermic process. Δ_mix H is calculated over the entire composition region by the Bonnier-Caboz method.     

Thermodynamic properties of liquid metals and alloys

A modified cellular model of liquids is used to develop a method for calculating thermodynamic properties of liquid metals and their alloys, which are widely used in compact modern heat exchange equipment operating at high thermal flux densities. The approach presented herein permits calculation of the properties of metals from various groups by a single consistent technique. Good agreement was found with experimental data for a significant number of liquid metals and their alloys.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, pp. 832–840, November, 1990.     

Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.     

Thermodynamic analysis of thermite reactions for synthesizing Ni-based alloys

The equilibrium compositions of a thermite reaction system (Fe₂O₃+Cr₂O₃+CrO₃+NiO+O₂+N₂+TiO₂+C+S+CaO+Al), which was used to prepare Ni-based alloys, were predicted by means of the free energy minimization method. The effects of the reactant contents on the equilibrium compositions and the adiabatic temperature of the thermite reaction system were analyzed. The microstructure of the Ni-based alloys was investigated. The thermodynamic analysis indicated that the products of the thermite reaction system contained primarily Al₂O₃, Fe, Ni, Cr, Ni₃Al and NiAl, due to impurities C, N₂ and S in the raw materials, there could also exist TiC, Cr₇C₃, CrN, CaS and TiN in the equilibrium products. The amount of Ni₃Al and NiAl in the products increased with increase of the aluminum content in the starting materials. The microstructure investigation showed that the Ni-based alloys consisted of austenite, ferrite, Ni₃Al and NiAl. The existence of Ni₃Al and NiAl in the Ni-based alloys reconciled with the predictions of thermodynamic analysis, suggesting the free energy minimization method is useful in designing advanced materials with multiple components by thermite reaction technique.     

Thermodynamic properties of alloys and phase equilibria in the In-Sn-Sb system

We report the thermodynamic properties of alloys and phase equilibria in the In-Sn-Sb system studied by emf measurements at temperatures from 600 to 830 K. Different types of exchange reactions in the electrochemical cell (?) W|In|In⁺ in electrolyte |In _x Sb _y Sn _z | W (+) are considered, and the ways of reducing their influence on the accuracy of emf measurements are analyzed. A number of T?x sections of the In-Sn-Sb phase diagram are refined.     

Thermodynamic assessment of high-temperature oxidation of Fe—Cr alloys in air

A thermodynamic technique is proposed for assessing the variation of composition with depth in systems graded in composition. The technique is used to evaluate the compositions of scales forming on Fe, Cr, and Fe—Cr alloys during high-temperature oxidation in air. The coexistence of inclusions and extended precipitates of several phases in scales is interpreted in terms of oxidation thermodynamics kinetics.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1476–1482.Original Russian Text Copyright © 2004 by Goncharov.     

The effect of cobalt, sulfur, and tellurium on the liquidus-solidus gaps of Fe-Ni-Al-Co-Ti alloys

The values of liquidus and solidus temperatures were measured for various permanent magnet alloys including Alcomax III and Hycomax II, III, and IV. The effect of sulfur on these temperatures was studied for each of the alloys, and the effect of tellurium on the temperatures for Hycomax IV was also studied. The effect of liquidus-solidus gaps on columnar crystal formation is discussed. An attempt is made to relate the values found to those obtained from equilibrium diagrams.     

The effect of trace additions of tellurium on oxidation and some other properties of aluminium-lithium base alloys

Alloys of various chemical compositions with and without tellurium (Te) have been prepared in this research. Several types of tests were carried out, namely, tensile, ageing (hardness versus ageing time), erosion-corrosion, and oxidation, in addition to the evaluation of oxidation resistance by thermal shock. X-ray diffraction was also used to analyse the oxidation scales built on the samples. The microstructures of the alloys and oxides have been studied as well. It was found that the addition of small amounts (0.2 and 0.4 wt %) of Te led to drastic increases in oxidation resistance compared to the corresponding Te free alloy at both testing temperatures (300 and 500 °C). The great increase in resistance to environmental attack displayed by Te containing alloys was due to impediment of the interaction of Li with oxygen and enhancement of oxide plasticity and adhesion. Oxides grown on Te containing alloys remained intact when subjected to thermal shock. Te containing alloys also showed a great improvement in corrosion-erosion resistance compared to the other investigated Te free alloy. It was also noticed that Te enhanced the ductility of the base alloys themselves.     

Evaporation characteristics of TeO2 in the formation of tellurium oxide thin films

Thin films of tellurium oxide have been formed by evaporating TeO₂ with different boat materials (molybdenum and tantalum) in a high vacuum and at different oxygen partial pressures. The optical and structural properties of the films have been studied. From the variations in the properties with respect to the formation conditions the evaporation characteristics of TeO₂ have been discussed.     

Thermodynamic analysis of equilibria in Fe-Cr-C alloys and evaluation of their dusting stability in aggressive carboniferous atmospheres

Thermodynamic analysis of phase transformations and reactions in Fe-Cr-C alloys is carried out with allowance made for all possible states, both stable and metastable, in the temperature range of metal dusting in aggressive carboniferous atmospheres. The results are compared with data on the corrosion resistance of chromium steel pipes under oil refining conditions. It is shown that, at high temperatures, one of the equilibrium phases in this system is the cementite solid solution (Fe_1-y Cr_y)₃C. Below 930 K, this phase becomes unstable to the decomposition into mixtures containing other carbides, but the tendency for chromium to stabilize cementite persists. The metal dusting mechanism changes in the composition range 0.03 y 0.055 (2.8–5.1 wt % Cr). This conclusion is supported by the results of tests on chromium steel (15Kh5M and 15Kh2M1) pipes under oil refining conditions, which indicate that the introduction of chromium into Fe alloys can only inhibit the degradation of Fe-Cr-C alloys under metal dusting conditions, without fully preventing it.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 177–184.Original Russian Text Copyright © 2005 by Alshevskii, Baklanova, Zaitsev, Maltsev, Rodionova, Rybkin, Shaposhnikov.     

Thermodynamic properties of silver–palladium alloys determined by a solid state electrochemical method

Silver–palladium alloys have various industrial applications, such as in hydrogen permeation and hydrogen storage materials. The objective of the present study is to determine experimentally the thermodynamic properties of solid state silver–palladium alloys in low temperatures. Thermodynamic measurements of silver–palladium alloys have been performed over a temperature range of 450–750 K by the electromotive force method with superionic conductor AgI as the solid electrolyte. The activity and partial molar Gibbs energy of silver were obtained for Ag–Pd alloys over the whole composition range, and the thermodynamic properties of palladium were calculated using the Gibbs–Duhem equation. The results show that activities of silver exhibit fairly large negative deviations over most of the composition range, and the activities of palladium are characterized by both negative and positive deviations from the ideal Raoultian behavior. The results also show the minimum integral enthalpy of mixing of Ag–Pd alloys to locate at around 60 at.% Ag.     

Pressure - temperature relations in the silver - tellurium system at 75 at-% tellurium

The p,T-relations in the silver-tellurium system at 75 at-% Te were studied by high pressure DTA and quenching experiments at pressures up to 3.2 GPa. Fitted data for the phase boundaries are given. AgTe₃ becomes a stable phase in the system above 0.4 GPa. In quenching melts with a nominal composition AgTe₃ from high temperatures, stable and metastable equilibria can be observed. Conditions for the formation of homogeneous AgTe₃ from the melts in the low-pressure region are discussed.     

Thermodynamic assessment of Co–Al–W system and solidification of Co-enriched ternary alloys

Based on the assessed three constituent binary systems and reported phase diagram data, the Co–Al–W ternary system was thermodynamically optimized by using CALculation PHAse Diagram (CALPHAD) method. The newly reported ternary phase γ′ was described with a two sublattice model, (Co,Al,W)_0.75:(Co,Al,W)_0.25, while liquid Fcc, Hcp(Co), and Bcc(W) were treated as substitutional solution phases. In order to describe the ternary solubility in the binary phases Co₇W₆ and CoAl, the models (Al,Co,W)₇W₂(Al,Co,W)₄ and (Al,Co,W)_0.5(Al,Co,W)_0.5(Va)₃ were, respectively, adopted. The rest 11 phases in the ternary systems were simply treated as stoichiometric compounds because their homogeneous ranges are small and can be neglected. A set of self-consistent parameters were obtained, which reasonably reproduced the phase relationship in Co-enriched corner. Finally, to confirm the reliability of the present assessment,the so-called Scheil–Gulliver model was used to simulate the solidification processes of three typical Co-based alloys. It was shown that the as-cast microstructures of the three ingots were well explained or predicted.     

Thermodynamic analysis of processability of Mg–Al–Zn–Mn alloys for rheocasting

A two-stage thermodynamic calculation procedure was developed to analyse alloy compositions for rheocasting. Based on Kazakov's criteria, the processability of AZ91, AZ61 and AM60B for rheocasting was investigated. The results show that these alloys do not satisfy selected criteria fully, and only AZ91 is possible to be used for rheocasting with semisolid slurry having lower fraction solid (0.2–0.4). In microstructure produced by rheocasting, the β-Mg₁₇Al₁₂ phase forms continuous network along primary α-Mg gain boundary and is concentrated in this limited contiguous space. The routes to modify and optimise alloy composition were proposed.     

Basic spectroscopic properties of bismuth tellurium oxide,Bi2TeO5

Optical absorption measurements in the visible and infrared and reflectivity measurements in the visible and ultraviolet are reported for single crystals of bismuth tellurium oxide (Bi₂TeO₅). Strong dependence of these measurements on the light polarization is found for this recently grown non-linear material. Deviation from the Urbach rule (see Appendix) is believed to result from non-stoichiometry; approximate values of the Urbach parameters are given for the [0 1 0] polarization.