Thermodynamic properties of Cr Mo solid alloys

The activity of Cr in solid Cr-Mo alloys has been measured at 1873 K using a metal-oxide-gas equilibrium technique. Thin foils of Mo were equilibrated with solid Cr203 under flowing gas mixtures of argon, hydrogen and watervapourof known composition. The equilibrium concentration of Cr in Mo was determined by chemical analysis. These measurements indicate positive deviations from Raoult's law. The activity data obtained in the study at 1873 K are combined with free energy of mixing at 1471 K, calorimetric enthalpy of mixing at 1673 K, and experimental evidence of phase separation at lower temperatures, reported in the literature, to obtain an optimised set of thermodynamic parameters for the Cr-Mosystem in the solid state.

Casting of titanium and titanium alloys

Titaniuni and its alloys have many applications in aerospace, marine and other engineering industries. Titanium requires special melting techniques because of its high reactivity at elevated temperatures and needs special mould materials and methods for castings. This paper reviews the development of titanium casting technology.

Synthesis from zinc oxalate, growth mechanism and optical properties of ZnO nano/micro structures

We report the synthesis of various morphological micro to nano structured zinc oxide crystals via simple precipitation technique. The growth mechanisms of the zinc oxide nanostructures such as snowflake, rose, platelets, porous pyramid and rectangular shapes were studied in detail under various growth conditions. The precursor powders were prepared using several zinc counter ions such as chloride, nitrate and sulphate along with oxalic acid as a precipitating agent. The precursors were decomposed by heating in air resulting in the formation of different shapes of zinc oxide crystals. Variations in ZnO nanostructural shapes were possibly due to the counter ion effect. Sulphate counter ion led to unusual rose-shape morphology. Strong ultrasonic treatment on ZnO rose shows that it was formed by irregular arrangement of micro to nano size hexagonal zinc oxide platelets. The X-ray diffraction studies confirmed the wurzite structure of all zinc oxide samples synthesized using different zinc counter ions. Functional groups of the zinc oxalate precursor and zinc oxide were identified using micro Raman studies. The blue light emission spectra of the various morphologies were recorded using luminescence spectrometer. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Physical chemistry of the reduction of calcium oxide with aluminium in vacuum

The physical chemistry of "aluminothermic" reduction of calcium oxide in vacuum is analyzed. Basic thermodynamic data required for the analysis have been generated by a variety of experiments. These include activity measurements in liquid AI-Ca alloys and determination of the Gibbs energies of formation of calcium aluminates. These data have been correlated with phase relations in the Ca-AI-0 system at 1373 K. The various stages of reduction, the end products and the corresponding equilibrium partial pressures of calcium have been established from thermodynamic considerations. In principle, the recovery of calcium can be improved by reducing the pressure in the reactor. However,, the cost of a high vacuum system and the enhanced time for reduction needed to achieve higher yields makes such a practice uneconomic. Aluminum contamination of calcium also increases at low pressures. The best compromise is to carry the reduction up to the stage where 3CaO-Al,O, is formed as the product. This corresponds to an equilibrium calcium partial pressure of 31.3 Pa at 1373 K and 91.6 Pa at 1460 K. Calcium can be extracted at this pressure using mechanical pumps in approximately 8 to 15 hr, depending on the size and the fill ratio of the retort and porosity of the charge briquettes.

Thermodynamics of Aluminium -Strontium Alloys

The activity of strontium in liquid Al-Sr alloys (X(Sr) less-than-or-equal-to 0.17) at 1323 K has been determined using the Knudsen effusion-mass loss technique. At higher concentrations (X(Sr) greater-than-or-equal-to 0.28), the activity of strontium has been determined by the pseudoisopiestic technique. Activity of aluminium has been derived by Gibbs-Duhem integration. The concentration - concentration structure factor of Bhatia and Thornton at zero wave vector has been computed from the thermodynamic data. The behaviour of the mean square thermal fluctuation in composition and the thermodynamic mixing functions suggest association tendencies in the liquid state. The associated solution model with Al2Sr as the predominant complex can account for the properties of the liquid alloy. Thermodynamic data for the intermetallic compunds in the Al-Sr system have been derived using the phase diagram and the Gibbs' energy and enthalpy of mixing of liquid alloys. The data indicate the need for redetermination of the phase diagram near the strontium-rich corner.

A new matched thermochemical diagram for vacuum refining of nickel and cupronickel alloys

Sulfur and oxygen dissolved in nickel and cupronickel melts can be remwed as gaseous oxides of sulfur by a vacuum treatment. Presented in this paper is a new matched thermcxhemical disgran~ that permit.. direct evaluation of the equilibrium partial pressure of SO, as a function of temperature wer an alloy of specified compition. The matched thermochemical diagram consists of a central plot which shows the integral Gibbs' energy of mixing for the binary system SO, at different temperatures. The central plot is flanked on either side by terminal plots of the chemical potentials of oxygen and sulfur, as functions of temperature, for different alloy compositions. By projecting the chemical wtentials of oxygen and sulfur from the terminal lots on to the central diagram, ihe equilibrium partial pressure of S0,can be directly ;cad on the nomograms on the central plot at different temperatures. The matched therrnochemical diagrams are useful in assuring the efficiency of vacuum refining.

Phase equilibria and thermodynamics of the system Dy-Mg-Cl at 1073 K

The phase relations in the system Dy–Mg–Cl at 1073 K have been established by isothermal equilibration and chemical analysis of quenched samples. Liquid Mg-rich alloy was found to be in equilibrium with molten DyCl2. Therefore, DyCl2 can be synthesized by reduction of MgCl2 with excess of metallic Dy at 1073 K. The Gibbs energy of formation of DyCl2 at 1073 K was evaluated by two different methods. From voltammetric determination of decomposition voltage, the upper limit for the standard Gibbs energy of formation of DyCl2 was estimated to be −505(±20) kJ mol−1. A value of −543(±10) kJ mol−1 was deduced from phase relations using Gibbs–Duhem integration. The value for the standard Gibbs energy of DyCl2 indicates that the Dy2+ ion has a potential capability for reducing TiCl4 to metal titanium. At the same time, Mg is a reductant for Dy3+ produced during the reduction of TiCl4. Thus, it is thermodynamically confirmed that reduction of TiCl4 by magnesium using a reaction mediator in the salt phase is feasible.

Production of niobium powder by electronically mediated reaction (EMR) using calcium as a reductant

Explored in this study is an electronically mediated reaction (EMR) route for the production of niobium powder using calcium as a reductant for niobium oxide (Nb2O5). Feed material, Nb2O5, and reductant calcium alloy containing aluminum and nickel were charged into electronically isolated locations in a molten salt (e.g. CaCl2) at 1173 K. The current flow through an external path between the feed and reductant locations was monitored. A current approximately 0.4 A was measured during the reaction in the external circuit connecting cathode and anode location. Niobium powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. This clearly demonstrates that niobium metal powder can be produced by an electronically mediated reaction (EMR), without direct physical contact between feed (Nb2O5) and reductant (calcium). Mechanism of calciothermic reduction of Nb2O5 in the molten salt is discussed using an isothermal chemical potential diagram.

Solute-solute and solute-solvent interactions in transition metal alloys: Pt-Ti system

An isothermal section of the phase diagram for (silver + rhodium + oxygen) at T = 1173 K has been established by equilibration of samples representing twelve different compositions, and phase identification after quenching by optical and scanning electron microscopy (s.e.m.), X-ray diffraction (x.r.d.), and energy dispersive analysis of X-rays (e.d.x.), Only one ternary oxide, AgRhO2, was found to be stable and a three phase region involving Ag, AgRhO2 and Rh2O3 was identified. The thermodynamic properties of AgRhO2 were measured using a galvanic cell in the temperature range 980 K to 1320 K. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa was used as the reference electrode. The Gibbs free energy of formation of the ternary oxide from the elements, ΔfGo (AgRhO2), can be represented by two linear equations that join at the melting temperature of silver. In the temperature range 980 K to 1235 K, ΔfGo(AgRhO2)/(J . mol-1) = -249080 + 179.08 T/K (±120). Above the melting temperature of silver, in the temperature range 1235 K to 1320 K, ΔfGo(AgRhO2)/(J . mol-1) = -260400 + 188.24 T/K (±95). The thermodynamic properties of AgRhO2 at T = 298.15 K were evaluated from the high temperature data. The chemical potential diagram for (silver + rhodium + oxygen) at T = 1200 K was also computed on the basis of the results of this study.

A thermodynamic study of liquid Cu-Cr alloys and metastable liquid immiscibility

he thermodynamic acitivity of chromium in liquid Cu-Cr alloys is measured in the temperature range from 1473 to 1873 K using the solid state cell: Pt, W, Cr + Cr2O3 |(Y2O3) ThO2|Cu - Cr + Cr2O3, Pt The activity of copper and the Gibbs energy of mixing of the liquid alloy are derived. Activities exhibit large positive deviations from Raoult's law. The mixing properties can be represented by a pseudo-subregular solution model in which the excess entropy has the same type of functional dependence on composition as the enthalpy of mixing: ΔGE = XCr(1 - XCr)[60880 - 18750 XCr)-- T(16.25 - 7.55 XCr)]J mol-1 Pure liquid Cu and Cr are taken as the reference states. The results predict a liquid-liquid metastable miscibility gap, with TC = 1787 (±3) K and XCr = 0.436 (±0.02), lying below the liquidus. The results obtained in this study are in general agreement with experimental information reported in the literature, but provide further refinement of the thermodynamic parameters.

Relation between Local Structure and Glass Forming Ability of Liquid Alloys

An attempt has been made to describe the glass forming ability (GFA) of liquid alloys, using the concepts of the short range order (SRO) and middle range order (MRO) characterizing the liquid structure.A new approach to obtain good GFA of liquid alloys is based on the following four main factors: (1) formation of new SRO and competitive correlation with two or more kinds of SROs for crystallization, (2) stabilization of dense random packing by interaction between different types of SRO, (3) formation of stable cluster (SC) or middle range order (MRO) by harmonious coupling of SROs, and (4) difference between SRO characterizing the liquid structure and the near-neighbor environment in the corresponding equilibrium crystalline phases. The atomic volume mismatch estimated from the cube of the atomic radius was found to be a close relation with the minimum solute concentration for glass formation. This empirical guideline enables us to provide the optimum solute concentration for good GFA in some ternary alloys. Model structures, denoted by Bernal type and the Chemical Order type, were again tested in the novel description for the glass structure as a function of solute concentration. We illustrated the related energetics of the completion between crystal embryo and different types of SRO. Recent systematic measurements also provide that thermal diffusivity of alloys in the liquid state may be a good indicator of their GFA.

Low‐voltage varistors based on zinc antimony spinel Zn7Sb2O12

It is possible to prepare low‐voltage varistors from the zinc antimony spinel Zn7Sb2O12 with breakdown voltages in the range of 3–20 V and nonlinearity coefficient α=7–15. The varistor property is due to the formation of high ohmic potential barriers at the grain boundary regions on low‐ohmic n‐type grain interiors of the polycrystalline samples. The method of preparation of the spinel, synthesized by coprecipitation followed by annealing under restricted partial pressures of oxygen, controls the mixed valence states for antimony, namely, Sb3+ and Sb5+. This is critical in attaining high nonlinearity and lower breakdown voltages.

Evolution of sub-micron grain size and weak texture in magnesium alloy Mg-3Al-0.4Mn by a modified multi-axial forging process

This is the first successful attempt to produce simultaneously ultrafine grain size and weak texture in a single-phase magnesium alloy Mg-3Al-0.4Mn through an optimal choice of processing parameters in a modified multi-axial forging (MAF) process. An average grain size of similar to 0.4 mu m and a weak texture could be achieved. This has led to an increase in the strength as well as room-temperature ductility (55%). The plot of the yield loci shows a decrease in anisotropy after MAF. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.