METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 27B, No. 6, December 1996

This Month Featuring: Electrometallurgy; Transport Phenonema; Physical Chemistry; Solidification; Mechanical Working; Solid State Reaction; Surface Treatment; Mathematical Modeling. View December 1996 Table of Contents.

ELECTROMETALLURGY

Application of Centrifugal Fields in Fused Salt Electrowinning with a View to Reducing Electrolytic Energy Consumption
ANTONY COX and DEREK J. FRAY
A high-temperature, laboratory scale electrochemical cell was designed, constructed, and commissioned to investigate the use of centrifugal fields in fused salt electrolysis production of light metals. Fused salt electrowinning of zinc was initially investigated due to the simpler physical and chemical nature of the Zn(l)|ZnCl2(l):KCl(l)|Cl2(g) system. Current efficiencies of 93 pct were obtained for an electrode spacing of 8 mm using 70-mm-diameter plane disc electrodes rotating at 100 rpm, significantly reducing the resistive contribution to the cell voltage. By reducing the immersion depth of the electrodes from 25 to 10 mm, current efficiencies of 88 pct were obtained for an electrode spacing of only 4 mm for the same operating conditions, further decreasing the resistive contribution to the cell voltage.

Preparation of Pure Silicon by Electrowinning in a Bytownite-Cryolite Melt
JAN R. STUBERGH and ZHONGHUA LIU
Pure silicon was prepared by electrowinning in a bytownite-cryolite molten mixture at about 970°C. In the electrolysis cell, the bottom of a graphite crucible was used as the anode. The cathode was also made of graphite. The reaction was carried out at the cell voltage of about 2.7 V, and the cathode current density was either 0.027 or 0.053 A/cm2 for 10, 50, and 100 pct of the time for all Si deposited. The obtained silicon, which was analyzed by electron microprobe analysis (EMPA), contained 99.79 to 99.98 pct Si, which is a specially pure quality. CO2 gas formed at the anode flushed silicon deposited at the cathode and purified it by oxidizing the impurities that ordinarily would be deposited at the cathode.

TRANSPORT PHENONEMA

Solid-State Contributions to Densification During Liquid-Phase Sintering
JOHN L. JOHNSON and RANDALL M. GERMAN
Densification via liquid-phase sintering generally requires transport of substantial amounts of dissolved solid through the liquid. However, in composite systems, such as W-Cu, solid solubility in the liquid is almost negligible, and densification is hindered by the low amount of total mass transport. In this case, solid-state sintering of the skeletal solid structure in the presence of the liquid is a significant densification mechanism. In this article, the relative contributions to densification of both liquid and solid mass transport mechanisms are considered. A computer simulation is constructed to predict the densification behavior and concurrent microstructural development of liquid-phase sintered composites for realistic heating cycles. Governing differential equations for densification are derived from idealized models of the microstructure, considering grain size, diffusion distance from vacancy source to sink, pore size, and pore morphology. Temperature-dependent terms, including the diffusivity, solubility, and surface energy, govern densification and microstructural parameters, such as the grain size, dihedral angle, and contiguity. Predictions for the sintered density, grain size, and contiguity are compared to experimental results for the W-Cu and W-Cu-Ni systems with approximately 20 vol pct liquid. For W-Cu, which has almost no intersolubility, solid-state sintering of W in the presence of liquid Cu is the dominant densification mechanism. Nickel additions increase solid solubility in the liquid and improve typical liquid-phase sintering contributions to densification. Alternatively, high sintered densities can be achieved in the absence of solubility with a sufficiently small particle size due to the solid-state contribution.

Studies of Interface Deformations in Single- and Multi-Layered Liquid Baths Due to an Impinging Gas Jet
F. QIAN, R. MUTHARASAN, and B. FAROUK
An impinging gas jet on a molten bath having a slag layer on top is encountered in various metal processing operations. The impinging region was studied using a physical model consisting of an air jet and water bath. Kerosene and corn oil were used as the second layer to investigate the role of the slag layer properties on interface shape and bath circulation. The interface shapes were measured both photographically and by using a surface-tracking resistance probe. The limiting condition at which the jet breaks through the kerosene or corn oil layer and reaches the water layer was determined experimentally. A phenomenological model for the prediction of penetration depth is developed for both short and long jet heights for liquid baths with and without a second liquid layer on top.

PHYSICAL CHEMISTRY

Thermodynamic Investigations of the Ternary Au-Sn-Zn System
S. KARLHUBER, A. MIKULA, and F. SOMMER
The thermodynamic properties of liquid ternary Au-Sn-Zn alloys were measured with an electromotive force (emf) method at four cross sections with a constant Au to Sn ratio of 9:1, 3:1, 1:1, and 1:3, respectively. At the last three cross sections, calorimetric measurements were also carried out in order to compare the integral enthalpy of mixing from the direct measurements with the data of the emf measurements which were derived by Gibbs=nDuhem integration.

The Measurement of Hydrogen Activities in Molten Copper Using an Oxide Protonic Conductor
NORIAKI KURITA, NORIHIKO FUKATSU, SATOSHI MIYAMOTO, FUMIAKI SATO, HIROYUKI NAKAI, KAZUHIKO IRIE, and TERUO OHASHI
We have developed a hydrogen sensor for in situ measurements of hydrogen activities in molten copper. The sensor consists of a concentration cell utilizing a proton conductor, CaZr0.9In0.1O3-, as the solid electrolyte. The electromotive force (emf) of the cell was generated by both hydrogen and oxygen activity gradients across the cell in a high-temperature region simulating the fire refining processes of copper. However, accurate hydrogen activity in molten copper could be evaluated from the emf if oxygen activity in molten copper was determined simultaneously by another concentration cell and if the hydrogen and the oxygen activities at the reference electrode were known. The performance of the sensor was studied under various conditions. The observed good response and reliability of the sensor show that it should be a powerful tool for improvement of the fire refining process of the molten copper. Theoretical treatment of the calculation of the emf of the concentration cells using a mixed ionic conductor, i.e., protonic and oxide ionic conductor, as solid electrolytes is also discussed briefly.

Thermodynamics of Phosphorus in Molten Silicon
TAKAHIRO MIKI, KAZUKI MORITA, and NOBUO SANO
Removal of phosphorus is one of the major problems on the purification of molten silicon for solar cell. The Gibbs energy change of phosphorus dissolution into molten silicon was determined in the temperature range from 1723 to 1848 K by equilibrating a molten silicon-phosphorus alloy in a controlled phosphorus partial pressure and is expressed by the following equations:

P2 (g) = P (mass pct, in Si)

G° = -139,000 (±2000) + 43.4 (±10.1T)(J/mol)

The possibility of removing phosphorus from silicon melts by vacuum treatment and the accompanying yield of silicon during the refining process are discussed.

Activities in CaO-SiO2-Al2O3 Slags and Deoxidation Equilibria of Si and Al
HIROKI OHTA and HIDEAKI SUITO
By using the data in previous and present slag-metal equilibrium experiments, the activities of SiO2 along the liquidus lines in CaO-SiO2-Al2O3 slags were determined at 1823 and 1873K from the reaction Si+2O=SiO2(s), in which the oxygen activities were estimated from the measured oxygen contents or from the combination of nitrogen distribution ratios (LN) and nitride capacities (CN). The activities of Al2O3 were also determined from the reaction 2Al+3O = Al2O3 (s), in which the oxygen activities were estimated from the values for LN and C;zN, or from the reaction 3SiO2 (s) + 4Al = 2Al2O3 (s) + 3Si, in which the activities of SiO2 and the contents of Al and Si along with the respective interaction coefficients were used. The activities of Al2O3 and CaO in the entire liquid region were estimated from the Rein and Chipman's activities of SiO2 by using the method of Schuhmann. On the basis of these activities, the deoxidation equilibria of Si and Al in steels were discussed.

A Thermodynamic Evaluation of the Ti-Mo-C System
JAE-HYEOK SHIM, CHANG-SEOK OH, and DONG NYUNG LEE
A thermodynamic assessment of the Ti-Mo-C system has been made, employing a two-sublattice regular solution model for the solid solution and carbide phases and an ordinary subregular solution model for the liquid phase. A set of thermodynamic parameters describing the Gibbs energy of each individual phase in the Ti-Mo-C as well as the Ti-Mo systems was evaluated from thermochemical and phase equilibria information available in the literature through a computer-aided optimization procedure called the CALPHAD method. The comparison between the calculated and experimental results was made and practically important phase diagrams are also presented.

Thermodynamics of Calcium and Oxygen in Molten Titanium and Titanium-Aluminum Alloy
FUMITAKA TSUKIHASHI, EIJI TAWARA, and TOSHIYUKI HATTA
The deoxidation equilibrium of molten titanium and titanium-aluminum alloys saturated with solid CaO has been measured in the temperature range from 1823 to 2023 K. The equilibrium constant of reaction CaO (s) = Ca (mass pct in Ti,Ti-Al) + O (mass pct in Ti,Ti-Al) and the interaction parameter between calcium and oxygen were determined for Ti, TiAl, and TiAl3. The standard Gibbs energy of reaction for TiAl was obtained as follows:

G°= 279,000-103T J/mol

The possibilities for the deoxidation of titanium and titanium-aluminum alloys by using calcium-based fluxes are discussed.

Thermodynamic Properties of Complex Oxides in the Sm-Ba-Cu-O System
XIANRAN XING, ZHIYU QIAO, and SHOUKUN WEI
An assembly for performing electromotive force (EMF) measurements using either a CaF2 single crystal or a MgO partially stabilized zirconia as a solid-state electrolyte has been constructed. Heat capacities of member complex oxides in the Sm-Ba-Cu-O system were measured by an adiabatic scanning calorimeter. From the EMF data, the standard Gibbs energies, standard enthalpies of formation, standard entropies, and decomposition pressures of the complex oxides were calculated. The standard Gibbs energies of reaction of complex oxides from relevant simple oxides Sm2O3, BaO, and CuO were derived. It was found that all complex compounds in this system were thermodynamic stable. The tendency of the thermodynamic stability of the solid solution compound Sm1+xBa2-xCu3Oy decreases with the solubility x from 0 to 0.4.

SOLIDIFICATION

Inverse Melting in Binary Systems: Morphology and Microscopy of Catatectic Alloys
R. FERRO, A. SACCONE, S. DELFINO, A.M. CARDINALE, and D. MACCIÒ
General characteristics of the reaction solid1solid2 + liquid (catatectic equilibrium) are summarized and some statistical information about its occurrence in binary alloy systems is reported. The data obtained in the analysis of a number of binary alloys involving rare earth metals (La-Al, Ce-Al, Gd- Au, Nd-Au, and Sm-Mn) showing catatectic equilibria are presented. General features of the micrographic appearance of these alloys are shown and discussed.

MECHANICAL WORKING

Van der Waals Approximation for Potassium Bubbles in Tungsten
A. NAGY
The finely dispersed potassium bubbles are responsible for the high-temperature sag resistance of lamp filaments; thus, the understanding of development of bubble structure is of practical importance. Usually, the ideal gas theory is used for describing the potassium bubbles in tungsten. Instead, we used the van der Waals approximation which can account for phase changes and describes a two-phase coexistence region. It is shown theoretically that bubbles in the practically important size range (some tenth of microns) can exist in a two-phase state. Some experimental findings are also better explained by the real gas approximation.

SOLID STATE REACTION

Real-Time Observations of the Oxidation of Mild Steel at High Temperature by Neutron Diffraction
H. ABULUWEFA, J.H. ROOT, R.I.L. GUTHRIE, and F. AJERSCH
The in situ characterization of the phase composition of iron oxides, "scale" that form on low carbon steel during oxidation at elevated temperatures was carried out using the neutron diffraction technique. Growths in the intensities of diffraction peaks from the crystal planes of the various oxides (FexO, Fe3O4, and Fe2O3) were monitored on-line. The volume fractions of the oxides in the scale were calculated on the basis of ideal structure factors and measured relative intensities of diffraction peaks. These were selected from a small region of the diffraction pattern. Calculated volume fractions of these oxides in the scale layer were in agreement with the area fractions obtained from scanning electron microscopy (SEM) analysis of the scale.

SURFACE TREATMENT

Communication: Characterization of Titanium Thin Films Prepared by Bias Assisted Magnetron Sputtering
H.S. VIJAYA, G.K. MURALIDHAR, G.N. SUBBANNA, G. MOHAN RAO, and S. MOHAN

MATHEMATICAL MODELING

Modeling of the Peritectic Reaction and Macro-segregation in Casting of Low Carbon Steel
M. EL-BEALY and H. FREDRIKSSON
Macro-microscopic models have been developed to describe the macrosegregation behavior associated with the peritectic reaction of low carbon steel. The macrosegregation model has been established on the basis of previously published work and experimental data. A microscopic model of a three-phase reaction L + has been modeled by using Fredriksson's approach. Four horizontal and unidirectional solidified experimental groups simulating continuous casting have been performed with a low carbon steel containing 0.13 wt pct carbon. The extent of macrosegregation of carbon was determined by wet chemical analysis of millings. It is confirmed, by comparing calculated results with experimental results, that this model successfully predicts the occurrence of macrosegregation. The results indicate that a peritectic reaction which is associated with a high cooling rate generates high thermal contraction and a high tensile strain rate at the peritectic temperature. Therefore, the macrosegregation, particularly at the ingot surface, is very sensitive to the cooling rate, where extremely high positive segregation was observed in the case of a high cooling rate. However, in the case of slow cooling rate, negative segregation was noted. The mechanism of macrosegregation with peritectic reaction is discussed in detail.

A One-Phase Model of the Mixing of Al-SiC Composite Melt
D. KOCAEFE and R.T. BUI
Good mixing of silicon carbide (SiC) particles with liquid aluminum is an important component of the fabrication process of Al-SiC composites and the casting of mechanical parts from composite melts. A one-phase flow model has been built in which the mixture is considered as one fluid, and the SiC flow is differentiated from the main mixture flow by a slip velocity, calculated from the balance of forces exerted on the particles. The impeller blades' action on the fluid is seen as a quadratic source of momentum. Sedimentation is simulated by imposing an increased viscosity on the fluid and setting gravity to zero once the SiC volume fraction reaches a critical value. The model is first applied to a water-SiC system for which some experimental data exist permitting a validation of the model. It is then applied to a Al-SiC system for which some parameter studies are carried out. Compared to the two-phase flow model published earlier, this one-phase model offers two advantages: it takes much less computing time and can accommodate a distribution of particle sizes instead of being limited to one size.

Theoretical Analysis of the Particle Gradient Distribution in Centrifugal Field During Solidification
QINGMIN LIU, YUNING JIAO, YUANSHENG YANG, and ZHUANGQI HU
A theoretical analysis is presented to obtain gradient distribution of particles in centrifugal field, by which the particle distribution in gradient composite can be predicted. Particle movement in liquid is described and gradient distribution of particles in composite is calculated in a centrifugal field during the solidification. The factors which affect the particle distribution and its gradient are discussed in detail. The theoretical analysis indicates that a composite zone and a blank zone exist in gradient composite, which can be controlled to the outside or inside of the tubular composite by the density difference of particle and liquid metal. The comparison of the SiC particle distribution in Al matrix composite produced by centrifugal casting between the theory model and the experiment denotes that the theoretical analysis is reasonable.

Eco-Techno-Economic Synthesis of Process Routes for the Production of Zinc Using Combinatorial Optimization
S. SUDHÖLTER, M.A. REUTER, and J. KRÜGER
The demands placed on the environmental and social acceptability of metallurgical processing technology are rising steadily. Of particular importance are the production techniques, products, and disposal of residues. These aspects are affected by the varying compositions of the primary and secondary raw materials processed in the plants and the rapidly changing market situations in the metallurgical industry. Metallurgical engineers have to select "optimal" processes from a vast number of existing technologies for the primary production of zinc and for the processing of zinc containing residues. To enable the engineer to compare these techniques and to choose the right combination of unit operations, a process design methodology is presented here, which has been adapted from methodologies developed in chemical engineering and minerals processing. In a previous article by the authors, a structural parameter approach was introduced, that implements a synthesis model, which includes all unit operations currently implemented in zinc metallurgy. At the basis of this model is a data base containing the details of the unit operations included in the model. In this article, this methodology is expanded to incorporate an unlimited quantity of different components by introducing the simulated annealing optimization technique to generate optimal flow sheets for the production of zinc under varying constraints which include operation costs, metal prices, environmental costs, and split factors for Zn, Pb, Ag, and Fe. Case studies demonstrate the functionality of this metallurgical tool for the hydrometallurgical recovery of zinc including numerous unit operations for the processing of by-products and residues. It will also be demonstrated how this model can be extended to a "waste management" tool that generates processing routes not only for the residues from the zinc industry but also for zinc containing residues from other processes, e.g., EAF dusts.

Analysis of Shell Thickness Irregularity in Continuously Cast Middle Carbon Steel Slabs Using Mold Thermocouple Data
J.P. SUNI and H. HENEIN
Thermocouples buried in the mold wall of a continuous caster are used to investigate the nature and source of shell thickness irregularity in middle carbon steel slabs. Fourier analysis is used in conjunction with digital filters to determine the power spectra of time series mold temperature and mold level measurements. Direct evidence is obtained on the physical dimension of irregularity, as well as the phase relationships between neighboring thermocouples in both the transverse and longitudinal directions. In addition, mold thermocouple readings are used to set the boundary heat flux conditions for use in self-consistent mathematical modeling of mold thermal profiles. Temperature readings-average, minimum, and maximum-allow for the calculation of an envelope of shell thicknesses around the average distribution. These techniques are used to help explain a mechanism for the occurrence of shell thickness irregularity, in terms of both meniscus disturbances and shell deflections in response to such disturbances.

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