METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 29B, No. 3, June 1998

This Month Featuring: The 1997 Edward DeMille Campbell Memorial Lecture; Composite Materials; Hydrometallurgy; Pyrometallurgy; Transport Phenomena; Physical Chemistry; Solid State Reactions; Materials Processing; Mathematical Modeling. View June 1998 Contents.

THE 1997 EDWARD DEMILLE CAMPBELL MEMORIAL LECTURE

Grain Boundary Cracking
PAUL G. SHEWMON
A chronological summary is given of the various types of grain boundary fracture found in metals. In each case, there is an impurity that adsorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphorous on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger reduction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fracture in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (no void nucleation), in which diffusion of solute to the new surface limits crack growth. Numerous examples of this are also discussed.

COMPOSITE MATERIALS

Chemical Reactions between Aluminum and Fly Ash during Synthesis and Reheating of Al-Fly Ash Composite
R.Q. GUO and P.K ROHATGI
Thermodynamic analysis indicates that there is the possibility of chemical reactions between aluminum melt and cenosphere fly ash particles. These particles contain alumina, silica, and iron oxide, which, during solidification processing of aluminum-fly ash composites or during holding of such composites at temperatures above the melting temperature of aluminum, are likely to undergo chemical reduction. These chemical reactions between the fly ash and molten aluminum have been studied by metallographic examination, differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and X-ray analysis after holding the aluminum-fly ash composites for different periods above the liquidus temperature. The experiments indicate that there is progressive reduction of silica and mullite in the fly ash, and formation of alumina with holding time of composites at a temperature of 850°C. The walls of the cenosphere fly ash particles progressively disintegrate into discrete particles as the reaction progresses. The rate of chemical reaction was high at the start of holding the composite at a temperature of 850°C, and then the rate significantly decreased with time. The reaction was almost complete after 10 hours.

HYDROMETALLURGY

Physicochemical and Structural Factors in the Sulfuric Acid Leaching of Nickel- and Copper-Bearing Synthetic Birnessites
RAKESH KUMAR and S. DAS
A large number of nickel- and copper-doped samples of birnessite (0.7 nm phase), a layered-structure manganese mineral, were synthesized by dehydration of respective buserites (1 nm phase). The samples were characterized in terms of chemical composition, specific surface area, phase constituents, crystallinity, strain, morphological features, and structural complexity, in order to study the influence of the physicochemical characteristics of the samples on the leachability of doped elements and manganese in sulfuric acid. In contrast to manganese, the leaching behavior of doped nickel and copper is found to be more sensitive to the structural characteristics of the host birnessite phase. The leachability of the doped elements does not show any correlation with the specific surface area of the samples. Significant parameters affecting leachability are the interlayer spacing of the parent buserite phase used in the synthesis and the microcrystalline dimension and strain in the <001> crystallographic direction of the birnessite phase. In addition, leachability is also controlled by the crystal field stabilization energy (CFSE) of the doped metal ion. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies on leach residues indicated the transformation of birnessite phase into other minerals such as nsutite (-MnO2). A significant fraction of the doped nickel and copper (20 to 40 pct) remains unleached, even after prolonged leaching up to 6 days, and this is attributed to the compact structure of the newly formed phases during leaching.

PYROMETALLURGY

Experimental Study of Phase Equilibria in the Systems PbOx-CaO and PbOx-CaO-SiO2
EVGUENI JAK, NAIGANG LIU, and PETER C. HAYES
The reported experimental work on the systems PbOx-CaO and PbOx-CaO-SiO2 in air is part of a wider research program that combines experimental and thermodynamic computer modeling techniques to characterize zinc/lead industrial slags. Extensive experimental investigation by high-temperature equilibration and quenching techniques followed by electron probe microanalysis was carried out in the temperature range 640°C to 1500°C (913 to 1773 K) and in the composition ranges 0 to 65 mol pct SiO2 and 0 to 42 mol pct CaO. Liquidus and solidus data were reported for most of the primary phase fields. Liquidus surfaces in the systems CaO-Pb-O and PbOx-CaO-SiO2 in air were completely reconstructed. Extensive solid solutions of PbO in ' dicalcium silicate and Ca2Pb3Si3O11 were measured.

Tellurium Distribution in Copper Anode Slimes Smelting
D.R. SWINBOURNE, G.G. BARBANTE, and A. SHEERAN
Tellurium is a common minor constituent of copper anode slimes. The distribution of tellurium between the phases during slimes smelting is an important consideration, both in terms of metal quality and the capture of the oxidized tellurium. In this work, the oxidation by oxygen at 1100°C of a silver-copper selenide matte containing 2 pct tellurium has been examined. The distribution of tellurium between the phases was determined as the extent of oxidation increased, and the system was modeled using a computational thermodynamics package. Oxidized tellurium was found to report to the slag, with none being removed with the gas. The thermodynamic model predicted, to an acceptable level, the tellurium content of all phases as oxidation progressed. It was used to show that oxidation by air rather than oxygen results in higher residual tellurium levels in silver metal and that the lower the smelting temperature, the greater the extent of tellurium elimination from silver to the slag.

TRANSPORT PHENOMENA

Liquid-Solid Mass Transfer from a Wall in Contact with a Gas/Liquid Interface Undergoing Wave Motion
Z.D. CHEN and J.J.J. CHEN
Experimental data are presented for mass transfer from a discrete area on a vertical wall near an air/water interface undergoing wave motion. The local mass transfer rate near the interface was found to be well correlated by Sh = Re0.51/3, which is much higher than that for a steady unidirectional flow over a flat plate with the same mean velocity considering the contact time between the wall and the liquid. Visualization studies showed that the high local mass transfer rate was related to the periodic renewal of the boundary layer near the wall and the extremely high initial mass transfer rate during the establishment of a new concentration field.

Mixing Time and Fluid Flow Phenomena in Liquids of Varying Kinematic Viscosities Agitated by Bottom Gas Injection
MANABU IGUCHI, KEI-ICHI NAKAMURA and RYOJI TSUJINO
A model experiment was carried out to investigate the mixing condition and related fluid flow phenomena in a slag layer of metal-refining processes agitated by bottom gas injection. Silicone oil was used as a model for the molten slag. Mixing time in a silicone oil bath was measured with a newly developed laser optical sensor. Measured mixing time values increased with an increase in the kinematic viscosity of the silicone oil. In order to explain the relation between mixing time and the kinematic viscosity of silicone oil, the rising velocity of bubbles and the vertical and horizontal velocities of silicone oil flow were measured with an electroresistivity probe and a laser Doppler velocimeter, respectively. The increase in the mixing time with the kinematic viscosity of silicone oil was caused mainly by the suppression of upward motion of bubbles and silicone oil in the bubbling jet region. An empirical correlation for the mixing time was derived as a function of the kinematic viscosity of silicone oil, in addition to conventionally used parameters such as the gas flow rate, bath diameter, and bath depth.

PHYSICAL CHEMISTRY

A Thermodynamic Study of Ru-Sn Binary Alloys
RYO KAWABATA, MUNETAKA MYO-CHIN, and MASANORI IWASE
An electrochemical technique incorporating stabilized-zirconia as the solid electrolyte and a Mo + MoO2 mixture as the reference electrode has been applied for the determination of the activities of Sn in {Ru-Sn} alloys. At 1573 K and XSn > 0.60, the activities of Sn were very close to the ideal behavior. Nevertheless, the heat of mixing showed relatively large negative values.

Effects of CaO, Al2O3, and MgO Additions on the Copper Solubility, Ferric/Ferrous Ratio, and Minor-Element Behavior of Iron-Silicate Slags
HANG GOO KIM and H.Y. SOHN
The effects of CaO, Al2O3, and MgO additions, singly or in combination, on the copper solubility, the Fe3+/Fe2+ ratio in slag, and on the minor-element behavior of silica-saturated iron silicate slags were examined at 1250°C and a p02 of 10-12 to 10-6 atm. The results indicated that copper solubility in slag was lowered with the addition of CaO, MgO, and Al2O3, in decreasing order. The Fe3+/Fe2+ ratio in the slag decreased with the additions, but this effect was smaller at lower oxygen potentials. The presence of small amounts (about 4 pct) of CaO, Al2O3, and MgO in the slag resulted in increased absorption of Bi and Sb into molten copper, but had a smaller effect at large additions (about 8 to 11 pct). The distribution behavior of Pb was a function of oxygen partial pressure, which indicates the oxidic dissolution of Pb in the slag as PbO, while the behavior of Bi, Sb, and As was found to be independent of oxygen potential, supporting the atomic (neutral) dissolution hypothesis of these elements in the slag. The distribution behavior of Pb and As was not significantly affected by the additions. The activity coefficients of Bi and Sb in the slags were determined to be as follows: (1) for no addition, Bi = 40 and Sb = 0.4; (2) for small additions (about 4.4 pct), Bi = 70 to 85 and Sb = 0.8; and (3) for large additions (about 8 to 11 pct), Bi = 60 to 75 and Sb = 0.5 to 0.7.

Thermodynamic Modeling of Liquid Fe-Ni-Cu-Co-S Mattes
FLORIAN KONGOLI, YVES DESSUREAULT, and ARTHUR D. PELTON
The modified quasichemical model for short-range ordering is described for liquid metal-sulfur solutions. Available thermodynamic data for molten Fe-S, Ni-S, Cu-S, and Co-S solutions are collected, critically evaluated, and optimized by means of the model. Very good descriptions of the thermodynamic properties are obtained with few parameters. Using only these binary parameters, the model predicts the thermodynamic properties of Fe-Ni-S, Fe-Cu-S, Ni-Cu-S, and Fe-Ni-Cu-S mattes over a wide range of composition and temperature within experimental error limits.

SOLID STATE REACTIONS

Investigation of Mechanochemical Reactions Between Zircon (ZrSiO4) and Alkaline Earth Metal Oxides
N.J. WELHAM
Alkaline earth metal oxides have been mechanically milled with zircon for up to 200 hours with the aim of forming zirconia or zirconates, either within the mill or during subsequent annealing at up to 1200°C. At a 1:1 molar ratio, all samples reacted during milling, forming zirconia and a metal silicate. Increased fractions of metal oxides resulted in reaction of the zircon to zirconates and metal silicates, except for magnesium, which formed zirconia and magnesium silicate. The zirconia formed was the metastable tetragonal form, which reverted to the low-temperature monoclinic form after heating to 1200°C. The powders milled for 200 hours were leached for 6 hours at 25°C in 18 pct HCl; their solubility was very high, ranging from 88 to 92 pct for the lower oxide ratio (except for magnesium) and >95 pct for all powders of the higher oxide ratio; the only remaining phase after leaching was zircon. Milling zircon in the absence of metal oxide showed a solubility of 9 pct, confirming that reaction occurred during milling.

MATERIALS PROCESSING

Modeling the Microstructural Changes during Hot Tandem Rolling of AA5XXX Aluminum Alloys: Part I. Microstructural Evolution
M.A. WELLS, D.J. LLOYD, I.V. SAMARASEKERA, J.K. BRIMACOMBE, and E.B. HAWBOLT
A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow, plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400°C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling processes, i.e., deformation temperatures of 350°C to 500°C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where Z = exp and time at the recrystallization temperature.

Modeling the Microstructural Changes during Hot Tandem Rolling of AA5XXX Aluminum Alloys: Part II. Textural Evolution
M.A. WELLS, D.J. LLOYD, I.V. SAMARASEKERA, J.K. BRIMACOMBE, and E.B. HAWBOLT
In Part II of this article, the experimental work undertaken to measure the effect of deformation parameters (temperature, strain, and strain rate) on the texture formation during hot deformation and the evolution during subsequent recrystallization is described. In addition, the isothermal kinetics of development of individual texture components were also determined. A neutron diffractometer was used to measure the texture in the as-hot-deformed aluminum samples, and the samples were then heat treated in a 400°C salt bath for various lengths of time, with the texture being remeasured at various stages in the recrystallization process. Using data from the experimental program, the texture evolution during recrystallization was modeled by applying a modified form of the Avrami equation. Results indicated that, of the deformation parameters studied, textural development was most sensitive to the deformation temperature for both alloys. In addition, modeling results revealed that the Cu component ({112}<111>) was the first to recrystallize, typically followed by the S ({123}<634>) and Bs ({110}<112>) components. This is in agreement with earlier work which indicated that the Bs component was the hardest to recrystallize, possibly because it is able to deform on very few slip systems and, hence, the dislocation interaction may be low.

The Mechanism of Formation of TiB2 Particulates Prepared by In Situ Reaction in Molten Aluminum
B. YANG, Y.Q. WANG, and B.L. ZHOU
By making use of a novel technique in which TiB2 particulates are fabricated by an in situ reaction in molten aluminum, we have successfully produced TiB2/Al composites. In order to reveal the characteristic of the technique, the mechanism of formation of TiB2 particulates obtained by this method is studied in this article. Both theoretical and experimental results have shown that the TiB2 particulates are formed by a diffusion mechanism when the molar fraction of aluminum in the preform is higher than 43.5 pct. In this case, the TiB2 particulates are generally spherical, and their mean size is less than 2.0 µm. On the contrary, the TiB2 particulates are formed by a solution-precipitation mechanism when the fraction is lower than 43.5 pct. In this case, the TiB2 particulates are multifaceted, and the size of most particulates lies between 3.0 and 6.0 µm.

MATHEMATICAL MODELING

Investigation of Inclusion Re-Entrainment from the Steel-Slag Interface
D. BOURIS and G. BERGELES
The mechanism of inclusion elimination from continuous steel casting is investigated at the steel-slag interface. Inclusion impact at the interface is considered under the concept of energy balance, with buoyancy forces, fluid dynamic forces, interfacial adhesion, and rebound forces determining whether the particle will pass through the interface or be retained by it. The effects of the inclusion, slag, and steel properties, as well as the effect of inclusion impact velocity, are considered at the interface. The interfacial tension between the slag and the inclusion should be smaller than that between the steel and the inclusion (negative wettability), so that the inclusions can pass into the slag layer and avoid re-entrainment. The inclusion particles that reach an equilibrium state at the steel-slag interface are subject to re-entrainment back into the steel, due to lift forces applied to them by the turbulent boundary layer at the interface. A removal criterion dependent upon the shear stress is introduced, and then the removal rates are calculated from the turbulent burst theory. It is found that the smaller diameter inclusions are trapped at the interface. Of the particles that remain at the interface, it is the larger ones that are more easily removed by the lift forces due to the turbulent shear stress. High slag viscosity is desirable, since it makes inclusion re-entrainment into the casting product more difficult.

Model for Temperature Profile Estimation in the Refractory of a Metallurgical Ladle
T.P. FREDMAN and H. SAXÉN
Modeling of the transient thermal state of metallurgical ladles is motivated by the need for estimating the drop in temperature of the liquid metal in the ladle. On-line estimation of the state is required, since the same ladle is used in a number of casting cycles with rapid changes in boundary conditions for the temperature field, and the conditions in the current as well as previous cycles affect the thermal state. Although a large number of methods for the numerical solution of conduction-diffusion partial differential equations have been developed, there are still advantages to keeping thermal field computations at a relatively simple level, in contrast to the situation in the design process of ladles, where two-dimensional modeling may be required. Extensive computations under nonverifiable boundary and initial parameter values are not especially suited for real-time simulation of industrial processes. This article presents a novel approach to the solution of the one-dimensional transient heat conduction problem applied to ladle linings, relying on classical analytical techniques in combination with numerical methods. The performance of the model was validated by a comparison of predictions to thermocouple measurements from the refractory of a steelmaking ladle during a campaign of 26 casting cycles. Reasonable agreement between the measured and simulated variables could be established, which demonstrates the feasibility of the approach.

A Computational Model for the Prediction of Steel Hardenability
M. VICTOR LI, DAVID V. NIEBUHR, LEMMY L. MEEKISHO, and DAVID G. ATTERIDGE
A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy, et al.

Constrained Sintering of a Sphere with Radial Density Gradient: Viscous Model
JIE LUO, YUHE REN, and R. STEVENS
The viscous model has been used to investigate the constrained sintering of a sphere with a radial density gradient. The governing partial differential equations (PDEs) for the sintering process have been derived. The differential-algebraic equation method has been applied to solve the PDEs numerically. The temperature gradient generated on heating during sintering is one of the reasons for the presence of a density gradient in the fired ceramic; this phenomenon occurs in sintering large fine ceramic components or in parts fabricated using a fast-firing technique. The temperature gradient generated during heatup is also incorporated into an analysis of the constrained sintering.

Fluid Flow in Casting Rigging Systems: Modeling, Validation, and Optimal Design
ROBERT M. McDAVID and JONATHAN A. DANTZIG
In this work, a methodology for the optimal design of flow in foundry casting rigging systems is discussed. The methodology is based on a novel, fully analytical design sensitivity formulation for transient, turbulent, free-surface flows. The filling stage of the casting process is modeled by solving the time-averaged form of the Navier-Stokes equations via a turbulent mixing-length model, in conjunction with the volume-of-fluid (VOF) method for modeling the free surface. The design of the runner and gating system of a simple block casting is presented as an example application for using sensitivity information in design. The analytical sensitivity routine is coupled to a numerical optimizer to yield an automated method for optimal design of the casting rigging system. The finite element model of the filling process is verified through physical experimentation. Solutions of glycerol and water are used to perform laminar and turbulent flow filling experiments, which are used to verify independently the VOF algorithm and the adequacy of the mixing-length model. The results show that the numerical model accurately reproduces the main features of the flow, and filling times agree with acceptable accuracy. Finally, the analytical techniques are used to obtain an optimal runner shape, which eliminates air entrainment, and the design is verified experimentally.

Simplified Simulation of the Transient Behavior of Temperatures in the Upper Shaft of the Blast Furnace
HENRIK SAXÉN
A one-dimensional dynamic model of the upper part of the blast furnace shaft is applied to study the evolution of gas and burden temperatures, mainly in order to shed light on the transient phenomena after charging dumps of burden. The effects of irregularities in the burden descent and charging are also studied briefly. The simulations demonstrate that the temperatures of the burden layers in the lower part of the simulated region assume a quasi-steady state, indicating that the changes in the top gas temperature experienced immediately after a dump of burden arise primarily because of heat transfer between the gas and the dump. These results support the idea that such temporary changes can be interpreted in terms of distribution of the dumps on the burden surface.

A Three-Dimensional Model of the Spray Forming Method
HYUN-KWANG SEOK, DONG-HUN YEO, KYU HWAN OH, HO-IN LEE, and HYUNG YONG RA
A three-dimensional model has been formulated to calculate the shape of the general preform, using vector calculus. The shape of a rod, tube, plate, or irregular preform can be calculated at given spray forming conditions. The shape of a spray-formed rod was analyzed at various spray forming conditions using the three-dimensional model. The effects of spray forming parameters, such as spray distribution parameters, angular velocity of rotation, withdrawal velocity, spray angle, and eccentric distance on rod shape, were analyzed. The most important parameters affecting the shape of rods are the spray distribution parameters and the withdrawal velocity. The dynamic evolution of rod shape with a stepwise variation of the withdrawal velocity during spray forming was investigated. The effect of a stepwise change of the withdrawal velocity was the same as that of the scanning atomizer. The calculated surface profiles were compared with those of spray-formed 7075 aluminum alloy rods prepared on a pilot scale. The calculated results for the surface profiles were in agreement with those of the spray-formed rods.

Modeling the Microstructural Changes during Hot Tandem Rolling of AA5XXX Aluminum Alloys: Part III. Overall Model Development and Validation
M.A. WELLS, D.J. LLOYD, I.V. SAMARASEKERA, J.K. BRIMACOMBE, and E.B. HAWBOLT
Part III of this article presents the overall mathematical development for the microstructural and textural evolution during industrial hot tandem rolling of AA5182 and AA5052 alloys and validation of the mathematical model, by comparison to both industrial data and information from the literature. The model consists of a plasticity module to simulate the temperature and deformation in the roll bite and an interstand module to characterize the changing microstructure, texture, and temperature in the strip between the rolling stands. The plasticity module was developed using a commercial finite-element package, DEFORM, a two-dimensional transient Lagrangian model which couples the thermal and deformation phenomena and is able to predict the temperature, strain rate, and strain distribution in the strip at any position in the roll bite. The interstand module incorporates semiempirical equations, developed in this study, which quantify the microstructural (percent recrystallization and recrystallized grain size) and textural changes in the strip between the rolling stands. The interstand model also includes a temperature module to predict the through-thickness temperature distribution in the strip based on one-dimensional heat conduction. Validation of the model against industrial data indicated that it gave reasonable predictions for the temperature, grain size, and volume fraction of some of the deformation texture components after recrystallization was completed. However, the model overestimated the mill loads in the last stands for both the AA5182 and AA5052 alloys and underestimated the amount of cube ({100}<uvw>) and S ({123}<634>) texture in the recrystallized strip.

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