Sponsored by: EPD Process Fundamentals Committee, MSD Thermodynamics &
Phase Equilibria Committee, Japan Institute of Metals
Program Organizers: Prof. R.Y. Lin, University of Cincinnati; Prof. Y. Austin Chang, University of Wisconsin-Madison; Prof. R. Reddy, University of Navada-Reno and Dr. C.T. Liu, Oak Ridge NL
Tuesday, PM Room: B2
February 6, 1996 Location: Anaheim Convention Center
Session Chairpersons: Y. A. Chang. Dept. of Material Science And Engineering,University of Wisconsin-Madison, Madison, WI 53706; Herbert Ipser, Institute of Inorganic Chemistry, University of Vienna, Wahringerstr. 42, A-1090 Wien, Austria
2:30 pm Invited
ON THE SITE REFERENCES OF TERNARY ADDITIONS TO TRIPLE -DEFECT B2 INTERMETALLIC COMPOUND: L. M. Pike, C. R. Kao, S. L. Chen, Y. A. Chang, Dept. of Material Science And Engineering, University of Wisconsin-Madison, Madison, WI, 53706
A knowledge of the site preference of ternary solute additions is essential to developing and understanding of how these solutes affect the properties of B2 intermetallic compounds. A quasi-chemical model will be presented which is able to predict the site preferences of dilute solution additions to triple-defect B2 compounds. The only parameter required is the enthalpy of formation at the stoichiometric composition. General equations are developed which can be used to determine site occupancies and defect concentrations for dilute as well as non dilute solution additions. These equation use atom pair bond enthalpies as the parameters. It is found that the site preferences of dilute additions are not always in agreement with predictions based on the solubility lobes in ternary Gibbs isotherm. Predictions for dilute additions to NiAl, CoAl and FeAl are compared to experimental results found in the literature. Satisfactory correlation is found between the predictions and the experimental results. In addition, the predictions from the model on site occupations and defect concentrations in the (Ni, Fe)Al system are compared to recent experimental results by the authors.
2:55 pm Invited
A MODEL FOR INTERMETALLIC/CERAMIC COMPOSITE MICROSTRUCTURES FROM SOLID STATE DISPLACEMENT REACTIONS: C. H. Henager, Jr., Pacific Northwest Laboratory, Richland, WA 99352; L. N. Brush University of Washington, Dept. of Materials Science and Engr., Seattle, WA 98195
The formation of intermetallic/ceramic composite microstructures from solid state displacement reactions is reviewed and a diffusion/nucleation model for the development and growth of these microstructures is discussed. While a wide variety of complex microstructures are produced from displacement reactions, the most likely microstructure to form is a particulate-type microstructure where one of the phases is continuous and the second phase is discontinuous and dispersed homogeneously within the first phase. The formation of these microstructures can be rationalized by a model that allows interdiffusion and nucleation at moving boundaries to simulate solid state displacement reactions in diffusion couples. The implications of the results for tailoring complex microstructures for various applications, such as ceramic joining will also be addressed. The discussion and model will focus on research in the Mo-Si-C and Ti-Si-C ternary systems. PNL is Operated for the U.S. DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
3:20 pm Invited
PROCESSING/MICROSTRUCTURE CONTROL FOR HIGH STRENGTH, DUCTILITY, AND TOUGHNESS AT ROOM TEMPERATURE IN P/M FeAl: P.J. Maziasz, V. Sikka, D. J. Alexander, A.Goyal, J.L. Wright. Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008 Oak Ridge, TN 37831-6114
A B2-phase FeAl (Fe-38 Al, at%) alloy with minor alloying additions of Mo, Zr, and C was produced as N2 atomized powder, and consolidated by extrusion at 950, 1000 and 1100deg.C. Both the properties and the processing-induced microstructures were quite sensitive through the extrusion conditions. P/M FeAl extruded at 1100deg.C had an equiaxed grain structure (6-30 m grain size): which produced total elongation of 9.2%, YS of 500 Mpa, UTS of 940 Mpa and Charpy impact energy of 20 J for test in air at room temperature. Extrusion at 950[[ring]]C significantly refined the grain size (1-7 um) and improved the ductility to 13 %, YS to 670 Mpa, UTS to 1080 Mpa and the impact toughness to 100 J. More in depth analysis of microstructure and texture will be presented, and effect of microstructural changes on property improvement in FeAl will be discussed.
3:45 pm Invited
COMPUTATIONAL THERMODYNAMICS IN MATERIALS SCIENCE: Hans Jurgen Seifert, Hans Leo Lukas, Gunter Petzow, Max-Planck-Institut f. Metallforschung, Inst. f. Werkstoffwissenschaft, Heisenbergstr. 5, D-70569 Stuttgart, Germany
The CALPHAD method (CALculation of PHAse Diagrams) has been extensively developed in the last 25 years. Thermodynamic descriptions for the functions of state of unary, binary and ternary phases are derived by the optimization of phase diagrams. These functions are stored in computerized thermochemical databases and can be combined to extrapolate to multicomponent systems. Depending on special demand, the actually most advantageous presentations of the calculated results can be chosen: tables or in a clear graphical form as phase diagrams, phase amount diagrams or volatility diagrams. The method will be described by the system Ti-Si-C-N-O including the ceramic subsystem Si3N4-SiC-TiCxN1-x-C-N-O which is most important for the development of ceramic composites. The Wagner-Schottky model expressed by the compound energy formalism proves to be an efficient description for solution phases of this system. The calculations give the thermodynamic stabilities of the condensed phases as function of temperature, concentration and partial pressure of different gas species. Adjusting the sintering conditions to these results the formation of unfavorable silicides or liquid phases can be suppressed.
4:10 pm BREAK
4:20 pm Invited
STATISTICAL THERMODYNAMICS A TOOL FOR UNDERSTANDING POINT DEFECTS IN INTERMETALLIC COMPOUNDS: Herbert Ipser, Regina Krachler, Institute of Inorganic Chemistry, University of Vienna, Wahringerstr, 42, A-1090 Wien, Austria
Statistical-thermodynamic methods are used to interpret the composition dependence of partial thermodynamic properties in non-stoichiometric intermetallic compounds. With these it is possible to derive the defect mechanism responsible for non-stoichiometry, but also to obtain concentrations of individual point defects. Two examples are given: 1) For [[beta]]'-FeAl with the cubic CsCl-structure different experimental values for defect concentrations, determined by different independent methods, are compared with those obtained from a statistical treatment of thermodynamic data. 2) For Rh3Te4 with the hexagonal NiAs-structure defect mechanism and vacancy concentrations are derived from tellurium vapor measurments.
4:45 pm Invited
FORMATION AND STABILITY OF Al3Ti IN THE HYPER-PERITECTIC Al-Ti ALLOYS AFTER MECHANICAL ALLOYING. Dong-Wha Kum, Hye-Sung Kim, Gyeungho Kim and Dong-Su Shur, Korean Institute of Science and Tech., 136-791 P.O. Box 131, Cheongryang, Seoul, Korea
Mechanical alloying is an effective way to finely distribute inert dispersoids in Al-TM(TM is a transition metal element) systems. It is considered that high melting point aluminides are formed by precipitation from supersaturated aluminum matrix. The analysis is based on the fact that much higher content of TM than the solubility limit can be dissolved in alpha aluminum during the high energy ball milling. Nonetheless, the role of undissolved TM particles from mechanically alloyed Al-Ti alloys in the hyper-peritectic region. This study showed that L12-type Al3Ti (a metastable form) formed first during annealing at elevated temperatures. The results indicated undissolved Ti particles of nanosize should have played an important role initiating the formation of Al3Ti phase. Thermal stability of the tri-aluminide phase is also discussed.
DIFFUSION GROWTH OF TITANIUM CARBIDE FILM ON Ni-Ti BINARY SUBSTRATES: S. Lesokhin, L. Levin, Dept. of Mat. Eng., Israel Institute of Technology -Technion, Haifa 32000, Israel
The formation of titanium carbide during hot filament assisted carburizing of Ni-Ti alloys was investigated. Phase content and morphology of the diffusion zone were examined by x-ray diffraction, x-ray microanalysis, SEM and Auger electron spectroscopy. The growth of stable and adherent TiC film was observed. A non-planarity of the interphase boundaries was noticed. The applicability of the chemical diffusion approach to the analysis of the experimentally observed interface instabilities is discussed. A new criterion, taking into account the influence of growth stresses on the stability of the flat interface, is developed by means of linear perturbation analysis. The predictions of the new criterion are compared with the experimental observations.
THERMODYNAMICS OF THE TERNARY SYSTEM Fe-Ni-Cr: Josef Tomiska, Jan Vrestal, Institute of Physical Chemistry, University of Vienna, Waehinger-Strasse 42, A-1090 Vienna, Austria
Thermodynamic investigations on liquid as well as on fcc solid ternary
Fe-Ni-Cr alloys have been performed by means of the computer-aided Knudsen cell
mass spectrometry. The "Digital Intensity-Ratio" -method in combination with an
overall best fit technique has been applied for the thermodynamic evaluation.
Ternary T.A.P.-series (Thermodynamically adapted power series) are used for the
algebraic representation of the thermodynamic excess properties. Based upon
these data, and on the results of mass spectrometric investigations on the
binary boundary systems, the phase diagram of the system Fe-Ni-Cr has been
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