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
Wednesday, PM Room: B2
February 7, 1996 Location: Anaheim Convention Center
Session Chairpersons: P.R. Taylor, Dept. of Met. & Min. Eng., College of Mines, Univ. of Idaho, Moscow, IH 83843; S.G. Kumar, Dept. of Chem. & Met. Eng., Univ. of Nevada, Reno, NV 89557
2:00 pm Invited
PROCESSING OF A Nb3Al-Ti TERNARY ALLOY FROM ELEMENTAL AND PREALLOYED POWDERS: S. G. Kumar, R. G. Reddy, Department of Chemical and Metallurgical Engineering, University of Nevada, Reno, NV 89557
Niobium aluminides offer the advantage of higher high temperature capability compared to other intermetallic aluminides due to the refratory nature of niobium. However the reaction sequence in Nb-Al system is slugish and currently there is a great interst in the processing aspects of these materials. In the present study, a systematic investigation was undertaken to compare the effect of different powder metallurgy processing routes on the phase relations and microstructure of 66Nb-22Al-12Ti (in atom %) alloy. The three different processing routes adapted: (I) cold pressing followed by reaction sintering; (ii) cold pressing and hot pressing followed by sintering; and (iii) arc melting, hydride-dehydride process to make the alloy powder followed by cold pressing and then sintering. The samples were analyzed by X-ray diffraction, optical microscopy, scanning electron microscope with energy dispersive spectrometer and electron microprobe analyzer. The analysis showed that alloyed pellets made from prealloyed powders i.e, arc melting followed by hydride-dehydride process, gave good densification.
2:25 pm
SYNTHESIS OF ZIRCONIUM CARBIDE FROM ZIRCON CONCENTRATES IN A THERMAL PLASMA REACTOR: E. E. Vidal1, M. Manrique, Department of Materials Science, Simon Bolivar University Caracas, Venezuela; Patrick R. Taylor, Department of Metallurgical and Mining Engineering, College of Mines, University of Idaho, Moscow, Idaho 83843
Ultra fine powder of zirconium carbide has been synthesised from zircon concentrate and methane using a thermoplasma reactor. The effect of several experimental variables such as powder used and zircon particle size has bee studied. A complete thermodynamic analysis of possible reactions and stable compounds was performed. A mathematical that describes in-flight decomposition of zircon particles was developed based on a description of the temperature and velocity profiles calculated in the reactor. Based on the results obtained, it is thermodynamically possible to form zirconium carbide from zircon and methane. Complete vaporization of zircon in the reactor is predicted when the particle size is below 45m and the effective power entering the reactor is 15 kW. The powders collected was characterized by XRD, SEM, TEM and chemical analysis using ICP. The results indicated the presence of zirconium carbide, zirconium oxide, silicon carbide and graphite.
2:50 pm
THE PHYSICAL CHEMISTRY OF NUCLEATION OF SUB-MICROMETER NON-OXIDE CERAMIC POWDERS VIA SUB-OXIDE VAPOR PHASE REDUCTION REACTION: Animesh Jha, Department of Material Technology, Brunel University, Kingston Lane, Uxbridge UB8 3PH, England
Fine ceramic powders (< 500 nm) exhibit exceptional physical and mechanical properties in engineered structural ceramics. The production of fine powders, in particular the non-oxide ceramics, via a cheaper route than the organic solvent route has been rather elusive. This paper examinines the physical chemistry of sub-oxide vapour-phase reduction reaction for the nucleation of non-oxide ceramic phase. Well known vapour species eg SiO and BO in the production of technical ceramic powders (SiC, BN) are particularly discussed for understanding the nucleation process of SiC and BN ceramic phases respectively. The regimes of partial pressures and temperatures are parlicularly identified and, the calculaled nucleation rate as a function of the temperature is compared with the experimental results on powder morphology. The produclion of amorphous and nanocrystalline h- BN powders is discussed in the context of substrate structure and thermodynamic parameters.
3:15 pm
KINETIC STUDIES OF REDUCTION OF SOME TRANSITION METAL TUNGSTATES: J. A. Bustness, Du Siehen, S. Seetharaman, Royale Institute of Technology, Division Theoritical Metallurgy, S-100 44 Stockholm, Sweden
The present study deals with the kinetics studies of reduction of some selected transistion metal tungstates with hydrogen. The measurement were carried by the isothermal thermogravimetric method. Shallow powder beds were employed in order to avoid the effects due to transport phenomena affecting the reaction rate. In this way, the chemical reaction rates could be selectively stidied. The experiments were carried out in the temperature range 873-1173K. From the experimental data the activation energies were evaluated and were found to be around 88kJ/mol in the case of both Fe as well as Co-tungstates indicating the similarities in W-O bonds in both cases. The activation energies obtained are discussed in the light of the position of the transient metal in the periodic table as well as the thermodynamic stabilities of the various tungstates.
3:40 pm BREAK
4:00 pm
EVALUATION OF THERMODYNAMICS OF SOLUTES OXYGEN AND SULPHUR IN TERNARY LIQUID ALLOYS USING INTERACTION PARAMTER FORMALISM: Singareddy R. Reddy, Ramana G. Reddy, Department of Chemical and Metallurgical Engineering, University of Nevada, Reno, NV 89557
A thermodynamic formalism for interpreting excess Gibbs energy of a ternary system is used in the present study to interpret activity coefficient of a dilute solute, 2, in a binary solvent 1-3. The formalism is based on Gibbs energy interaction parameters is deduced from the integral function of a ternary system. The derived expression is used to interpret logarithmic activity coefficient of oxygen in seven metallic systems an of sulfur in three metallic systems. The present model calculation of logarithmic activity coefficient of solute are compared with experimental data. Self interaction parameter of the solute is derived from the partial function of solute using constant compositional path. The predicted thermodynamic values are in excellent agreement with the experimental data and also our predictions are far more accurate than the predictions made by other models. The derived equation is used successfully to deduce thermodynamics of self interaction of oxygen as a interstitial atom in two metallic systems.
4:25 pm
MECHANISMS OF REDUCTION - NITRIDATION OF SYNTHETIC KAOLINITE IN THE PRESENCE OF CARBON: Catherine Davey, Animesh Jha, Department of Material Technology, Brunel University, Kingston Lane, Uxbridge, Middlesex UB8 3PH, England
Carbothernic reduction and nitridation reactions using synthetic kaolinite mineral were investigated for the formation of silicon based nitrogen ceramics. The temperature dependence of the reaction rate and its effect on phases forrned are reported. The effect of the reactivity of carbon on the kinetics of formation of AIN and Si3N4, is also discussed. Using the standard Gibbs free energy values for the formation of nitrides and oxides of aluminium and silicon, a log p02 versus l/T diagram is constructed. The experimental results are discussed and compared with the predicted phase equilibria. We also report the influence of sub-oxide gaseous species on the microstructure of the ceramic phases produced.
4:50 pm
AN EVOLUTION METHOD ON DEGREE OF GRAPHITIZATION OF CARBON MATERIALS UNDER HIGH TEMPERATURE AND HIGH PRESSURE: Yuan-Haun Lee, Jonq-Ren Tarng, Ke-Ming Lin, Gin-Haw Chen, Wen-Ku Chang, Institute of Materials Science and Engineering, National Taiwan University, 1 Roosevelt Rd. Sec 4, Taipei, Taiwan
The majority of early studies used X-ray diffraction, but this requires a model to interpret the diffraction processes. A turbostratic model has been applied by Warren, Franklin and Bacon to a wide range of materials including both soft and hard carbons. Maire and Mering applied the same methods to the interpretation of the (00l) lines of a series of graphitic carbons. In our case, graphitic carbons undergone high temperature and high pressure graphitization processes have been evaluated by the same methods. It were found that most technological XRD data were obtained by a Rietveld method with the least-squares refinments could be in the simplest way since the full correction technique is very laborious. The behaviour of materials as it procedes through the graphatization process is characterized by changes in the measured interlayer spacings (d) as a function of temperature. A series of graphitic carbons including the transformation of diamond to graphite through the process under high temperature and high temperature have been evaluated. A simple calibration equation has also been established.
5:15 pm
FLUX ASSISTED DISPERSION OF CERAMIC PHASES IN MOLTEN ALUMINUM ALLOYS: Chris Dometakis, A. Jha, Dept of Matls Tech., Brunel Univ., Uxbridge, Middlesex, UB8 3PH England
Without the use of a flux, only a poor dispersion of ceramic phase can be
achieved. A molten-flux-assisted ceramic phase dispersion technique for
producing aluminum alloy metal-matrix composites has been developed. In this
paper, we report the physical chemistry for the dispersion of TiB2 in molten
aluminum. In particular, aspects of the interfacial energy and the
themodynamics of the dispersion reactions are briefly discussed for enhancing
the volume of dispersion phase in the molten metal and for controlling their
morphology. This investigation also addresses the role of molten flux in the
fabrication of Al-alloy mmc using a casting technique. The morphological
aspects of the dispersion are explained on the basis of the classical theory of
nucleation.
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