Program Organizers: Dr.Robert A.Schiffman, R.S. Research Inc., Barton, VT 05822; Professor Carlo Patuelli, Universita di Bologna, I-40126 Bologna, Italy
Tuesday, AM Room: Orange County 2
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: Dr. R.W. Smith, Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada, K7L 3N6
COMPOSITION DEPENDENCE OF THE SURFACE TENSION OF VANADIUM-TITANIUM ALLOYS NEAR LIQUIDUS TEMPERATURE: S. Bannerjee, A.B. Hmelo, T.G. Wang, Department of Applied and Engineering Sciences, Box 1593-B, Vanderbilt University, Nashville, TN 37235
We report the surface tension of V-Ti alloys as a function of temperature and composition. We've gathered high resolution data using the oscillating drop technique by simultaneously viewing the drop from three orthogonal directions, over a temperature range of 150-200 C. above the liquidus down to the undercooled regime. The composition dependence of the surface tension of V-Ti deviates from ideality. The d gamma/dT values of the alloys imply some surface adsorption effects at and below the melting temperature, as corroborated by surface composition calculations based on the d gamma/dC values we've obtained. There is clear evidence of clustering of like-atoms in the region of the congruent melting composition. Both surface tension and undercooling data support this conclusion. As expected, the clustering effect is dominant at temperatures below the liquidus. The surface composition becomes approximately equal to the bulk composition at superheated temperatures. The congruent melting composition appears to be V-60 wt.%Ti rather than at V-68 wt.%Ti as has been reported earlier.
DYNAMIC REDOX REACTIONS AND THE FORMATION OF FINE-GRAINED, POLYCRYSTALLINE OXIDE/SILICATE CERAMICS FROM INVISCID MELTS: Reid F. Cooper, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706
The dynamics of oxidation or reduction reactions in multicomponent, transition-metal-cation-bearing oxides involves the coupled diffusion of electron holes (polarons) and component cations. As a consequence, internal reactions, resulting in the nucleation/crystallization at a reaction front of higher-order oxides or the more-noble metal component, respectively, dominate dynamic behavior. In the amorphous state, such reactions result in homogeneous nucleation of this product phase, suggesting an approach to the preparation of fine-, uniform-grained ceramics directly from inviscid melts: the redox-reaction-front metal or oxide phase can act as a dispersed heterogeneity for nucleation of other, "majority" phases. This approach, for which microgravity containerless processing is ultimately required, will be illustrated with results from Fe2+ bearing aluminosilicate melts and glasses. For example, oxidation of the melt at 1400 C. results in "isothermal undercooling": the liquidus temperature and primary phase are a function of Activity of O2 and thus less than or equal to 1 micron magnetite (Fe3O4) forms at the reaction front.
PARTICLE REDISTRIBUTION DURING REMELTING OF Al/SiCp COMPOSITES AN EXPERIMENTAL AND COMPUTATIONAL STUDY: B.K. Dhindaw, P. Ganguly, F. Juretzko, D.M. Stefanescu, Department of Metallurgical and Materials Engineering, The University of Alabama, Box 870202, Tuscaloosa, AL 35487-0202
Low gravity processing of particulate metal matrix composite samples includes two distinct phenomena: (a) the remelting of the originally prepared sample when the fluid flow within the melt plays a dominant role in the particle redistribution and (b) the solidification stage when the pushing of the particles by the solidification front is the determining factor. The effects of these two sub-processes on the ceramic phase distribution in the final composite microstructure are distinctly and relatively independent of each other. The present work aims at the analysis of the effects of the melt fluid flow occurring during the melting of the cylindrical composites specimens and comparison of predictions with experiments. The gravity driven buoyancy and the surface tension driven Marangoni flows have been assumed to be the dominant flows. The model analyzes the flow patterns at three gravity levels viz., 1g, 0.1g and 0.01g. The model has been validated using Al-2/3% SiCp composites specimens.
GRAVITATIONAL EFFECTS IN LIQUID PHASE SINTERING: R.M. German, 118 Research West, Penn State University, University Park, PA 16802-6809
Liquid phase sintering is applied to high solid contents, since rigidity is needed to retain compact shape during sintering. Experiments have been performed under microgravity to determine the difference in microstructures due to gravitational compression of contacting grains. The conditions present during microgravity processing allow liquid phase sintering over a wider range of liquid-solid ratios. A widely held assumption is that solid grains will remain dispersed in the absence of gravitational forces. This paper shows the action of a weak agglomeration force couples with Brownian motion causes solid grain agglomeration, but that the rate of grain coarsening is actually slower under microgravity conditions for all solid contents. One consequence of this work is a clear demonstration of a coalescence contribution to grain coarsening during liquid phase sintering. Other unique aspects of the microgravity experiments will be described, including shape distortion and pore growth.
9:50 am BREAK
SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF POROUS CERAMICS IN MICROGRAVITY: T. Mattor, J. Moore, Colorado Center for Advanced Ceramics, Colorado School of Mines, Golden, CO 80401
Porosity formation and distribution in boron carbide - alumina foamed ceramics is studied in a microgravity environment. Porous boron carbide - alumina is created through a self-propagating high-temperature synthesis (SHS) process in an inert environment. The variables under investigation are: reaction chamber pressure, additions of excess B2O3-SiO2 mixtures, additions of excess aluminum, form of carbon (graphite or lampblack), green pellet diameter, acoustic levitation level, and gravitation level. Each variable is studied independently under a variety of reaction chamber pressures. Porosity and distribution is established as a function of the above variables, and the underlying mechanism is explained.
CONVECTIVE FORCE EFFECTS ON THE CRITICAL VELOCITY OF ENGULFMENT OF PARTICLES BY A PLANAR SOLID/LIQUID INTERFACE: S. Sen, USRA, MSFC, Huntsville, AL 35812. H. Pang, D.M. Stefanescu, B.K. Dhindaw, Solidification Laboratory, The University of Alabama, Tuscaloosa, AL 35487. P.A. Curreri, NASA, MSFC, Huntsville, AL 35812
Directional solidification experiments were performed on mixture of transparent succinonitrile and spherical polystyrene particles. Ground based experiments performed using a thermal gradient stage indicated a decrease in critical velocity of engulfment with increase in particle size between 1-10 microns. Further, for a given particle size there was a variation in critical velocity with variations in experimental cell thickness. Results from low-gravity experiments performed during aircraft parabolic maneuvers will be discussed. The effect of convective forces on the critical velocity for particle engulfment will be quantified with respect to several theoretical models describing particle pushing and engulfment.
CRYSTALLIZATION STUDIES ON HEAVY METAL FLUORIDE GLASSES USING PARABOLIC FLIGHT AIRCRAFTS AND SOUNDING ROCKETS: S. Varma, S.E. Prasad, Sensor Technology Limited, B.M. Hi-Tech Div., P.O. Box 97, Collingwood, Ontario, Canada L9Y 3Z4; A. Ahmad, T.A. Wheat, MTL/CANMET, Natural Resources Canada, 405 Rochester Street, Ottawa, Ontario, Canada K1A 0G1
The predicted low losses in heavy metal fluoride glass (HMF) fibres have not yet been realized in spite of over 20 years of world wide research. The main reason is believed to be excess scattering due to homogeneous and heterogeneous nucleation during glass synthesis and fibre drawing. The primary reason for heterogeneous nucleation is container contamination, while gravity driven density segregation could trigger homogeneous nucleation. Although containerless glass synthesis on the ground could minimize heterogeneous nucleation, homogeneous nucleation could still cause excess scattering. Heat treatment of these glasses for fibre drawing can cause further nucleation and optical degradation due to density segregation. A microgravity environment could, therefore, reduce homogeneous nucleation and optical degradation during fibre drawing. This was examined with the help of ground based experiments, and microgravity experiments aboard the T-33 aircraft and the CSAR-I and CSAR-II sounding rockets. The results of these experiments, which indicate that microgravity helps in reducing crystallization and optical degradation in these glasses during their heat treatment for fibre drawing, are presented in this paper.
EFFECTS OF COMPOSITION ON THE PORE BEHAVIORS IN MICROGRAVITY PROCESSED FE-CU COMPACTS: Z. Xue, A.K. Kuruvilla, J.E. Smith, Jr., The Department of Chemical and Materials Engineering, The University of Alabama in Huntsville, Huntsville, AL 35899
Several liquid phase sintered samples from the Fe-Cu, Co-Cu and W-Ni-Cu
systems have been processed aboard STS-57, STS-60 and STS-63. Processing times
range from 75 seconds to over 66 minutes for samples with aspect ratios of 0.5
to 1.0. Samples processed under 12 psia of argon did not exhibit slumping but
did show shape distortion as a function of the amount of liquid present during
the melt. Extensive pore formation was observed for the microgravity processed
samples that underwent either pore metamorphosis to smaller pores or
coalescence to large pores as a result of instabilities within the compacts.
The particular type pore behavior was found to depend on the amount of liquid
present during the melt. Microgravity results from Fe-Cu system with the Cu
concentration of 33, 43 and 53 weight percent will be presented to show the
various behaviors. The results demonstrate the use of microgravity to study
pore metamorphosis within systems that undergo large scale sedimentation
effects during unit gravity processing.
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