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Session Chairpersons: Dr. S. Krishnamurthy, UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432; Prof. Carlos G. Levi, Materials Department, University of California, Santa Barbara, CA 93106
2:00 pm INVITED
SYNTHESIS AND PROCESSING OF CERAMICS, INTERMETALLICS, AND COMPOSITES BY FIELD-ACTIVATED COMBUSTION SYNTHESIS: Zuhair A. Munir, Division of Materials Science & Engineering, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294
Abstract not available.
2:30 pm
SYNTHESIS OF IN-SITU TiAl-BASED COMPOSITES FROM ELEMENTAL POWDERS: D.E. Alman, J.A. Hawk, U.S. Department of Energy, Albany Research Center, Albany, Oregon 97321
Alloys and composites based on the intermetallic compound TiAl are emerging as an important class of light-weight, high-temperature structural materials. Recently, it has been recognized that these alloys have applications in industries, such as the automotive industry, where cost is frequently a major concern in materials selection. However, for these alloys to be used in this type of application, new low cost methods for high volume component fabrication are required. One potential fabrication approach is reactive synthesis (also termed combustion synthesis). This technique involves initiating an self-propagating, high-temperature synthesis (SHS) reaction within an intimate mixture of elemental powders. This process has been used to fabricate intermetallics, ceramics and in-situ composites in the form of powders and dense monoliths. SHS reactions tend to initiate at low homologous temperatures of the forming compound (for aluminides near or at the melting point of Al, 660°C), and tend to go to completion in a short period of time (i.e., a few seconds). For some compounds, particularly aluminides, the reaction is ac companied by the formation of transient liquid phases. These factors can reduce the required processing parameters (time, temperature and pressure) needed to produce dense products by reactive synthesis techniques compared to conventional powder metallurgical approaches. This paper characterizes the reactions that occur and resultant microstructures of TiAl based composites fabricated from ternary mixtures of elemental Ti, Al and B or Si powders. Mixtures of the elemental powders were prepared corresponding to TiAl reinforced with 0, 10 25, 60 and 100 vol. pct. Ti5Si3 or TiB2. The powders were consolidated by reactive hot-pressing (at 1000°C and 20 MPa for 1 hr). It was found that the composites produced from Ti, Al and Si powders were dense, and the elemental powders transformed to the target phases of TiAl and Ti5Si3. Whereas, composites produced from the Ti, Al and B powders were porous and inhomogeneous, that is several aluminide (TiAl, Ti3Al and TiAl3) and boride phases (TiB2, AlB12, TiB) formed during hot-pressing. The different behavior observed by the two ternary systems can be attributed to both reaction sequence and phase diagram considerations. First, Differential Thermal Analysis (DTA) revealed that an endothermic reaction associated with the formation of Al-Si eutectic occurs prior to the initiation of an SHS reaction within the mixtures of Ti, Al and Si powders. No such pre-reaction melting occurred within the mixtures of Ti, Al, and B powders. Thus, the "extra" transient liquid phase that formed during the reaction between Ti, Al and Si systems enhances diffusion (hence homogenization) and densification within this system during reaction processing. Also, an examination of phase diagrams reveals that there exists no Al-Si compounds to compete with the formation of titanium-aluminide and titanium-silicides during reactions between Ti, Al and Si powders. However, there are several aluminum-boride phases that can compete with the formation of titanium-aluminide and titanium-boride during reactions between Ti, Al and B powders. The implications of this study is that TiAl-based composites can be designed for densification during reactive processing.
2:55 pm
REACTIVE SYNTHESIS OF NiAl-Nb COMPOSITE FROM ELEMENTAL POWDERS: L. Farber, A. Lawley, I. Gotman, Department of Materials Engineering, Drexel University, Philadelphia, PA 19104; I. Gotman, E. Y. Gutmanas, Department of Materials Engineering, Technion, Haifa 32000, Israel
A NiAl matrix composite reinforced with Nb particles was synthesized in the solid state from blends of ultrafine elemental Ni, Al and Nb powders. The fabrication method involved consolidation of elemental powder blends to full density followed by heat treatment. The maximum processing temperature did not exceed 800°C. Kinetics and the sequence of phases formation during synthesis were investigated. For Ni-Al-Nb blends, consumption of Al with the formation of Ni-Al intermetallic phases only was detected in the temperature range 425°C-550°C. Subsequent heat treatment at 800°C resulted in rapid completion of the synthesis reaction with the formation of the NiAl matrix. No reaction occurred between Nb particles and the matrix during synthesis. The phase stability of the composite in the 800°C-1100°C temperature range was investigated. Mechanical properties of the synthesized material are discussed in the context of resulting microstructure.
3:20 pm
FeAl-TiC AND FeAl-WC COMPOSITES - MICROSTRUCTURE AND MECHANICAL PROPERTIES: R. Subramanian,, J.H. Schneibel, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115
For applications of TiC- and WC-based cermets in corrosive environments, a potential binder material is an intermetallic, iron aluminide. In this investigation, it is shown that iron aluminide (Fe-40 at %Al) bonded TiC and WC composites can be processed to almost full density (> 99%) with carbide volume fractions ranging from 0.3 to 0.85 by conventional liquid phase sintering and pressureless melt infiltration techniques. The melt infiltration process was successful in the fabrication of composites with carbide volume fractions greater than 0.7 and important aspects of this technique will be discussed. Mechanical property data such as bend strength, hardness and fracture toughness will be presented and interpreted in terms of the composite microstructures. For FeAl-WC composites containing 60 vol.% WC, room temperature three-point bend strengths and fracture toughness values reached 1680 MPa and 20 MPa.m1/2, respectively. Consistent with the high fracture toughnesses, the fracture surfaces showed evidence of ductile deformation of the FeAl binder. Research sponsored by the Laboratory Directed Research and Development Program of the Oak Ridge National Laboratory, and by the Division of Materials Sciences, U.S. Department of Energy, under Contract No. DE-AC05-960R22464 with Lockheed Martin Energy Research Corporation, Inc. This research was also supported in part by an appointment to the ORNL Post-Doctoral Research Associates Program administered jointly by the ORISE and ORNL.
3:45 pm BREAK
3:55 pm
DEOXYGEN IN SILICIDE FORMATION: Chi-Fung Lo, Darryl Draper, Materials Research Corporation, Orangeburg, NY 10962
A preliminary study on the deoxygen behavior of tungsten-, molybdenum- and tantalum-silicide formations using powder technique was performed. During the synthesis under vacuum, the transformation of amorphous to crystalline silicon and the formation of silicides were monitored by X-ray diffraction. The oxygen content in the materials at various phase-transformation stages was measured. The results indicated that, independent of the synthesized metals, no significant change in the oxygen content was found until the formation of metal-disilicides. Via the formation of disilicides, the oxygen decreased from 1000-3000 ppm to less than 500 ppm. In this study, the exothermic behavior of silicon phase transformation and the silicide formations was also investigated.
4:20 pm
TITANIUM/TITANIUM CARBIDE COMPOSITE FORMATION BY GAS-SOLID IN-SITU REACTION: Yong Jin Kim, Hyungsik Chung, Department of Materials Processing, Korea Institute of Machinery and Materials, 66 Sangnam Dong, Changwon, Kyungnam 641-010, S-Korea; Suk-Joong L. Kang, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-Dong, Yusong-Gu, Taejon 305-701, S-Korea
Sponge titanium powder was die compacted and reacted with carbonaceous (CH4) gas at the temperature range of 700-1,000°C. Layered TiC film was formed uniformly on the surface of the powders in the green compact. The thickness of the TiC layer varied with the reaction temperature and time. The reacted compacts were sintered in a vacuum up to 1450°C. During the sintering, the TiC layer in the power surface was broken into small fragments and the fragment changed gradually into round shaped particles with increasing the sintering temperature. The relative sintered density over 94% was obtained at the sintering temperature of 1350°C for 2hrs. Ti/TiC composite containing up to 50 v/o of TiC was successfully made by the in-situ reaction and sintering. The volume of TiC in the sintered body mainly depends on the reaction temperature, time and Ti powder size. But the gas flow rate during the reaction affected little to TiC volume in the sintered composite.
4:45 pm
PROCESS-STRUCTURE RELATIONSHIPS FOR TAPE CASTING OF CONTINUOUS FIBER-REINFORCED MMC'S: Shin Yu and Dana M. Elzey, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903
The tape casting process offers a potentially cost-effective manufacturing route for continuos fiber-reinforced metal matrix composites. However, the ultimate performance is limited by the presence of microstructural defects, which evolve to an extent which depends sensitively on the constituent materials and processing conditions used. Results of an experimental study are reported in which the evolution of several important microstructural defects have been investigated for various processing conditions. These observations have been used as a basis for the development of predictive process-structure models for the tape casting of MMC's. The models may be used to explore processibility and cost issues for hypothetical matrix/fiber composite systems and processing conditions.
5:10 pm
FIBER FRAGMENTATION DURING PROCESSING OF METALLIC MATRIX COMPOSITES: Nicole M. Gorey, Donald A. Koss, John R. Hellmann, Department of Materials Science and Engineering, Penn State University, University Park, PA 16802
Fiber fragmentation can be a serious problem during the processing of metallic matrix composites. This research focuses on the fracture of continuous sapphire fibers during composite consolidation. During the latter stages of consolidation, matrix flow along the fibers may cause fiber fracture even in the absence of fiber bending. Fiber fragmentation by this mechanism has been examined using a theoretical analysis which predicts the extent of composite flow as a function of processing parameters and the resulting fiber fragmentation lengths. In order to validate the analysis, a model composite system, which consisted of a tin matrix and degraded sapphire fibers, has been "hot pressed" at room temperature to simulate elevated temperature consolidation of sapphire-reinforced Ni-base composites. A comparison of observed and predicted fiber fragmentation lengths indicate good agreement. The analysis can readily be applied to predicting conditions that should be used to prevent extension-induced fiber fracture during high temperature consolidation of structural composites. The research was supported by NASA.
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