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Session Chairpersons: Dr. Edward S. Chen, U.S. Army Research Office, 4300 S. Miami Boulevard, Research Triangle Park, NC 27709; Dr. Douglas B. Gundel, Systran Co., Inc., 4126 Linden Ave., Dayton, OH 45432
8:30 am INVITED
INTERMETALLIC MATRIX COMPOSITES PREPARED BY MECHANICAL ATTRITION: A REVIEW: Carl C. Koch, Department of Materials Science & Engineering, North Carolina State University, Raleigh, NC 27695
The interest in intermetallic matrix composites has grown in recent years due to the realization that monolithic intermetallics would not likely satisfy the balance of properties needed for advanced aerospace systems. Processing of intermetallic matrix composites is a critical issue and a number of methods have been explored to fabricate such composites with either continuous or discontinuous reinforcements. While the latter category in general provides less improvement in mechanical behavior, this is the type of composite where mechanical alloying may provide an inexpensive processing route. Intermetallic systems to be discussed include Ni3Al, NiAl, Ti3Al, TiAl, and MoSi2. Early work on oxide dispersions in intermetallics by mechanical alloying, dispersions introduced by cryomilling, and nanocrystalline intermetallic composites will be covered by this review. The possibility of superplastic forming of nanocrystalline intermetallic matrix composites will also be considered.
MECHANICAL ALLOYING OF INERT GAS ATOMIZED Al-Li-Cu-Mg-Zr ALLOY/SiC SHORT FIBERS REINFORCED MMC POWDERS: S. Özbilen, Gazi University, Faculty of Technical Education, Department of Metals Education, Teknikokullar, Arkara, Turkey
MMC powders of Al-Li-Cu-Mg-Zr alloy base with variable amount SiC short filamentary reinforcement were produced in a pilot plant down-draught atomizer with a Mannessman type nozzle under Ar. Powder based alloy matrix composite powders were ground in a mechanical alloying attritor not only for introducing deformation but also to investigate the influence of this new and unique processing route on the microstructure and properties of the material system under investigation.
SYNTHESIS OF NANOSTRUCTURED SiC/Si3N4 COMPOSITE POWDERS THROUGH REACTION MILLING: Z.-G. Yang, L. Shaw, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269
In this study, synthesis of SiC/Si3N4 nanocomposite powders through reaction milling was investigated. Graphite and silicon powders were used as the source of carbon and silicon respectively, while the source of nitrogen was from either nitrogen or ammonia gases. Various compositions of the starting powder mixtures for forming nanopowders spanning from pure SiC to pure Si3N4 were investigated. It was found that nanocrystalline SiC powders could be synthesized at ambient temperature by milling silicon and graphite powders in argon atmosphere. However, the formation of SiC was retarded when milling was conducted in nitrogen or ammonia atmosphere. Crystalline Si3N4 formed only after post annealing the milled powder mixtures and the annealing temperature had a strong effect on the formation of the composite powders. Based on the results from XRD, TEM, DTA and TGA, the formation mechanisms of the composite powders will be discussed.
9:50 am INVITED
SYNTHESIS OF METAL MATRIX COMPOSITES BY MECHANICAL ALLOYING: F.H. Froes, C. M. Ward-Close, E.G. Baburaj, A. Vassel, College of Mines & Earth Research, University of Idaho, Moscow, ID 83844
Abstract not available.
10:20 am BREAK
AN INVESTIGATION OF THE VACUUM HOT PRESSING BEHAVIOR OF SILICON CARBIDE FIBERS COATED WITH NANOCRYSTALLINE Ti-6Al-4V: Joseph M. Kunze, Triton Systems, Inc., 114 Turnpike Road, Chelmsford, MA 01824; Haydn N. G. Wadley, Intelligent Processing of Materials Laboratory, Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22903
The vacuum hot pressing of silicon carbide monofilaments coated with nanocrystalline Ti-6A1-4V has been studied and modeled. In the experiments, surprisingly high identification rates were observed, even at processing temperatures and pressures well below those used for processing conventional Ti-6A1-4V. From the cross sections of partially consolidated specimen, the evolution of coated fiber-fiber contacts and pore shapes were determined. The pores were found to be cusp-shaped throughout the consolidation process. Columns of coated fibers were observed to form which resulted in regions of locally high fiber volume fraction. In the model, the initial densification was based upon a micromechanical contact analysis for a metal coated fiber. Final stage densification was analyzed by modifying the Qian et al strain rate potential for a power law creeping body containing isolated cusp-shaped pores. Simulations of the VHP experiments were performed using this model which incorporated time and temperature dependent microstructure relations. Overall, the simulations compared well with the experimental density data, although the load supported by the regions of locally high fiber volume fraction resulted in the model slightly overestimating the observed densification time response.
MODEL-BASED SIMULATION OF THE CONSOLIDATION PROCESSING OF METAL COATED FIBERS: D.M. Elzey, R. Vancheeswaran, H.N G. Wadley, Intelligent Processing of Materials Laboratory, Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22903
The metallization of structural ceramic fibers by physical vapor deposition, sputtering, etc., followed by consolidation (e.g. hot isostatic pressing) offers an attractive route for the manufacture of continuous fiber-reinforced metal matrix composites (MMC's). Recent models for describing the evolution of key microstructural features (such as porosity, interfacial reaction zone thickness and fiber microbending/fracture) during consolidation are described, and are combined to simulate changes in the composite's microstructural "state" during arbitrary consolidation process schedules. Results are presented for PVD Ti-6Al-4V -coated SiC monofilament fibers consolidated by vacuum hot pressing which illustrate the presence of optimal solutions to the process path planning problem. An optimization scheme is briefly described which allows identification of the consolidation process cycle providing maximum relative density with minimum fiber and interfacial damage. In addition to the process schedule, optimal quality is shown to depend strongly on the fiber/matrix combination, the initial coated fiber packing geometry and the (evolving) matrix microstructural state.
MODEL-BASED SIMULATION OF THE CONSOLIDATION PROCESSING: R.Vancheeswaran, H. N. G. Wadley, Intelligent Processing of Materials Laboratory, Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22903
The performance of fiber reinforced titanium matrix composites (TMC) made by consolidation of spray deposited monotapes is strongly influenced by the processing conditions used. This high temperature consolidation step must simultaneously minimize fiber microbending/fracture, the interfacial reaction product layers at the fiber-matrix interface and at the same time eliminate matrix voids (i.e. increase the relative density). These three microstructural variables have conflicting dependencies upon the consolidation process variables (temperature, pressure and time), and it has been difficult to identify process pathways by trial and error that lead to composites of acceptable quality (where the fiber damage and reaction layer thickness are kept below some bounds, while matrix porosity is eliminated). Models for predicting the microstructure's dependence upon process conditions (i.e. the time varying temperature and pressure) are combined with consolidation equipment dynamics to simulate the microstructure evolution and to assess the relative "processability" of several silicon carbide fiber/titanium alloy matrix systems during their consolidation. We introduce the idea of process failure surfaces and show how this simulation tool in conjunction with a model predictive control (algorithm), is able to design "locally" optimal process cycles that minimize fiber damage, reaction product layer thickness and porosity. The approach is then used to path plan process schedules that will steer away from these damage surfaces for a variety of TMC systems.
STUDIES ON SINTERED ZIRCON-REINFORCED ALUMINUM ALLOY MATRIX COMPOSITES: J. U. Ejiofor, Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487; B.A. Okorie, Department of Metallurgical and Materials Engineering, Enugu State University of Technology, P.M.B. 01660, Enugu, Nigeria; and R. G. Reddy, Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487
Zircon, ZrSiO4, is a thermally stable mineral. Owing to its abundance, high hardness, excellent abrasion/wear resistance and low coefficient of thermal expansion, researchers are investigating its use for medium strength, tribological applications. In the present study, the conventional low-cost, double compaction powder metallurgy route in the synthesis of Al-13.5Si-2.5 Mg alloy(wt%) reinforced with ZrSiO4, was investigated. The mechanical, physical and tribological properties were determined following the development of optimum conditions of compaction and sintering. At 0.15Vf, the UTS, 0.2%Y.S. and hardness improved by 4%, 12.8% and 88% respectively while the adhesive wear rate and the coefficient of friction reduced by 99.55 and 35.5% respectively. At a critical volume fraction of zircon, between 0.03 and 0.05, a significant improvement in wear resistance was observed. The use of optical microscopy, EPMA, SEM and x-ray analysis revealed the phases and possible reactions at the matrix-reinforcement interface, fracture mode and compositions of fractured surfaces which are related to measured mechanical properties and, the influence of the reinforcement phase on wear rate. Further structural analysis showed that the improvement in mechanical properties is attributed largely to the load-bearing ability and intrinsic hardness of zircon than to particulate dispersion effects. An attempt was made to model the strength of the composites.
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