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well as other significant advantages, enormous effort has been devoted to powder metallurgical techniques. In this category we include reactive sintering of elemental or elemental plus prealloyed powders, 3 " 7 reactive hot pressing, 8 reactive HIPing, 5 " 7 injection molding, 4 " 9 the XD process, °"14 dynamic compaction of powders, 15 " 18 mechanical alloying, 19 ' 21 the powder cloth method, 2,24 ' 25 and, of course, traditional hot pressing techniques. 22,23 Most recently, a process that combines alloy rod or tubing with matrix alloy powder to provide macroscopic plus microscopic toughening has been developed by United Technologies Research Center (UTRC) and labeled "microstructurally toughened composites." 26 Among ail the powder processes, only injection molding, reactive hot pressing, and the microstructural toughening procédure hâve produced aligned composites. In order to overcome the inhérent limitations of most powder techniques in providing aligned composites, two other major catégories of composite préparation hâve been studied and are reviewed in this article. They are vapor déposition processing (CVD, CVI, 27 " 29 or plasma spraying 3 ") to produce monolithic, fibrous or laminated structures, and liquid métal infiltration.31"34

P o w d e r Processing Reactive Consolidation Several variants of reactive sintering hâve been employed to consolidate interThis article is reprinted from Intermetallic metallic matrix composites. Ail incorpoMatrix Composites, D.L. Anton, R. McMeeking, rate fibers or particles in a mixture of at least some elemental powders, although and D. Miracle, and P. Martin, eds. (Volume prealloyed powders may also be présent 194, MRS Symposium Proceedings). Copyto dampen the réaction. The basis of ail right 1990, Materials Research Society.

MRS BULLETIN/DECEMBER1990

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Ni+AI were mlxed to stoichlometrlc proportions.

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Compacts were ClPed to a green density o( approximately 70% theoretical.

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Vacuum encapsulated in 304 stalnless steel.

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RHIPed 1200

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Innovative Processing Techniques for Intermetallic Matrix Composites

reactive sintering processes is the formation of a liquid phase as a resuit of an exothermic reaction between elemental powders présent in the mixture. The liquid phase accélérâtes consolidation and is consumed during the process. 3 The reaction may proceed with no pressure (reactive sintering, RS), with isostatic pressure (reactive hot isostatic pressing, RHIP) or unidirectional pressure (reactive hot pressing, RHP). A flow chart for RHIP of NiAl is shown in Figure 1. This procédure has generally been utilized to produce particulate-reinforced and random-fiberreinforced intermetallics. A typical microstructure of NiAl-20 vol%TiB2 composites is shown in Figure 2, while compressive flow stresses versus température for several composites are shown in Figure 3. 7 Al3Ta composites with 10 vol% Safill fibers, randomly oriented, hâve been Consolidated by RHIP at 1200°C, with a pressure of 172 MPa. However, the