Pressure casting of a zirconia-toughened alumina fiber-reinforced NiAl composite
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I.
INTRODUCTION
C U R R E N T L Y , a considerable interest exists in developing materials with high-temperature capabilities beyond those of nickel base superalloys. The intermetallic compound NiAI, due to its high melting temperature (1640 ~ low density (5.86 g / c c ) , and high elastic modulus (297 GPa) combined with an excellent resistance to oxidation, is considered to be a suitable material for use at elevated temperatures. [L2'3] However, the main obstacles which have prevented the utilization of NiA1 base alloys are the lack of adequate room-temperature ductility and toughness, low high-temperature strength, and creep resistance. [4,5[ These difficulties can, to some extent, be overcome by incorporating suitable continuous ceramic fibers into NiA1 to form an intermetallic matrix composite material. There are several routes that can be employed to incorporate fibers into a given matrix. The selection of a processing route has a strong beating on the quality of the composite material. [6] The aim of the present study is to characterize the microstructure of an NiA1 composite reinforced with continuous PRD-166* fibers fabricated by pressure-casting technique. The outcome of this investigation will determine the suitability of pressure casting as a processing route and the PRD-166 as a fiber for reinforcing NiAI base matrices. Pressure casting was selected on the basis of its prior success in producing continuous fiber-reinforced materials. [7) The PRD-166 fiber, which is a 20-~m-diameter zirconiatoughened alumina fiber, was used because its coefficient of thermal expansion (9 x 10 -6 ~ is closer to NiAI (12.5 x 10 -6 ~ than SiC (4.86 • 10 -6 ~176 fibers, and it also is chemically more stable in nickel aluminide-based matrices than the SiC fibers. [12] This fiber is more resistant to high tempera-
S. NOURBAKHSH, Associate Professor, O. SAHIN, Postdoctoral Research Associate, W.H. RHEE, Graduate Student, and H. MARGOLIN, Professor, are with the Department of Metallurgy and Materials Science, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted January 29, 1991. METALLURGICAL TRANSACTIONS A
tures than the single-phase polycrystalline a-A1203 fiber, Fiber FP,* which undergoes appreciable grain growth. [131 *PRD-166 and Fiber FP are trademarks of E.I. DuPont de Nemours and Company, Inc., Wilmington, DE. PRD-166 contains 0.2 weight fraction of partially stabilized ZrO2.
II.
EXPERIMENTAL PROCEDURE
The NiA1 base intermetallic alloy used in the investigation had a nominal chemical composition of Ni-45AI1Ti (all compositions in atomic percent). The alloy was produced by arc melting of its pure constituents under argon. Titanium was added to facilitate infiltration of the fibers by the molten NiA1. The fibers were supplied in the form of a continuous yam containing about 190 strands of fiber wound on a drum. The fibers were carefully rewound around a 280-mm-diameter glass drum and held together using a fugitive binder to form a thin sheet of fully aligned fiber preform. The fiber preform was then cut
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