Oxidation of a zirconia-toughened
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I.
INTRODUCTION
T H E efficiency of a heat engine increases with increasing operating temperature of the engine. Extensive research is being carried out to develop materials with high-temperature capabilities beyond those of nickel-based superalloys. Attention has mainly been focused on intermetallic compounds. Among the various intermetallic compounds, the Ni3Al-based compounds, due to their high ductility at room temperature, high strength, and high resistance to oxidation at elevated temperatures, have been the subject of numerous recent scientific investigations and alloy developments, tl-4] The main problem with NiaA1 is its inferior creep resistance I51 as compared with nickel-based superalloys. The creep resistance of NiaA1 can be improved by reinforcing the aluminide with a suitable ceramic fiber. Both SiC and Al203-based fibers have been tried. In contrast to SiC fibers, which tend to react extensively with Ni3Al,t6] A1EO3-based fibers are highly stable in Ni3Al. t7] Moreover, the coefficient of thermal expansion of A1203, 9 • 10 -6 ~ -itS] is closer to that of Ni3A1, 12.5 x 10 -6 ~ than that of SiC, 4.5 • 10 -6 ~176 Consequently, the A12Oa-based fibers appear to be more desirable. The fiber-reinforced composite materials intended for use at elevated temperatures should not only be able to withstand the thermomechanical stresses created during operation but also the environment to which they are exposed. The aim of the present work is to report on the effect of heat treatment, in vacuum and in air, on the development of microstructure in a NiaAl-based intermetaUic O. SAHIN, Postdoctoral Research Associate, S. NOURBAKHSH, A s s o c i a t e P r o f e s s o r , W.H. R H E E , Graduate Student, and H. MARGOLIN, Professor, are with the Department of Metallurgy and Materials Science, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted January 24, 1992. METALLURGICAL TRANSACTIONS A
composite reinforced with PRD-166 fibers.* PRD-166 is *PRD-166 fiber is a trademark of E.I. Du Pont de Nemours & Company, Inc., Wilmington, DE.
a 20-/xm-diameter polycrystalline ot-A1203 fiber which contains 20 wt pct partially stabilized ZrO2. E81The Ni3AI composite has been produced by pressure casting.t~ 1]The objective of this investigation was to provide an insight into the suitability of A1203 fiber-reinforced Ni3A1 composites for use at elevated temperatures. II.
EXPERIMENTAL PROCEDURE
The Ni3Al-based alloy used in the investigation was supplied by Armco Research Center, Middletown, OH. A small amount of Ti was added to the intermetallic alloy by arc melting under argon, to facilitate the infiltration of the fibers by the molten alloy during pressure casting. The matrix alloy had a nominal chemical composition of Ni-16.8A1-7.9Cr-I.2Ti-0.5Zr-0.1B (atomic percent). The PRD-166 fibers were supplied in the form of a continuous yam containing about 190 strands of fibers wound on a bobbin. The yam was passed through a solution (1000 cc ethanol + 200 cc water) containing 40 grams of fine A1203 particles, approximatel
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