Fabrication of Al 2 O 3 -reinforced Ni 3 Al composites by a novel in situ route
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I.
INTRODUCTION
NICKEL-BASED superalloys are already being used as load-bearing turbojet engine components at temperatures as high as 0.9 times their absolute melting temperature. Further progress in increasing operating temperature inevitably will be slow. Consequently, extensive attempts have been made to develop high-temperature structural materials with capabilities higher than those offered by nickel-based superalloys. Intermetallic compounds in general and aluminides in particular have received the most attention.[1–5] Among the aluminides, the B-doped Cr-containing Ni3Albased alloys, because of their appreciable ductility from room to high temperatures, are the most attractive candidates.[6] Although these alloys are noticeably stronger than the nickel-based alloys, they have an inferior creep resistance.[7] Reinforcing Ni3Al-based alloys with a suitable material has the potential of not only increasing the strength and reducing the mass density but also of improving the creep resistance. Among the various reinforcing materials that were employed, Al2O3 appears to be the chemically most compatible material. However, recent investigation indicated that the interfacial bond strength between single-crystal Al2O3 fiber and binary Ni3Al was weak, ;19 MPa.[8] Although alloying with 0.5 at. pct Cr increased the bond strength to 55 MPa, its addition resulted in a fiber/matrix reaction. The Weibull mean strength of the single-crystal Al2O3, which was 2.6 GPa in the as-received condition, decreased to 1.1 GPa when incorporated into a binary Ni3Al. The strength decreased to 0.85 GPa when the binary intermetallic was alloyed with 1 at. pct Ti.[8] Alloying with Cr led to extensive fiber cracking due to the deformation of the fiber by compression twinning.[8] In a recent study, it was shown that when Ni3Al com-
S. NOURBAKHSH, Professor, O. SAHIN, Postdoctoral Research Associate, and H. MARGOLIN, Distinguished Research Professor, are with the Department of Chemical Engineering, Chemistry and Materials Science, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted July 26, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
posites unidirectionally reinforced with ZrO2-toughened polycrystalline Al2O3 fiber were exposed to air at elevated temperatures, oxygen diffused rapidly along the grain boundaries of the Al2O3 fiber deep into the composite.[9] This resulted in the oxidation of Al of the Ni3Al matrix, which led to the formation of a layer of Al2O3 covering the surface of the fibers. When a Ni3Al alloy containing small amounts of Zr, 0.34 at. pct was used as the matrix and the composite was subsequently annealed in air, the oxygen reacted with the Zr of the matrix and formed ZrO2.[9] In contrast to the behavior of Al2O3, the ZrO2 did not precipitate as a layer on the surface of the fiber but rather as thin ribbons starting at the surface of the fiber and extending into the matrix. The diffusion of oxygen through the ZrO2 ribbons and its subsequent reaction with the Al of the matrix resulted in the formation of
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