Microstructure of AI 2 O 3 fiber-reinforced superalloy (INCONEL 718) composites
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I.
INTRODUCTION
NICKEL-based alloys, being ductile at ambient and elevated temperatures, are potential matrices for fiber-reinforced composites intended for use at high temperatures. Several fibers have been used in the past to reinforce nickelbased alloys. Boron and carbon fibers were dissolved when incorporated in the superalloys.ti] Fiber recrystallization and intermetallic compound formation had been reported when tungsten fibers were used.t2.3] Alumina fiber offers a few advantages when compared to tungsten fiber. It has a much lower mass density (3.95 v s 19.25 g/cc). The coefficient of thermal expansion of AI203 (9 • 10 -6 ~ is closer to that of nickel-based alloys (15.8 to 19.3 x 10 -6 ~ II than that of tungsten (5 • 10 -6 ~ However, A1203 is more brittle than the tungsten fiber. The fiber/matrix reaction, therefore, is expected to result in rapid degradation of A1203 fiber strength. This restricts the use of A1203 fiber to composite systems in which there is very little or no fiber/matrix reaction. Recently, it was demonstrated that fiber-reinforced hightemperature intermetallic matrix composites can successfully be fabricated by a liquid metal infiltration technique known as pressure casting,tSl The outcome of these investigations indicated that AI203 is chemically compatible with binary nickel aluminides, such as Ni3AI and NiA1.t6,71 The addition of 1 at. pct Ti to nickel aluminides was required in order to infiltrate successfully the fiber preform. The interfacial bond strength between Ni3A1 and A1203 fiber, however, was low, approximately 19 MPa.tS~ Individual additions of Zr and Y to Ni3AI to improve bond strength resulted in fiber/matrix reaction, t8,91 The aim of this study was to fabricate continuous AI203 fiber-reinforced nickel-based alloy composites by a pres-
S. NOURBAKHSH and H. MARGOL1N, Professors, O. SAHIN, Postdoctoral Research Associate, and W.H. RHEE, Graduate Student, are with the Department of Materials Science and Engineering, Polytechnic University, Brooklyn, NY t 1201. Manuscript submitted Februar 3, 21, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
sure-casting technique and to evaluate the suitability of the technique as a means for producing composite materials for high-temperature applications. This article will focus on the microstructure. The mechanical properties of the composites will be dealt with in a subsequent publication. The nickel-based superalloy used in the investigation was INCONEL* alloy 718. This alloy was selected for use *INCONEL is a trademark of INCO Alloys International, Huntington, WV.
on the basis of its composition. It contains several alloying elements such as Ti, which facilitate infiltration of the fiber preform, or Cr and Nb, which enhance bonding of the A1203 fiber to the matrix. The alloy does not contain reactive elements, such as Zr or Y, which were shown, in the case of Ni3A1, to react with A1203 .ts'91 II.
EXPERIMENTAL PROCEDURE
The composition of the matrix alloy used is (in at. pct) 20Fe, 20Cr, 3.2Nb, 1.8Mo, 1.2Ti, 1.0AI, 0.5Si, 0.2C
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