Interface characterization of fiber-reinforced Ni 3 Al matrix composites
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INTRODUCTION
N I C K E L aluminides based on Ni3AI are attractive materials for high-temperature structural applications due to their high stiffness and high strength retention at elevated temperature, combined with low density and good oxidation resistance.ill The yield strength of NisA1 shows an increase rather than a decrease with increasing temperatures, and the high-temperature strength of Ni3AI is superior to that of the nickel-based superalloys at 850 ~ However, the inherent brittle intergranular fracture had, so far, limited the wide-spread applications of polycrystalline aluminides as engineering materials. Nevertheless, significant improvement in the ductility and toughness has been achieved recently through various metallurgical techniques such as microalloying, macroalloying, grain refinement, e t c . 12-8] It has been found that the addition of a small amount of boron dramatically increases the ductility by suppressing brittle intergranular fracture at ambient temperatures. A tensile elongation greater than 50 pet with virtually 100 pet transgranular failure has been achieved in Ni-24 at. pct AI containing about 0.5 at. pct B. 161 The increased ductility of nickel aluminide alloys provides a unique opportunity to develop intermetallic matrix composites with better specific stiffness and strength than conventional nickel-based superalloys, i91 The incorporation of ceramic reinforcements into the intermetallic alloy matrix should produce a composite which possesses a lower density and improved tensile strength and stiffness, as well as enhanced creep resistance. 19) Potential applications of these novel intermetallic composites include hypersonic aircraft, space vehicles, propulsion system of jet engines, e t c . , where sufficient stiffness, corrosion resistance, and strength at J.-M. YANG. Assistant Professor, is with the Department of Materials Science and Engineering, University of California-Los Angeles, Los Angeles, CA 90024. W.H. KAO, Department Head, is with the Materials Science Laboratory, The Aerospace Corporation, El Segunde, CA 90245. C.T. LIU, Group Leader, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted November 21, 1988. METALLURGICAL TRANSACTIONS A
elevated temperature are required. Various approache: have been employed to consolidate nickel aluminide ma trix composites with continuous fibers and particulates such as diffusion bonding, u~ reactive sintering, u1'12] an( pressure casting. I~31 However, one of the major factor,, which limits the successful application of the composite: at high-temperature environments is the occurrence o: chemical reactions between the reinforcement and thc matrix. These reactions are generally diffusion controlled and occur during composite fabrication, as well as during service. Although the development of an interaction zone at interfaces is desirable for establishing a sound fiber/matrix bonding, overgrowth of the interaction layer is detrimental to the properties of the composite.
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