Microstructure of a pressure-cast Fe 3 AI intermetallic alloy composite reinforced with zirconia-toughened alumina fiber
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I.
INTRODUCTION
THEREis a distinct need for structural materials for use at elevated temperatures. The intermetallic compound, Fe3A1, due to its excellent oxidation and corrosion resistance, high melting temperature, and lower density than nickel-based superalloys, may have some utility at elevated temperatures. There are at least three major shortcomings associated with this compound which tend to limit its suitability. These are the lack of adequate roomtemperature ductility, a low tensile strength at temperatures above 600 ~ t~l and a low elastic modulus, t2! The low room-temperature ductility of Fe3A1 has been hypothesized to be due to weak atomic bonding across certain crystallographic planes, leading to a low cleavage strength, implied by transgranular cleavage fracture. t3] A substantial increase in ductility, from 3.7 pct to as much as 9.4 pct, has recently been noted when 2 to 6 pct Cr (all compositions in atomic percent) was added to a binary Fe-28A1 alloy, t31 Examination of the fracture surfaces indicated that the enhancement in ductility was accompanied by a transition from transgranular cleavage to a mixture of transgranular cleavage and intergranular failure. [3] With such levels of ductility, the room and elevated temperature strength of Fe3A1 can be vastly improved by reinforcing the alloyed compound with a suitable material. Unidirectionally strengthening the aluminide with continuous ceramic fibers may also significantly increase the elastic modulus and lower the density. Pressure casting offers distinct advantages over other techniques, such as powder metallurgyt41 and diffusion bonding, tS~ which have been used in the past to fabricate
S. NOURBAKHSH, Associate Professor, H. MARGOLIN, Professor, and F.L. LIANG, Postdoctoral Research Associate, are with the Department of Metallurgy and Materials Science, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted January 19, 1990. METALLURGICAL TRANSACTIONS A
continuously reinforced intermetallic matrix composites. Pressure casting is not only an economical but also a flexible technique, since it allows incorporation of various arrangements of fibers into a metal matrix. There are no limitations on the diameter of the fibers or size of the samples. However, since processing is carried out at high temperatures, there is a distinct possibility that exposure to molten metal at high temperatures may severely degrade the fibers and/or result in extensive fiber/matrix chemical reaction. The aim of the study undertaken was to determine the suitability of the pressure-casting technique as a means of fabricating fiber-reinforced iron-aluminide composites. The fiber selected for this investigation was DuPont's 20-1zm diameter PRD-166* fiber, t6,Tj The nominal com*PRD-166 is a trademark of E.I. DuPont de Nemours & Company, Inc., Wilmington, DE.
position of the iron-aluminide matrix employed was Fe-28A1-2Cr-lTi. Titanium was added to facilitate the infiltration of the fibers by the molten metal.tSl II.
EXPERIMENTAL PROCEDURE
The iron-aluminide in
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