Prospects, Promises and Properties of Refractory Intermetallics
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Prospects, Promises and Properties of Refractory Intermetallics Donald L. Anton* and Dilip M. Shah** *United Technologies Research Center, 410 Silver Lane, E. Hartford, CT 06108 "**Pratt & Whitney, 400 Main St., E. Hartford, CT 06108 ABSTRACT Extensive research activity, over the last 15 years, has been conducted on structure/property relationships and processing of intermetallic compounds for high temperature use. Progress has been made in improving a number of properties of these compounds; however, the demanding balance of properties required (high strength, good strength retention at temperatures exceeding 1000°C, low density, damage tolerance at ambient temperatures, good creep and stress rupture characteristics and environmental stability at high temperatures) are not likely to be achieved in a monolithic (single phase) compound. Initial work on intermetallic matrix composites has proven to be quite promising. It has already been shown that compounds, brittle at room temperature, may be toughened by the inclusion of appropriate reinforcements, either strong or tough and ductile. Both artificial and natural or in-situ composite fabrication techniques have been used to manufacture these composite systems. The properties of two specific intermetallic matrix composite systems, NiAI/Al,0 3 and Cr2Nb/Nb are summarized to elucidate their strengths, weaknesses and potential. Candidate composite systems are also discussed along with the rationale behind their selection. INTRODUCTION Superalloys, used to the limit of their temperature capabilities in both the turbine and combustor of advanced jet aircraft, are being considered for replacement by refractory intermetallics such as NiA! and MoSi2 . Higher gas path operating temperatures will lead to greater thrust-to-weight ratios, higher fuel efficiencies and lower NOX emissions. Take off noise levels will also be limited through new high temperature nozzle designs. All of these problems, if solved, will lead to a new generation of turbo-fan engines which will meet the ever challenging needs of our industrialized society [1]. This paper will endeavor to briefly review the state-of-the-art of refractory intermetallic materials being considered for aerospace applications [2]. For our purposes, refractory intermetallics will be defined as those compounds with melting points in excess of 1600"C. In particular, the compounds NiAl, Cr , Nb, MoSi 2, MoSi,, Cr3Si and NbAI, which hold the greatest potential at this time for use as structural materials, will be discussed. A number of shortcomings will become evident in all of these materials and both artificial and in-situ composite approaches put forth to solve these obstacles. This will be followed by a description of the fundamental aspects encountered in high temperature composites, with special attention paid to the particular problems encountered with intermetallic matrix composites. MONOLITHIC PROPERTIES Creep Strength
A significant amount of creep data on refractory intermetallics has been generated over the past eight ye
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