Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites
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I.
INTRODUCTION
TITANIUM aluminides, particularly those based on Ti3AI (alpha-two, as), have been the subject of numerous investigations as a result of their projected potential for excellent specific properties in the temperature range of 590 ~ to 760 ~ [1'21 The most advanced alloys of this class are generally two-phase mixtures of the ordered a2 phase (D019) and a disordered/3 phase, produced through alloying with Nb. Representative of this class is the c~2 + /3 alloy, Ti-24Al-llNb (all compositions by atom percent). Composites formed by the reinforcement of these alloys with continuous SiC have been a natural extension of the monolithic studies because of the significant increases in strength and stiffness offered by the reinforcement. Indeed, extensive research has been performed on the SiC/Ti-24Al-I 1Nb composite system. [3-81Although the predicted increases in strength and stiffness have been obtained when the material has been properly processed, this composite system suffers from a number of problems owing, at least in part, to deficiencies in the matrix alloy. Most severe among these matrix deficiencies are poor room-temperature ductility and toughness, relatively high reaction rates with the SiC reinforcement, low tensile strength, and poor resistance to interstitial embrittlement when exposed in an air environment at elevated temperature. 19'1~ While some mechanical properties, such as ductility, strength, and
toughness, of a2 + fl alloys may be improved by selecting specific processing techniques (e.g., powder vs foil-matrix consolidation), environmental resistance and reactivity with the fiber reinforcement are relatively insensitive to the processing route selected for composite fabrication. Recently, a new class of Ti aluminides based around the ordered orthorhombic (O) phase, Ti2A1Nb, have been studied for their potential use as monolithic materials for elevated temperature applications. 1~21Alloys of this class are generally two- or three-phase materials comprising either an "O + ordered beta (/30)," or an "O +/30 + as" microstructure, respectively. Initial studies of these alloys have demonstrated that this class of aluminides exhibits improved room-temperature ductility and toughness as well as significant increases in tensile strength when compared to a2 + /3 alloys, even on a specific basis. ~ In the present study, characteristics of an orthorhombicbased alloy, Ti-21A1-22Nb, are examined and compared with those of the baseline alpha-two-based composition, Ti-24Al-llNb, as potential matrices for reinforcement with SiC fiber. Throughout this article, the Ti-21AI-22Nb alloy is referred to alternatively as the "orthorhombic alloy," while the Ti-24AI-11Nb alloy is referred to as the "alpha-two alloy." lI.
EXPERIMENTAL PROCEDURE
A. Foil Processing J.A. GRAVES, Program Manager, and C.G. RHODES, Member Technical Staff, are with the Department of Materials Science, Rockwell International Science Center, Thousand Oaks, CA 91360. P.R. SMITH, Materials Research Engineer, is with the Materials Behavio
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