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I.

INTRODUCTION

INTERMETALLIC titanium aluminides are excellent candidates for advanced high-temperature structural applications. Their low density and excellent high-temperature oxidation resistance make them particularly attractive when compared to superalloys on a strength to weight basis.[~] During the last decade, Ti3A1 and TiAI alloys with limited room-temperature ductility have been developed,t~.21 but they still lack toughness and creep strength. Second phase strengthening, particularly in the form of fibrous reinforcement, is a viable approach that can improve both toughness and creep strength. Several techniques have been used to fabricate titanium aluminide composites, including powder cloth,t3] foil-fiberfoilt4] processing of SiC/Ti-24Al-11Nb composites and the powder metallurgy technique for fabrication of 3,-TiA1 with continuous A1203 fiber reinforcement.t5] However, composites processed by these routes showed less promising mechanical properties owing to the weak bonding between the matrix and reinforcement phase tr] and reactions between the matrix and fibers, tT] One method to circumvent these problems is to grow the reinforcement phase by in situ reaction. Hyman et al. produced ~/-TiA1 reinforced with TiB/TiB2 by conventional solidification,tS] in which the titanium borides were formed in situ from the matrix, but the technique could be used with only a limited number of reinforcement phases and the morphology and size of the reinforcement phases changed in the scaled-up version as a result of nucleation at different stages of solidification. The XD* tech*XD is a trademark of Martin Marietta Corporation, Bethesda, MD.

nology has also been used to produce titanium aluminide composites.t ~s] In a recent study, t9] Ti3AI(Nb)/TiB composites were synthesized directly from elemental powders, using the exothermic reaction that occurs between the constituents. This process is known as combustion synthesis or self-propagating high-temperature synthesis. This technology pro-

W.Y. YANG, Postdoctoral Fellow, on leave from the University of Science and Technology Beijing, Beijing, People's Republic of China, and A. PETRIC, Associate Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L85 4L7. H.C. YI, Research Fellow, is with Guingn6 International, Paradise, Newfoundland AlL IC1. Manuscript submitted August 16, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A

duces composites with good bonding and a stable interface between reinforcement phase and matrix. This article reports on the characterization of the microstructure of such Ti3AI(Nb)/TiB composites using transmission electron microscopy (TEM) and other techniques. II.

EXPERIMENTAL DETAILS

Elemental powders of titanium (Johnson Matthey, -325 mesh, 99 pet), aluminium (Johnson Matthey, -325 mesh, 99.5 pct), niobium (Anachemia Canada, -325 mesh, 99.8 pct), and amorphous boron (Johnson Matthey, amorphous, -325 mesh, 92 pct) were used in this study. The powders were mixed in two stoichiometrie