Autogenous gas tungsten arc weldability of cast alloy Ti-48Al-2Cr-2Nb (atomic percent) versus extruded alloy Ti-46Al-2Cr
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I. INTRODUCTION A. Gamma Titanium Aluminide
INTERMETALLIC compounds are of interest because they are light in weight and maintain high strength at elevated temperatures. One such intermetallic compound, gamma-based titanium aluminide (g-TiAl), has been of interest in both cast and wrought forms, because its desirable properties imply excellent performance in aerospace, structural, and high-temperature applications. Gamma titanium aluminide is an ordered, equiatomic compound of the elements titanium and aluminum.[1,2] The commercially cast gamma titanium aluminide of composition Ti-48Al-2Cr2Nb (at. pct) was developed by Huang, General Electric.[3] An additional characteristic that makes these materials interesting is their oxidation resistance at high temperatures, above 760 7C.[4] The high aluminum content of this compound improves the oxidation resistance. The only disadvantage of this material is that it lacks the ductility and toughness that is essential for structural applications. This brittle behavior of g-TiAl is inherently related to the lack D.J. BHARANI, Graduate Assistant, is with the Department of Statistics, Old Dominion University, Norfolk, VA 23508. V.L. ACOFF, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
of dislocation mobility in its face-centered tetragonal (L10) crystal structure.[2] Problems such as room-temperature brittleness and poor hot workability have delayed the rapid development of many practical applications of intermetallics.[5] However, recent breakthroughs in processing and alloy design have tremendously increased the interest in intermetallic compounds and alloys.[5–12] For example, the desires to double the thrust-to-weight ratio and to slash fuel consumption by 30 to 50 pct in missile engines and to reduce commercial air transport fuel by as much as 30 pct have contributed to this increased interest and have aided recent practical developments.[13] In accordance with the partial Ti-Al phase diagram shown in Figure 1, three distinctly different TiAl structures can be obtained.[14,15] For alloys with .52 at. pct aluminum, a single-phase g-TiAl microstructure is obtained. Alloys between 46 and 50 at. pct aluminum result in a ‘‘duplex’’ microstructure that consists of g grains and a lamellar structure that contains alternating a2 and g platelets (good lowtemperature ductility). Alloys with less than 48 at. pct aluminum can be processed to achieve a fully lamellar microstructure (highest fracture toughnesses and creep resistances). Thus, alloys of current engineering significance contain 47 to 49 at. pct aluminum with ternary and multielement additions such as chromium, niobium
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