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INTRODUCTION

INTERMETALLIC alloys based on the TiA1 matrix have received considerable interest in recent years because of their potential as high-temperature structural materials. ~1-51Recent studies have demonstrated that TiAIbased alloys within a certain composition range can be heat-treated to exhibit two-phase microstructures containing the following features: tl,4,6-~~ (1) equiaxed T (TiAI) grains and a2 (Ti3A1) particles, (2) lamellar colonies of alternating layers of a2 and 3' platelets, and (3) a combination of equiaxed 3' grains and lamellar colonies. These microstructures are often referred to as fully lamellar, nearly fully lamellar, duplex, and fully gamma, depending on the relative components of lamellar colonies and equiaxed T g rains-t6J A particular TiAl-based alloy that can be manipulated by heat treatment to obtain these various microstructures is the Ti-47A1-2.6Nb-2(Cr + V) alloy, tl0] Recent work on the Ti-47AI-2.6Nb-2(Cr + V) alloy and other similar alloys has indicated that microstrncture plays an important role in the tensile and fracture behaviors of two-phase TiAl-based alloys. [6,1~ In particular, the fully and nearly fully lamellar microstructures have been found to exhibit relatively good fracture toughness and excellent creep resistance t6,11.13,14] at both ambient and elevated temperatures but poor tensile ductility, [6,9'1~ even at temperatures as high as 800 ~ [e.g., 1.5 pct tensile ductility for Ti-47AI-2.6Nb-2(Cr + V) at 800 ~176 On the other hand, duplex microstructures consisting of a significant volume fraction of ffme equiaxed T grains and some lamellar grains exhibit relatively low fracture toughness and creep resistance but moderate tensile ductility at ambient and elevated temperatures [e.g., ~ 3 . 7 pct for Ti-47AI-2.6Nb-2(Cr + V) at 25 ~176 K.S. CHAN, Staff Scientist, is with Southwest Research Institute, San Antonio, TX 78228-0510. Y.-W. KIM, formerly with MetcutMaterials Research Group, Wright-Patterson AFB, Dayton, OH 454330511, is Chief Scientist, Material Research Division, Universal Energy Systems, Dayton, OH 45432-1894. Manuscript submitted January 27, 1992. METALLURGICAL TRANSACTIONS A

Most of these observations were based on tests performed in air and, in limited cases, in vacuum, tl4J A recent comparison L121 of tensile results obtained for the Ti-47Al-2.6Nb-2(Cr + V) alloy at 800 ~ under two different strain rates revealed significant differences in the flow and fracture behaviors in air and in vacuum, suggesting that elevated temperature deformation and fracture in this material are sensitive to strain rate, environment, or both. Previous s t u d i e s [15'16! o n Ti-24A111Nb, an a2-based titanium aluminide alloy, have also shown that the tensile behavior is sensitive to the test environment (air vs vacuum) at relatively slow rates (1 • 10 -4 S-1 o r l e s s ) [15] but is insensitive to environment at a moderate strain rate (1 x 10 -3 S-l) at 600 ~ tlrJ Whether the test environment and strain rate exert similar influence on the fracture and plastic flow behav