An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide al
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INTRODUCTION
IN recent years, a considerable amount of research has been conducted to develop high-temperature, lightweight structural materials capable of extended service at temperatures in the range of 600 7C to 750 7C. Much of this effort has focused on a class of titanium aluminide alloys based on the orthorhombic (Ti2AlNb) phase. The orthorhombic titanium aluminide alloys possess higher ductility, toughness, and tensile strength without a significant loss in creep resistance compared to earlier alloys based on the alphatwo phase. In addition, the orthorhombic alloys exhibit excellent strength retention after thermal cycling, are less sensitive to interstitial embrittlement, and have enhanced thermomechanical fatigue resistance.[1,2,3] A range of compositions with 22 to 28 at. pct aluminum and 12 to 30 pct niobium having a variety of microstructures has been investigated. In general, these microstructures have consisted of three phases: orthorhombic (O), ordered beta (B2), and alpha-two (a2). As for other titanium and titanium aluminide alloys, the specific microstructures and textures developed in orthorhombic titanium aluminide alloys depend greatly on composition, interstitial content, and thermomechanical processing route.[4,5] For example, Semiatin and Smith[6] examined microstructural evolution during rolling of a Ti-22Al-23Nb sheet. Their results showed that the B2 phase does not recrystallize during subtransus hot rolling or during thermomechanical processing, P.D. NICOLAOU, Research Scientist, is with the Materials and Processes Division, UES, Inc., Dayton, OH 45432. S.L. SEMIATIN, Senior Scientist, is with the Materials Directorate, Wright Laboratory, WL/MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted August 14, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
comprising supertransus preheating followed by rolling under decreasing temperature conditions through the transus, thus leading to elongated B2 grains. In addition, precipitation of alpha-two or orthorhombic phase from the solid solution of the crystallographically textured B2 phase gives rise to noticeable mechanical (microstructural) texture. In related work, Seetharaman[7] conducted a series of uniaxial compression tests on a Ti-21Al-22Nb alloy in order to establish its hot workability during bulk forming processes. Microstructures with a high volume fraction of B2 phase had low flow stresses, gradual work hardening, and flow instabilities. On the other hand, fine acicular microstructures showed high flow stress, flow softening, and relatively smooth flow. The sharp mechanical and crystallographic textures developed during primary processing of orthorhombic titanium aluminide alloys may be expected to have significant impact on plastic flow during secondary hot working as well as on service properties. For example, the room-temperature tensile properties of orthorhombic alloys have been observed to be very anisotropic as a result of texture; the tensile ductility in the longitudinal (rolling) direction of or
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