High-temperature deformation and failure of an orthorhombic titanium aluminide sheet material
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I.
INTRODUCTION
A new class of titanium aluminide alloys based on the ordered orthorhombic phase Ti2AlNb has been developed for high-temperature applications in both monolithic and composite form.[1,2,3] These alloys exhibit higher room-temperature strength, ductility, and fracture toughness than alpha-two base materials without a significant loss in creep resistance. In addition, compared to the alpha-two titanium aluminides, the orthorhombic alloys possess excellent strength retention after thermal cycling, are less sensitive to interstitial embrittlement, and have enhanced thermomechanical fatigue resistance.[4,5] A wide range of microstructures and hence properties can be developed in the orthorhombic titanium aluminide alloys through control of chemistry and processing.[4,6–9] Alloys with various volume fractions of orthorhombic, ordered beta, and alpha-two phase can be produced by varying the levels of aluminum (typically between 22 and 28 at. pct), niobium (between 12.5 and 30 at. pct), as well as other quaternary elements or interstitials (especially oxygen).[10] These alloys have been rolled, forged, and extruded at temperatures in the single-phase beta field or high in the two-phase (orthorhombic 1 ordered beta) or three-phase (orthorhombic 1 ordered beta 1 alpha-two) field. As for conventional alpha/beta titanium alloys and the alpha-two titanium aluminide alloys, issues related to interstitial contamination and flow behavior are important in the design of working processes. In this regard, Seetharaman[11] conducted a series of uniaxial hot compression tests on a Ti-22Al-23Nb (at. pct) alloy in order to establish its workability during bulk forming processes. The alloy exhibited a strong dependence of the flow stress on temperature, strain rate, and preform microstructure. For example, microstructures with a high volume fraction of ordered beta had low flow stresses and low-to-moderate levels of strain hardening or flow softenP.D. NICOLAOU, Research Scientist, is with the Materials and Processes Division, UES Inc., Dayton, OH 45432-1894. S.L. SEMIATIN, Senior Scientist, is with the Metals and Ceramics Division, Materials Directorate, Wright Laboratory, WL/MLLN, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted March 21, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing depending on temperature. On the other hand, the plastic flow of fine, acicular (orthorhombic 1 ordered beta) microstructures was characterized by high flow stresses and a marked amount of flow softening. The objective of the present work was to establish the deformation and failure behavior of a typical orthorhombic titanium aluminide alloy (Ti-21Al-22Nb) during secondary hot working involving tensile modes of loading. To this end, uniaxial tension and plane-strain compression testing were conducted to determine the flow stress dependence on strain and strain rate, the level of normal plastic anisotropy, and failure modes. This information is important not only for operations such as superplastic sheet forming but
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