Elevated temperature tensile behavior of Ti-25AI-10Nb-3V-1Mo
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I.
INTRODUCTION
T I T A N I U M aluminide alloys based on the a2 phase, an intermetallic phase with the D0t9 crystal structure, have undergone development in an attempt to increase the temperature capability of titanium-base alloys. The addition of approximately 25 at. pct aluminum to titanium stabilizes the a2 phase. Such aluminum concentrations were expected to increase the elevated temperature properties, such as strength and creep resistance, while promoting formation of a protective alumina layer at the material's surface. While elevated temperature properties did improve, only marginal and scattered success has been achieved in raising titanium's inherently low environmental resistance. The complete history of a2 titanium aluminide alloy development can be found in a number of comprehensive reviews, tt-7~ Most development and understanding of az alloys to date has concentrated on low-temperature ductility and fracture toughness. More recently, work has been done to examine the elevated temperature tensile behavior of these materials, i8-~5jThese studies have characterized the elevated-temperature tensile behavior and have shown a potential for elevated-temperature environmental degradation. The purpose of the present research is to examine elevated-temperature tensile behavior in greater detail and to attempt to understand microscopic effects. For this study, the alloy Ti-25A1-10Nb-3V- 1Mo (atomic percent) was used to represent the a2 + /3/B2 class of materials, lJr~ The designation fl/B2 is used since the bodycentered cubic (bcc) phase in these materials may either C.H. WARD, formerly Materials Scientist, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433, is Program Manager, Air Force Office of Scientific Research, Bolling AFB, Washington, DC 20332. A.W. THOMPSON, formerly Professor, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA 15213, is Scientist, Lawrence Berkeley Laboratory, Berkeley, CA 94720. J.C. WILLIAMS, General Manager, is with Engineering Materials Technology Laboratories, GE Aircraft Engines, Cincinnati, OH 45215. Manuscript submitted September 13, 1993. METALLURGICALAND MATERIALSTRANSACTIONSA
be disordered or ordered, depending on alloy composition. Tensile testing in vacuum was used to assess intrinsic material behavior by avoiding this particular alloy's reputation for poor environmental resistance.
II.
EXPERIMENTAL PROCEDURE
Three microstructures were produced in the Ti-25AI10Nb-3V-1Mo (hereinafter called Ti-25-10-3-1) alloy using hot-die forging and postforging heat treatments. The heat of material studied was found to have a/3 transus of approximately 1075 ~ as determined by metallography and differential thermal analysis (DTA). Processing was selected to provide microstructural variation in primary ct2 volume fraction and secondary a2 lath size. The starting billets for hot-die forging were cylindrical with 10-cm diameter • 11-cm height. Forging was carried out with a stroke rate of 1.25 c m / m i n to a fin
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