Biaxial creep testing of textured Ti-3AI-2.5V tubing
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INTRODUCTION
TITANIUM-based alloys are used in steam turbines, aircraft gas turbines and body structures, chemical or petrochemical heat exchangers, marine applications, and orthopedic prosthesis to replace damaged bones. The particular alloy under study, Ti-3A1-2.5V, is frequently used in air frames, jet engines, hydraulic and fuel lines, and as a foil material for manufacture into honeycomb cores. These high-temperature applications are limited by creep properties of the material. In addition, preferred orientations in these alloys have a significant effect on their properties, and it is seen that the mechanical properties of titanium alloys are greatly improved through texture strengthening, tL2,3~ Ti-3AI-2.5V is an a + / 3 alloy, with vanadium acting as a/3 stabilizer and aluminum as an a stabilizer. The addition of/3 stabilizers results in alloys of greater density than the base metal, while the addition of a stabilizers, like aluminum, results in an alloy of lower density, t41 According to Banerjee and Krishnan, t4] aluminum, in addition, may lower the stacking fault energy of the a phase and therefore might contribute to creep strength. Incorporation of a small volume fraction of/3 (5 to 10 pct) further strengthens the ct phase, t4] The operating slip planes that have been observed in hexagonal close-packed (hcp) metals tS-~~ with low c/a ratio (c/a < 1.633), such as Ti, Hf, and Zr, are various prismatic and pyramidal planes. The slip directions that have been identified in most cases are of the (1120) type. t"l However, evidence of {1122} (1123) slip in titanium has also been reported, t~2~The second important VASUDEV VENKATESAN, Graduate Research Assistant, Materials Science and Engineering Department, SHEIKH T. MAHMOOD, Postdoctoral Research Associate, and K. LINGA MURTY, Professor, Nuclear Engineering Department and Materials Science and Engineering Department, are with the North Carolina State University, Raleigh, NC 27695-7909. Manuscript submitted February 8, 1990. METALLURGICAL TRANSACTIONS A
mechanism by which hcp metals deform is twinning. Low
c/a ratio materials exhibit {10i2} and {1121} twins in tension along the basal pole and {1122} twins in compression. With the hcp (a) phase being in greater proportion, the deformation textures in the two-phase (or + /3) alloys arise during forming operations because of the restricted number of independent slip systems. The texture study in these alloys is all the more interesting because the incompatibility of deformation caused by the a phase is assumed to be accommodated by suitable strains in the /3 phase due to the abundance of possible slip systems in body-centered cubic (bcc), thus altering the texture. tlA3] The effect of aluminum on the deformation textures of titanium has been extensively studied, t14-17] The texture maintains a fairly constant degree of splitting of basal poles in the ND (normal direction)-TD (transverse direction) plane (like that of unalloyed Ti) up to an A1 concentration of 2 wt pct, but it transforms to an almost ideal b
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