High-temperature deformation processing of Ti-24Al-20Nb
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I.
INTRODUCTION
THE Ti3AI-Nb alloys are of interest both as monolithic materials and matrix materials for composites for high-temperature applications. These high-niobium alloys in the cast form consist of three phases, a2, O, and B2. Figure 1 shows a vertical section in the Ti-A1-Nb system.t1] The volume fraction of these phases is controlled by the prevalent cooling rates through the fl transus. Iz,21The formation of a 2 can be suppressed totally at high enough cooling rates, but slow cooling first results in the formation of a2 phase and subsequently the O phase. Thus, alloys with high niobium content, which lie in a two-phase O + B2 phase field at low temperatures, may in practice contain the a2 phase, as well (as the consequence of slower cooling rates during vacuum arc melting). There also exists a narrow three-phase region of o~2 At- O At- B2 and a2 + B2 phase field, which are estimated to lie above 975 ~ and 1030 ~ respectively, for the alloy under study5 L3~ The response of a2 aluminide alloys to processing conditions typical of hot working has received attention only recently. A study by Semiatin et al.t4] on a cast Ti-24AI11Nb alloy revealed three distinct hot-working regimes, a warm-working regime, a region of a2 recrystallization, and finally a region of hot-working of single-phase ft. A study by Long and Rackr51 on a similar alloy revealed that the stable flow regime as identified by the dynamic material model (DMM) [6] was associated with dissolution of Widmanst~itten a2, coarsening of primary a2, and dynamic "spheroidization" of lamellar a 2. A more recent study by Sagar et al.t71 on a similar alloy has revealed domains of dynamic recrystallization (DRX) of a 2 phase both in the
lath a 2 and equiaxed a2 structures. This study had also suggested that cross slip constitutes the activation barrier for DRX. Recrystallization of/3 phase during thermomechanical processing has received limited attention in conventional a + / 3 titanium alloys. Since the overall microstructural development in the a2(O) + B2 titanium aluminide alloys is similar to the conventional a + / 3 titanium alloys, it would be relevant to address this issue in these alloys also. Long and Rack observed DRX of/3 phase at high temperatures (1150 ~ to 1200 ~ at medium strain rates (0.01 s-t) and Sagar et al. observed/3 phase recrystallization after deformation and subsequent solution treatment for combinations of deformation temperatures and strain rates which were different from those corresponding to DRX of a 2 phase. In this study, we report the work carried out on a highniobium alloy, Ti-24A1-20Nb. This alloy is of interest because it offers improved creep resistancet~l and ambient
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P.K. SAGAR and D. BANERJEE, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 058, India. K. MURALEEDHARAN, formerly with the Defence Metallurgical Research Laboratory, is Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15
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