Structure, tensile deformation, and fracture of a Ti 3 Al-Nb alloy
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I.
INTRODUCTION
THE potential of the aluminides
of titanium, Ti3A1 (a2) and TiA1 (3'), for high-temperature applications is limited mainly by their lack of ductility. The last decade, however, has seen the development of Ti3Al-base alloys with some room-temperature plasticity. [11 The design of such alloys is essentially based on the principle of stabilizing the high-temperature, ductile phase of Ti into the structure through alloying. Niobium has found favor as the preferred/3 stabilizer since it increases the nonbasal slip activity in Ti3A1, [21 thereby ameliorating the critical problem of an insufficient number of slip systems in this aluminide. In addition, Nb also enhances oxidation resistance.t3] A typical composition of this class of alloys is Ti-24Al-11Nb. t41 However, the simple two-phase mixture envisaged by this design principle is complicated by the ordering of the phase in such compositions to a B2 structure (designated /3o). tS] More recently, we have shown that Nb additions also result in a phase separation of a2 into a Nb-rich phase which is slightly distorted to an orthorhombic symmetry, t6] A complex phase diagram with several ternary phase fields results in the Ti3A1-Nb section. 17] In this paper, we examine the microstructure of Ti24A1-11Nb from the perspective of these latter results on phase equilibria and then assess its room-temperature tensile properties for a variety of microstructural conditions in terms of structure, deformation behavior, and failure mode. In a companion paper, [8] we describe a detailed transmission electron microscopy (TEM) characterization of deformation in the ordered a2 and/30 phases.
A.K. GOGIA, D. BANERJEE, and T.K. NANDY, Scientists, are with the Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad 500 258, India. Manuscript submitted April 4, 1989. METALLURGICAL TRANSACTIONS A
We note that very limited work has been carried out on the tensile deformation of this alloy. [4.9] II.
EXPERIMENTAL
A 20-kg ingot of Ti-24AI-11Nb was melted by consumable electrode arc melting. Niobium additions were made as an A1-Nb master alloy. The alloy ingot was processed in a manner similar to that for conventional titanium alloys, that is, first forged in the/3 region from 150-mm-diameter ingot to 70-mm square billet and then rolled below the/3 transus (1398 K) down to 12.7-mmdiameter rods at about 1373 K. These rods were mill annealed at 1373 K. The detailed chemistry of the alloy is given in Table I. The heat treatments given to the alloy are listed in Table II and can be broadly divided into three categories: (1)/3 solution treatments: the samples were heattreated above the transus and cooled at different rates; (2) a2 + /3 solution treatments: the samples were solution treated below the transus at three different temperatures and water quenched to obtain different amounts of primary a2 phase; and (3) aging treatment: the samples were aged for 24 hours at temperatures ranging from 923 to 1123 K for a selected a2 + / 3 solution-reatment temperature. Room-
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