Transformations in a Ti-24AI-15Nb alloy: Part I. Phase equilibria and microstructure
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I.
INTRODUCTION
THE
development of Ti3AI base alloys for hightemperature applications [~'2"31 appears to have substantially preceded the recognition of the complexity of phase equilibria and transformations in the base Ti3A1-Nb system. It has only recently been shown that the addition of Nb to the Ti3AI composition results not only in ordering of the/3 phase (B2 structure, designated/30) but also in a distortion of the hexagonal a2 phase (TiaA1, D0w) to an orthorhombic symmetry (O, Cmcm). [4] Various other stable or metastable phases such as to and an to-like phase (B82 structure) 15'6"71 have also been identified, and a rigorous evaluation of high-temperature stability in the Ti-A1-Nb system is in progress. [8,91 Studies on phase equilibria and transformations in alloys along the Ti3A1Nb section, with Nb replacing Ti up to 25 at. pct, have also been carried out, [~~ but the most detailed work thus far appears to be in the Ti-24Al-11Nb (24-11) composition [~4,t5] whose microstructure is dominated by the a2 and /3o phases. The O phase appears in significant amounts at Nb levels exceeding 12 at. pct [4] for A1 contents of 24 to 25 pct. We have therefore chosen the Ti24AI-15Nb (24-15) composition for a detailed study of phase transformations involving the O phase, keeping in mind that at least three alloy compositions developed thus far contain a total of/3 stabilizing additions up to or greater than the 15 at. pct level. I2'3't61 II.
EXPERIMENTAL
Titanium-24 at. pct Al-15 at. pct Nb was melted by consumable double-vacuum arc remelting and hot rolled below the/3-transus into 12-mm-diameter bars. Table I shows the chemical composition of the alloy. The starting material for all of the heat treatments discussed in this article was the hot-rolled bar in a mill-annealed condition (slow furnace cool from 1333 K) whose microK. MURALEEDHARAN, Scientist, A.K. GOGIA, Scientist, T.K. NANDY, Scientist, and D. BANERJEE, Associate Director, are with the Defence Metallurgical Research Laboratory (DMRL), Hyderabad 500258, India. S. LELE, Professor, is with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India. Manuscript submitted June 12, 1991. METALLURGICAL TRANSACTIONS A
structure is shown in Figure 1. The/3-transus of the alloy, the temperature above which the alloy goes to singlephase/3, was determined as 1383 --+ 10 K. The heat treatments are divided into three categories. The first (Table II) consisted of solution treatment at temperatures ranging from 1173 to 1473 K followed by water quenching. These were carried out essentially to determine the phases present at various temperatures and their volume fractions (for reasonable solution-treatment times). The second set of heat treatments (Table III) consisted of solution treatment in the single-phase /3 field followed by cooling at different rates to a variety of temperatures (including room temperature) followed by a water quench. These are useful in assessing the microstructures both for/3 heat treatment
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