Transformations in a Ti-24AI-15Nb Alloy: Part I. Phase Equilibria and Microstructure K. MURALEEDHARAN, A.K. GOGIA, T.K. NANDY, D. BANERJEE, and S. LELE A variety of heat treatments have been employed to explore phase equilibria and the development of microstructure in a Ti-24AI-15Nb alloy. These include solution treatments both above and below the/3-transus, followed by controlled cooling and aging at temperatures high enough to preclude to-phase formation. The phase fields /3, /30, ct2 + /30, a2 + /30 + O, and /30 + O have been identified in the alloy, and schematic time-temperature-transformation (TTT) curves are proposed for continuous cooling transformations from the/3 phase. The composition of the az and/30 phases in the a2 + /30 region and of the a2, /30 and O phases in the ternary phase field have been obtained by analytical electron microscopy. I. INTRODUCTION THE development of Ti3AI base alloys for high- temperature applications [~'2"31 appears to have substan- tially preceded the recognition of the complexity of phase equilibria and transformations in the base Ti3A1-Nb sys- tem. It has only recently been shown that the addition of Nb to the Ti3AI composition results not only in or- dering 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 Ti3A1- Nb 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) com- position [~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 con- tents of 24 to 25 pct. We have therefore chosen the Ti- 24AI-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 start- ing material for all of the heat treatments discussed in this article was the hot-rolled bar in a mill-annealed con- dition (slow furnace cool from 1333 K) whose micro- K. 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. structure is shown in Figure 1. The/3-transus of the alloy, the temperature above which the alloy goes to single- phase/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) con- sisted of solution treatment in the single-phase /3 field followed by cooling at different rates to a variety of tem- peratures (including room temperature) followed by a water quench. These are useful in assessing the micro- structures both for/3 heat treatment as well as for welded structures in titanium alloys. The third set of heat treat- ments (Table IV) consisted of aging at various temper- atures ranging from 923 to 1123 K, samples which had been solution treated either above or below the fl-transus. Such an aging treatment is normally required in order to stabilize the microstructure for a given high- service temperature. The aging temperatures used in this study were high enough such that isothermal/3 decom- position to to or to-related phases has not been examined. Taken together, the set of heat treatments constitute a comprehensive investigation into the variety of micro- structures that may be developed in this class of alloys. Heat treatments were done in evacuated quartz capsules, back filled with argon gas in cases of section sizes less than 5 mm or in air with a protective coating when the section size was larger than 5 mm. Thin foils for transmission electron microscopy (TEM) were made by twin-jet electropolishing in a solution con- taining 6 pct sulfuric acid in methanol [~7] at 223 K. Transmission electron rrficroscopy observations were made in a PHILIPS* EM 430T analytical electron microscope *PHILIPS is a trademark of Philips Electronic Instruments Corporation, Mahwah, NJ. operating at 300 KV or in a PHILIPS CM12 analytical electron microscope operating at 120 KV, both attached with energy-dispersive spectroscopy (EDS) detectors. Fully quantitative microanalysis of individual phases was carried by analytical electron microscopy using tech- niques described in detail elsewhere. ItS1 METALLURGICAL TRANSACTIONS A VOLUME 23A, FEBRUARY 1992--401