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  • Part I. The Microstructural Evolution in Ti-Al-Nb O 1 Bcc Orthorhombic Alloys

    C.J. BOEHLERT, B.S. MAJUMDAR, V. SEETHARAMAN, and D.B. MIRACLE

    Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12Al- 38Nb O 1 bcc orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and a2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X- ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti 5 50 at. pct was modified. The aging- transformation behavior was studied in detail. For solutionizing temperatures between 875 8C and the bcc transus, the phase composition and volume fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and a2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstätten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstätten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for subtransus a2 1 B2 microstructures. Similarly, Widmanstätten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O 1 B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 8C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 8C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus- solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 8C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti- 12Al-38Nb, Widmanstätten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

    I. INTRODUCTION (designated as b). Henceforth, such alloys will be designated as “O 1 bcc” alloys. In order to optimize properties forTHE orthorhombic (O) phase (cmcm symmetry based specific applications, it is important to understand the phase

    on Ti2AlNb) has similarities with the hexagonal close- composition, morphology, and structure-property relations packed (hcp) a2 phase (Ti3Al, DO19 structure), yet differs of such alloys. This need formed the rationale for the work by the lattice arrangement of Nb with respect to Ti.[1,2] Alloys described herein. with a significant volume fraction of O phase have shown Several works have discussed the phase equilibria of O attractive mechanical properties,[3–10] owing to the excellent alloys.[9–19] Several of these studies have focused on the creep resistance of the O phase combined with some room- lower-temperature stability and aging transformation behav- temperature (RT) ductility of the O phase arising from the ior.[9,10,13,15–19] Past investigations revealed that alloy compo- existence of multiple slip systems. Compositions that have sition and the temperatures of processing, solutionizing, and received recent attention are Ti-25Al-17Nb, Ti-22Al-23Nb, aging all have significant influence on the mechanisms gov- Ti-25Al-25Nb, and Ti-23Al-27Nb.* Those containing Nb erning the transformation behavior. The metastable B2 phase

    decomposes into a2 and/or O through three distinct transfor-* All alloy compositions are given in atomic percent. mation modes. The first transformation mechanism is Wid-

    concentrations of 25 pct and higher show very little a2 phase manstätten precipitation of a2 and/or O that involves both and a much larger O 1 bcc phase field, where the bcc a shear transformation as well as diffusional transport. A phase may either be ordered (designated as B2) or disordered second decomposition mode is through a composition-

    invariant mechanism, which rapidly transforms the super- transus parent B2 to either a metastable a2 phase at low Nb contents or metastable O at higher Nb levels.[13,18] A thirdC.J. BOEHLERT, Postdoctoral Fellow, is with the Department of

    Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218. decomposition mode is through discontinuous precipitation B.S. MAJUMDAR and V. SEETHARAMAN, Senior Scientists, are with of a2 1 bcc or O 1 bcc; the resultant cellular microstructure UES, Inc., Dayton, OH 45432-1894. D.B. MIRACLE, Research Group replaces fine intragranular matrix precipitation.[6–9,19,20]Leader, is with the Air Force Research Laboratory, Wright-Patterson AFB,

    Independent of the aging transformation, the B2 phase tendsOH 45433-7817. Manuscript submitted March 26, 1998. to undergo a composition-induced disordering resulting in

    METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 30A, SEPTEMBER 1999—2305 NOT PROTECTED BY U.S. COPYRIGHT

  • Table I. Chemical Analysis of the Near Ti2AlNb Alloysthe disordered b structure at low temperatures.[17–19,21] This is attributed to the decreasing-Al and increasing-Nb content

    At. Pct Wt. (ppm)of the B2 phase in the ternary system as the temperature Material Ti Al Nb N Fe Odecreases. The temperature and B2 phase compositional

    dependence of these transformation modes has yet to be Ti-23Al-27Nb bal 23.2 27.2 200 1100 1160 Ti-25Al-24Nb bal 25.4 24.2 110 350 280determined, and the low-temperature stability of these alloys Ti-25Al-23Nb bal 24.7 23.3 150 290 930remains a concern. Ti-12Al-38Nb bal 13.2 39.2 70 255 575In this work, Ti-Al-Nb alloys containing approximately

    Ti-50 at. pct were studied. The Nb/Al ratios were unity and higher: nominally, Ti-25Al-25Nb, Ti-23Al-27Nb, and Ti- 12Al-38Nb. At the start of the investigation, available iso-

    temperature and then unidirectionally forged to an approxi-therms suggested that such a range of compositions might mately 3:1 ratio. After the initial forging, the workpiecesallow a systematic investigation of the relative effects of the were EDM cut, recanned, and forged in a direction perpen-O and bcc phases on the mechanical properties without dicular to the original forging direction under identical condi-significant complexity arising from the presence of a third tions. The canned material was then isothermally soaked atphase a2. Also, these O 1 bcc alloys are structurally attrac- 815 8C for 1 hour, followed by a soak at the rolling tempera-tive, since alloys containing any significant amount of the ture for 15 minutes prior to rolling. Unidirectional multipassa2 phase suffer from ductility problems, arising primarily rolling was carried out with interpass reheating at the rollingfrom the single ^a& slip of the a2 phase. Details of the temperature for 5 minutes. The reduction per pass rangedmechanical properties of these O 1 bcc alloys are available between 5 and 10 pct, and the total reduction per pancakein Reference 9. was approximately 60 pct. The total effective true shearThe objectives of this work were twofold: (1) to identify strain was calculated to be on the order of 3. After the finalthe temperature ranges of the separate phase regimes for pass, the assemblies were reheated at the rolling temperatureO 1 bcc alloys and (2) to understand the compositional and for 3 minutes and then cooled slowly in vermiculite. Thetemperature dependence of the separate aging transformation estimated cooling rate was 3 8C/min. The can was thenmodes and associated phase morphologies. Whereas most removed. The final thickness of the sheets was approxi-past studies have concentrated on processing O 1 bcc alloys mately 10 mm. Additional details on processing, experi-in the supertransus B2 or b phase field, the approach in this ments, and microstructural and mechanical results arework was to process the alloys in the lower-temperature available in Reference 9.subtransus domain. As will be shown in this article, this

    Samples for the phase evolution and aging study wereapproach provided significant flexibility in controlling the diamond-cut from the rolled sheet, then cleaned and wrappedresultant microstructures through postprocessing heat treat- in tantalum foils prior to being encapsulated in quartz tubesments. Special emphasis was focused on the lower-tempera- that were backfilled at a low pressure of high-purity argonture aging transformation behavior and identifying the order/ gas. The capsules were subjected to solution treatments atdisorder transition of the B2 phase. In addition to the descrip- different temperatures, followed by controlled cooling ortion of the microstructural evolution behavior, the opportuni- water quenching. Selected samples were then reheated at aties and limitations of using processing and heat-treatment lower temperature, followed by water quenching, in orderschedules for obtaining stable microstructures that contain to study the aging transformation behavior. The chemicaldifferent grain sizes, phase volume fractions, and morpholo- composition distribution between the different phases wasgies are discussed. measured using a JEOL* 733 electron microprobe analyzer.

    * JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. II. EXPERIMENTAL

    The grain size and phase volume fractions were determin