Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys
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INTRODUCTION
THE orthorhombic (O) phase (cmcm symmetry based on Ti2AlNb) has similarities with the hexagonal closepacked (hcp) a2 phase (Ti3Al, DO19 structure), yet differs by the lattice arrangement of Nb with respect to Ti.[1,2] Alloys with a significant volume fraction of O phase have shown attractive mechanical properties,[3–10] owing to the excellent creep resistance of the O phase combined with some roomtemperature (RT) ductility of the O phase arising from the existence of multiple slip systems. Compositions that have received recent attention are Ti-25Al-17Nb, Ti-22Al-23Nb, Ti-25Al-25Nb, and Ti-23Al-27Nb.* Those containing Nb * All alloy compositions are given in atomic percent.
concentrations of 25 pct and higher show very little a2 phase and a much larger O 1 bcc phase field, where the bcc phase may either be ordered (designated as B2) or disordered
C.J. BOEHLERT, Postdoctoral Fellow, is with the Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218. B.S. MAJUMDAR and V. SEETHARAMAN, Senior Scientists, are with UES, Inc., Dayton, OH 45432-1894. D.B. MIRACLE, Research Group Leader, is with the Air Force Research Laboratory, Wright-Patterson AFB, OH 45433-7817. Manuscript submitted March 26, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
(designated as b). Henceforth, such alloys will be designated as “O 1 bcc” alloys. In order to optimize properties for specific applications, it is important to understand the phase composition, morphology, and structure-property relations of such alloys. This need formed the rationale for the work described herein. Several works have discussed the phase equilibria of O alloys.[9–19] Several of these studies have focused on the lower-temperature stability and aging transformation behavior.[9,10,13,15–19] Past investigations revealed that alloy composition and the temperatures of processing, solutionizing, and aging all have significant influence on the mechanisms governing the transformation behavior. The metastable B2 phase decomposes into a2 and/or O through three distinct transformation modes. The first transformation mechanism is Wid¨ manstatten precipitation of a2 and/or O that involves both a shear transformation as well as diffusional transport. A second decomposition mode is through a compositioninvariant mechanism, which rapidly transforms the supertransus parent B2 to either a metastable a2 phase at low Nb contents or metastable O at higher Nb levels.[13,18] A third decomposition mode is through discontinuous precipitation of a2 1 bcc or O 1 bcc; the resultant cellular microstructure replaces fine intragranular matrix precipitation.[6–9,19,20] Independent of the aging transformation, the B2 phase tends to undergo a composition-induced disordering resulting in
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VOLUME 30A, SEPTEMBER 1999—2305
the disordered b structure at low temperatures.[17–19,21] This is attributed to the decreasing-Al and increasing-Nb content of the B2 phase in the ternary system as the temperature decre
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