Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)
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INTRODUCTION
THE phase equilibria of binary Ti alloys may be conveniently divided into a- and/3-stabilized systems, with further classification based on the principal invariant transformations.Ill In doing so, the/3-stabilized systems are categorized as either/3-isomorphous or/3-eutectoid types. The present work focuses on the former, consisting of the Ti-V, Ti-Mo, Ti-Nb, Ti-Zr, Ti-Hf, Ti-Ta, and Ti-Re systems, and for which the bulk of previous research has been confined to the Ti-V, Ti-Mo, and Ti-Nb systems. Relatively slow diffusivities, combined with rather low transformation temperatures and difficulty in alloy mixing, make the approach to equilibrium difficult in the other systems and may be the cause of the dearth of prior work. However, the same factors also allow for a variety of nonequilibrium phases and refined microstructures in such alloys and are therefore of considerable interest. The Ti-Ta system was chosen for study in the present case. The most recent Ti-Ta binary phase diagram is shown in Figure l(a). t21 This system is characterized by an isomorphous /3 (BCC) phase field extending from pure Ti at elevated temperatures to pure Ta. Below the (or + /3)//3 phase boundary (/3 transus), both a (HCP) and/3 phases are in equilibrium, with limited mutual solubilities. Some controversy surrounds the actual location of the/3 transus, as demonstrated by the data in Figure 2. Three prior experimental studiest3.4,sl have been conducted that include an assessment of this boundary, none of which, however, are in good agreement. These results were discussed by Murray, I2~ who suggested that such variation was probably due to interstitial impurities or inadequate quench rates and therefore suspect. Consequently, Murray12j thermodynamically modeled the /3 transus based on a regular solution model of a reported t61 metastable miscibility gap. The calculated /3-transus J.D. COTTON, J.F. BINGERT, and P.S. DUNN, Staff Members, and R.A. PATTERSON, Program Manager, are with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted February 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
boundary of Murrayt2a is shown in Figure l(a) and is generally 100 ~ lower in temperature than the earlier studies. Three nonequilibrium phases are observed in Ti-Ta alloys on quenching from the/3 field:tT-~~ HCP martensite (og), orthorhombic martensite (a"), and metastable /3. For/3-isomorphous Ti-X alloys in general, the compositional range of occurrence of each phase may be predicted by electron-to-atom (e/a) ratios tS'~l (e/a is defined as the weighted mean Group number), as shown in Figure l(b). Typically, a' occurs from an e/a of 4.00 to 4.07, above which a" occurs. At an e / a of approximately 4.15 to 4.20 to phase may be observed, and above an e/a of 4.2, 100 pct/3 formation usually occurs. The presence of to phase in Ti-Ta alloys has been observed as a result of the decomposition of metastable/3 during postquench annealing, t9j but differences exist in the literature as to whether Ti-Ta alloys form to
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