Phase equilibria investigations on the aluminum-rich part of the binary system Ti-Al
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NTRODUCTION
THE binary phase diagram Ti-Al is the basis for many alloys of technical importance, namely, for those based on the intermetallic compounds Ti3Al[1] and ␥-TiAl.[2,3] While the titanium-rich part of the system has been investigated extensively,[4–13] the aluminum-rich part is still not clear. Ogden et al.[14] and Bumps et al.[15] reported the initial versions of the Ti-Al phase diagram. These contain only two intermetallic compounds in the aluminum-rich part, TiAl and TiAl3. Po¨tzschke and Schubert[16] found the phase TiAl2 (HfGa2 type). Raman and Schubert[17] inserted the hightemperature phase Ti5Al11 and the low-temperature phase Ti9Al23 in the phase diagram. Afterward, Loiseau and Vannuffel[18] included the phase Ti3Al5 and found two structural modifications for the phase TiAl2: TiAl2 (I) (ZrGa2 type) and TiAl2 (II) (HfGa2 type). In the composition range TiAl2-TiAl3, a great number of long period structures were also observed by Loiseau et al.,[19] especially in the high-temperature but also in the lowtemperature region. Kaltenbach et al.[20] found only two intermediate phases between TiAl and TiAl3: TiAl2 and Ti5Al11. These authors also corrected the temperatures of the phase reactions. The most recent phase diagram version for the aluminumrich part of the Ti-Al system, which is based on extensive experimental work, has been elaborated by Schuster and Ipser.[21] These authors found seven intermetallic compounds in the composition range of 50 to 75 at. pct Al. They added the phase Ti1⫺xAl1+x , observed at 64 at. pct Al, into the hightemperature part of the phase diagram. The intermetallic compounds Ti5Al11 and Ti2Al5 were incorporated as two separate phases in the high-temperature region of this system. However, no phase equilibria investigations have been made in the low-temperature region below 970 ⬚C. Calculated phase diagram versions of the Ti-Al system have been reported by Murray,[22] Kattner et al.,[23] and Zhang et al.[24] However, Murray[22] considered only the J. BRAUN, Postdoctoral Fellow, and M. ELLNER, Senior Research Associate, are with the Max-Planck-Institut fu¨r Metallforschung, D-70174 Stuttgart, Germany. Manuscript submitted July 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
intermetallic phases TiAl and TiAl3 in the aluminum-rich part of this system. Kattner et al.[23] made a calculation that included the phases Ti2Al5 and TiAl2 with a low- and a high-temperature modification. Zhang et al.[24] yielded a calculation of the Ti-Al system based on an improved thermodynamic description of the intermetallic phases.
II. EXPERIMENT The binary Ti-Al alloys (with a mass of approximately 3 g) were made from metals with the following purity: titanium 99.99 pct (Johnson Matthey Co.) and aluminum 99.999 pct (Vereinigte Aluminium-Werke). The alloys were produced by melting metals in an arc furnace in an argon atmosphere (Messer-Griesheim 5N). Chemical analysis of the melted alloys was not carried out because the weight loss was smaller than 0.5 wt. pct. For the heat-treatment procedures,
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