Isothermal oxidation of TiAl alloy
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DUCTION
TITANIUM aluminides offer improved performance compared to the conventional high-temperature materials such as superalloys owing to their low density and good creep resistance.[1–4] However, the oxidation characteristics of titanium aluminides at high temperatures are a major concern. In general, unlike Ni-Al alloys, a protective Al2O3 layer does not form on all the Ti-Al alloys because both Ti and Al form oxides of very similar stability. Aluminum forms a very slow growing oxide (␣-Al2O3), while titanium forms several oxides (TiO, TiO2, Ti2O3, etc.), which have relatively high growth rates. It has been observed that a protective continuous layer of alumina is formed on Ti-Al alloys containing more than the stoichiometric amount of Al. TiAl3 is the only compound on which a protective, continuous ␣-Al2O3 was found to exist over a wide temperature range. [5–8] A nonprotective TiO2(rutile) phase was found to be the major oxide constituent on the surface of Ti3Al at elevated temperatures.[9,10] In the case of a TiAl stoichiometric compound, Ti-rich scales were formed at elevated temperatures resulting in an increase in the parabolic rate constant[9] by several orders of magnitude. Meier et al.[11] studied the oxidation behavior of TiAl in air and oxygen and have observed that TiAl does not form a protective layer of ␣Al2O3 but forms scales composed of TiO2 and Al2O3. Their results indicate that the alloys in the Al-rich portion of the TiAl field formed a protective layer over the temperature range 1100 ⬚C to 1300 ⬚C. In this study, isothermal oxidation of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range of 850 ⬚C to 1000 ⬚C. Oxidation rate constants were obtained from the weight gain curves using the parabolic law. The oxidation product was a mixture of TiO2 and Al2O3 at all the temperatures of measurement. The rate of oxidation was rapid at 1000 ⬚C and the scale spalled midway through the oxidation. II. EXPERIMENTAL PROCEDURE The alloy samples were prepared by arc melting 99.9 pct pure Ti wires and 99.9 pct pure Al rods in a copper hearth
R.G. REDDY, ACIPCO Professor, X. WEN, Graduate Student, and M. DIVAKAR, Postdoctoral Research Fellow, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. Manuscript submitted November 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
under the flow of purified argon using the tungsten electrodes. The ingots containing 51.4 at. pct Ti and 48.6 at. pct Al (numbers henceforth refer to atom percent in the paper) were homogenized at 1000 ⬚C for 18 hours in argon atmosphere and later cut into experimental samples of approximately 10-mm-diameter and 1-mm-thick discs using a low speed diamond saw. The samples were polished using 600 grit emery papers and ultrasonically cleaned in acetone. The oxidation kinetics of samples was studied by weight gain measurements using a thermogravimetric (TGA) setup detailed elsewhere.[12] The sample was suspended in a Lindberg furnace by a plati
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