Nitridation of Ti-Al alloys: A thermodynamic approach
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The authors express their gratitude to Mitsubishi Heavy Industries, Ltd. for providing the specimens used in this work. This work was conducted as part of joint research with Tohoku Electric Power Co., Ltd. REFERENCES 1. R. Viswanathan: Damage Mechanisms and Life Assessment of HighTemperature Components, ASM INTERNATIONAL, Metals Park, OH, 1989, pp. 425 and 453-57. 2. T.M. Maccagno, A.K. Koul, J.-P. Immarigeon, L. Cutler, R. Allem, and G. L’esperance: Metall. Trans. A, 1990, vol. 21A, pp. 3115-25. 3. C.T. Sims and W.C. Hagel: The Superalloys, John Wiley & Sons, New York, NY, 1972, pp. 52-73. 4. S.H. Ai, V. Lupinc, and M. Maldini: Scripta Metall. Mater., 1992, vol. 26, pp. 579-84. 5. B.A. Lerch and S.D. Antolovich: Metall. Trans. A, 1990, vol. 21A, pp. 2169-77.
Nitridation of Ti-Al Alloys: A Thermodynamic Approach R. SCHMID-FETZER and K. ZENG In the framework of the joint European action COST 507 (Development of a database for light alloys), an extensive thermodynamic assessment of the Ti-Al-N phase equilibria has been accomplished.[1] The purpose of this article is to demonstrate the application of that thermodynamic model to the nitridation of Ti-Al alloys. Figure 1 shows the calculated conventional ternary phase diagram and the corresponding stability diagram in terms of the nitrogen pressure, PN2. It is remarkable that TiAl3, dissolving only very little N, can withstand the highest PN2 (1.58 3 10214 bar) before R. SCHMID-FETZER, Professor, and K. ZENG, Postdoctoral Research Fellow, are with the Electronic Materials Group, Technical University of Clausthal, D-38678 Clausthal-Zellerfeld, Germany. Manuscript received April 25, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation to the nitrides AlN 1 d occurs, where d 5 TiN12x. On the other hand, aTi, showing the highest nitrogen solubility of the metal phases in the ternary system, is getting nitrided at the lowest values of PN2, either to the binary nitride ε-Ti2N or to the ternary phase t1, depending on its Ti/Al ratio. The nitridation limit of the intermetallic phase g(TiAl) depends strongly on its composition, being highest at 41.1 at pct Ti with PN2 5 1.8 3 10215 bar and more than four orders of magnitude higher than that at the Ti-rich end 51.4 at pct Ti. The formation of t1 is very sluggish at 1000 7C.[2] For real-time process applications, it is therefore useful to perform a metastable phase equilibrium calculation by suppressing the existence of t1. The result is shown in Figure 2. The metastable extension of those phase fields which previously had been saturated with t1 is most obvious for a and a2, but also for g at the Ti-rich end between 5.85 3 10220 and 4.07 3 10217 bar. Under these kinetically favorable conditions, virtually all compositions of g are initially precipitating the ternary nitride t2. Further nitridation yields TiAl2 1 t2 and finally AlN 1 d is obtained, as for all the other Ti-Al starting compositions. The reactions observed during in situ processing of TiAl‘‘Ti2AlN’’ composites[3] are the first example of a quantitative app
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