Thermodynamic activities in the alloys of the Ti-Al-Nb system
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I. INTRODUCTION
INCREASING demands on the efficiency of gas turbines and jet turbine engines, as well as environmental aspects, require the development of new high-temperature materials. Alloys based on Ti and Al are of increasing interest, particularly the intermetallic Ti3Al and TiAl phases, because of their potential use as construction materials for high-temperature components (e.g., References 1 through 3). These alloys can potentially reduce the mass of components, based on present-day superalloys, by about 50 pct, because of their low density (4.5 g cm23). Titanium aluminides are, therefore, also of interest as an automotive material for exhaust valves and turbocharger turbine rotors,[4] besides their usefulness in gas turbine and jet engine applications. The disadvantages are the low ductility[1] and the inadequate oxidation resistance[3] at the service temperatures of 700 8C to 900 8C. The alloys of technical interest contain 40 to 50 at. pct Al. These alloys form TiO2 and Al2O3 on the surface when they are exposed to oxidizing conditions. This mixed oxide scale is not protective, because the Ti oxides are fast growing, unlike Al2O3, which forms a protective barrier. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, the Gibbs free energy of formation of the titanium oxides and Al2O3, as well as the thermodynamic activities of Ti and Al in the alloy. It was found by various laboratories[5–11] that the addition of Nb to Ti-Al alloys increases their oxidation resistance. Different mechanisms were proposed for the explanation of this increase. One hypothesis is that Nb increases the thermodynamic activity of Al relative to that of Ti, thereby favoring the formation of a stable Al2O3 scale. Investigations for the determination of the thermodynamic activities of Ti and Al in the alloys of the Ti-Al-Nb system were, therefore, carried out for the first time. Emphasis was laid on the alloys in the a2-Ti3Al, (a2-Ti3Al 1 g -TiAl), and g -TiAl phase fields. Partial enthalpies and entropies of mixing in the alloys of the g -TiAl and b phase fields were M. ECKERT, formerly Postdoctoral Student, Research Centre Ju¨lich, is Project Manager, Chemetall GmbH, 38679 Langelsheim, Germany. D. KATH, Technical Engineer, and K. HILPERT, Group Leader and Professor, are with the Institute of Materials and Processes in Energy Systems, Research Center Ju¨lich, 52425 Ju¨lich, Germany. Manuscript submitted June 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
evaluated for the mean temperature of the measurements. The constitution of the samples used for the measurement of the activities was determined at the measurement temperatures. The experimental investigations for the determination of the constitution will be described in Reference 12. The final results on the constitution are given in this article in order to facilitate the use of the activities determined. Our experimental studies, for the determination of partial thermodynamic properties a
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