Examination of solidification pathways and the liquidus surface in the Nb-Ti-Al system
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INTRODUCTION
IN the desire to obtain higher efficiencies and greater power generation from gas turbine engines, attention has focused on intermetallic alloys based on the Nb-Ti-Al system for use in high-temperature structural components. The Nbrich intermetallic phases (d-Nb3Al and s-Nb2Al) contained in this system have demonstrated exceptional strength and creep resistance at high temperatures, but are equally known for their brittleness at low temperatures.[1,2] However, potential exists for resolving problems in ductility while retaining high-temperature properties through several possible strategies, which include ductile solid solution alloys, brittle phase reinforcement of a ductile matrix, and ductile phase toughening of a brittle matrix. To better understand the structural potential of these types of alloys and the conditions under which to develop them, accurate knowledge of the solidification pathways, solid-state transformations, and microstructural evolution is essential. The first complete investigation of the solidification phase fields of the Nb-Ti-Al system was conducted by Kaltenbach et al.[3] The liquidus projection was later refined through work by Perepezko et al., [4] following the establishment of the binary Ti-Al diagram to include the high-temperature a phase field. In addition, the latter study led to further adjustments to the nature and directions of the liquid phase reactions occurring in the system, especially in narrowing KEITH J. LEONARD, Postdoctoral Research Associate, JOSEPH C. MISHURDA, Visiting Research Associate, and VIJAY K. VASUDEVAN, Professor, are with the Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012. Manuscript submitted November 3, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
the primary solidification limits of the d and s phase fields. Through the examination of the solidification routes of alloys near the s-h, s-g, and g-h liquidus troughs, it was concluded that the ternary eutectic reaction proposed in Reference 3 was not possible. Instead, a ternary eutectic reaction involving the L 1 h → s 1 g was suggested. Through thermodynamic calculations, Kattner and Boettinger[5] reported that the primary solidification fields extend farther into the ternary diagram than previously proposed,[4] with an additional change in the reactions involving the liquid, h, s, and g phases. The most recent experimental liquidus projection published by Zdziobeck et al.[6] (Figure 1) again showed a decrease in the primary solidification range of the g phase into the ternary diagram. No changes in the nature and directions of the liquid phase reactions were made in comparison with Reference 4. Reports from other investigators give evidence for an extension of the primary b solidification field beyond that proposed in References 4, 5 and 6. The high-temperature b phase field was reported by Hoelzer and Ebrahimi[7] in a Nb-28Ti-30Al alloy and by Ebrahimi and De Aragao[8] in a Nb-33Ti-40Al alloy, though previous liquidus projections wou
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