Microstructure and phase relations in a powder-processed Ti-22AI-12Nb Alloy
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INTRODUCTION
THE advent of projects such as the Integrated High Performance Turbine Engine Technology (IHPTET) and the National Aerospace Plane (NASP) programs have given impetus to the development of materials with high specific strength, high specific modulus, and good environmental properties at elevated temperatures. [q The commonly used high-temperature materials such as superalloys suffer from the disadvantage of high density. Attention is focused on intermetallics, the phases that are responsible for the room temperature, as well as high-temperature properties of many of the commercial alloys, for example, CuAI 2 and MgZn in high strength Al-alloys, Ni3AI in Ni-based superalloys, NiA1 in alnico magnets, SbSn in bearing alloys, etc. t2~ For high-temperature oxidation, the formation of thermodynamically stable surface oxide phases is required, and AI203, SiO2, and BeO are some of these phases, t3] So aluminides, silicides, and beryllides are some of the important intermetallic systems for high-temperature applications. Among the aluminides, Fe base, Nb base, Ni base, and Ti base are the systems that received wide attention. The densities of intermetallic phases in these systems are as follows: FeA1 (5.6 g/cc), Fe3AI (6.7 g/cc), NbAl 3 (4.54 g/cc), Nb2A1 (6.85 g/cc), Nb3A1 (7.62 g/cc), NiA1 (5.9 g/cc), Ni3A1 (7.5 g/cc), TiAI (3.9 g/cc), and Ti3A1 (4.2 g/cc)J4,5,61The Ti-A1 system is identified as a potential candidate material for the IHPTET and NASP programs and considerable attention has been focused on this system in recent years. The alloys based on Ti3A1 (42) and TiAI (y) are found to be the potential candidates for improved performance as aerospace materials. The elastic moduli of these compounds are lower than those of superalloys, but their retention is high at high temperatures. I7] This could be attributed to the strong bonding in their ordered structure. High-temperature properties such as strength retention, creep/stress rupture S.G. KUMAR, Research Assistant Professor, is with Dept. of Chemical and Metallurgical Engineering, University of Nevada, Reno, NV 89557. R.G. REDDY, formerly Professor with the Dept. of Chemical and Metallurgical Engineering, University of Nevada, is now ACIPCO Professor with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL. Manuscript submitted February 8, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
life, and high-temperature fatigue resistance are improved over the conventional titanium alloys because of the slow diffusion kinetics in these ordered alloys. As in other aluminides, the room-temperature ductility of binary titanium aluminides is poor. Ternary additions of niobium to the alpha-2 phase are found to improve the ductility. Ti-24Al-I 1Nb is found to have a balance of roomtemperature ductility and elevated rupture resistance. Ti-25A1 has some degree of ductility only at 873 K and above, whereas Ti-24AI-I 1Nb exhibits good ductility at intermediate and high temperatures. Ti3Al-based alloys developed so far
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