Structure and stability of a new ternary hexagonal phase in Al 3 Ti-based Al-Ti-V alloys
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2) intermetallic Al3Ti has attracted special attention for potential intermediate structural applications because of its low density, high melting point, and good oxidation resistance.[1] However, the Alrich titanium aluminide offers such limited low-temperature ductility and restricted alloying potential that it remains impractical as an engineering alloy. The use of ternary alloying additions (e.g., Cr, Mn, Fe, etc.) to promote a change in crystal structure from DO22 to ordered cubic L12 has proven successful, but has produced no significant improvement in room-temperature ductility in tension.[2] In adopting an alternative approach to improving the low-temperature properties of monolithic Al3Ti, the present work has involved selection of the Al-Ti-V system as a ternary alloy system offering an attractive range of phase relationships near the composition Al3Ti, suitable for the design of multiphase intermetallic or duplex intermetallic/metallic-phase microstructures. Initial studies[3,4] of phase equilibria in the Al-Ti-V system concentrated on Ti-rich alloys and the titanium aluminides TiAl and Ti3Al and were derived from the available Ti-Al binary-phase diagrams,[5,6] which were then not well established. Tsujimoto[7] and Kornilov and Volkova[8] also limited their interest to the Ti-rich region of the ternary diagram, while Raman[9] established an isothermal section at 1373 K, identifying intermediate phases such as Al2Ti and Ti5Al11 between ␦ -Al3(Ti,V) and ␥ -TiAl. Volkova and Kornilov[10] WOONG-SEONG CHANG, Principal Research Scientist, is with Research Institute of Industrial Science and Technology, Pohang, 790-330, Korea. Contact e-mail: [email protected] B.C. MUDDLE, Professor and Head, is with the School of Physics and Materials Engineering, Monash University, Clayton, Victoria 3168, Australia. Manuscript submitted March 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
examined a series of alloys along the composition line between TiAl and Al8V5 in the ternary system and constructed a polythermal diagram, while Hashimoto et al.[11] re-examined the diagram of Tsujimoto[7] and prepared new isothermal sections at 1073 and 1273 K. More recently, Ahmed and Flower[12] established partial isothermal sections at 873, 1173, and 1473 K, and Paruchari and Massalski[13] examined the liquid-solid equilibria to produce a liquidus projection surface and also assessed the phase relationships at 1173 K. Finally, Hayes[14] has most recently generalized the Al-Ti-V ternary-phase diagrams, including the liquidus projection surface and a series of isothermal sections from 1073 to 1673 K. However, there is already evidence that current understanding of the ternary-phase equilibria is incomplete. The present authors have observed evidence of a eutectic reaction of the form liquid (L) → ␦ -Al3(Ti,V) ⫹ -Al8V5[15] that is not recognized on current phase diagrams, as well as an extended range of intermediate high-temperature intermetallic phases in the composition range between ␦ -Al3(Ti,V) and ␥ -TiAl.[16] Most established
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