A comparison study of microstructure and mechanical properties of Ti-24Al-14Nb-3V-0.5Mo with and without Si
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I. INTRODUCTION
OF the intermetallic alloys currently targeted for hightemperature structural applications, Ti3Al-based alloys are relatively mature, with potential weight-saving use in gas turbine engines up to 650 8C to 700 8C.[1–5] Improvement in room-temperature ductility and elevated-temperature strength has been achieved by the incorporation of the ductile body centered cubic (bcc, or b) phase by alloying with b stabilizers such as Nb, Mo, and V.[1–3] Recently, a new class of titanium aluminide alloy, based on the ordered orthorhombic phase Ti2AlNb, has been developed in both monolithic and composite form.[6,7] The ordered orthorhombic (O) phase based on Ti2AlNb was first discovered by Banerjee et al.[8] in a Ti-25Al-12.5Nb (at. pct) alloy and was later found in many alloys containing more Nb. These alloys contain a significant amount of the O phase as well as the ordered bcc (B2) phase and the ordered hexagonal (a2) phase. The superior creep resistance of the O phase,[9] together with the room-temperature ductility conferred by the multiple slip systems,[10] has aroused great interest. Orthorhombic alloys with microstructures of (a2 1 O 1 B2) or (O 1 B2) were developed and reported to have superior room- and elevated-temperature mechanical properties to those of the conventional (a2 1 b) alloys, even on a density-corrected basis.[6,11–15] Compositions with a promising combination of strength, ductility, toughness, and creep resistance were identified in the range of 20 B. LU, Postdoctoral Student, R. YANG, Professor and Head of Laboratory, Y.Y. CUI, Associate Professor, and D. LI, Professor Emeritus, are with the Titanium Alloy Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. Manuscript submitted June 10, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
to 26Al and 15 to 30Nb (at. pct). Recently, the evolution of orthorhombic alloys has been from first-generation alloys such as Ti-25Al-17Nb and Ti-22Al-23Nb to second-generation alloys such as Ti-22Al-27Nb.[16] However, the increasing amount of niobium is attended by the increased density and complexities of phase transformations in the alloys.[17] As a consequence, the establishment of relationships between phase constitution, properties, and solidification homogeneity becomes more difficult. Moreover, the maximum oxidation resistance was found in the Nb range of approximately 10 to 15 (at. pct) in various Ti-Al-Nb alloys; both lower and higher Nb levels resulted in accelerated oxidation.[18] Silicon has been considered to be beneficial to creep properties of titanium alloys in solid solution and in the form of silicide precipitates.[19,20] Ti5Si3 (z) phase was reported to have a complex D88 hexagonal structure (a 5 0.5143 nm and c 5 0.7444 nm), with a high melting point (2130 8C) and a Vickers hardness of 968 6 30 Hv.[21] This silicide is also a brittle phase, with a room-temperature fracture toughness as low as 2.1 MPa !m.[22] The approach of incorporating Ti5Si3 into the Ti3Al-based alloy
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