Changes in microstructure during primary creep of a Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si alloy
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THE Ti-Al intermetallic alloys are potential replacements for superalloys in some high-temperature applications in heat engines, turbine engine blades, and aerospace and automobile components, since it has high specific strength at high temperatures.[1,2,3] The microstructure is affected by composition and heat treatment, and creep properties are sensitive to microstructure. Gamma titanium aluminide can be heat treated to obtain four types of microstructure.[4] The best creep resistance has been observed in fully lamellar microstructures.[5] However, the duplex microstructure is preferred for many applications, because it provides a desirable combination of room-temperature ductility and toughness.[2] The additions of Cr, Nb, V, Mn, W, Mo, and Si in twophase Ti-Al alloys affect many material properties. Additions of 1 to 3 at. pct of Cr, V, or Mn can improve the ductility of duplex alloys,[6–11] but V generally reduces the oxidation resistance.[12] Niobium greatly enhances the oxidation resistance[13,14,15] and slightly improves the creep resistance.[14] The addition of 0.5 to 1 at. pct Si enhances the creep strength, oxidation resistance, and room-temperature fracture toughness of 48 at. pct Al two-phase alloys.[16–20] Silicon also increases fluidity and reduces the susceptibility to hot cracking.[21] Molybdenum provides a good balance D.Y. SEO, Graduate Student, and T.R. BIELER, Associate Professor, are with the Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226. S.U. AN, Principal Researcher, is with the Advanced Analysis Center, KIST, P.O. 131, Seoul, Korea. D.E. LARSEN, Manager, Materials Technology, is with the Advanced Technology Division, Howmet Corporation, Whitehall, MI 49461. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
of strength and ductility in TiAl, having very fine equiaxed gamma grains with the a2 and b phase.[22] The addition of W greatly improves oxidation resistance and enhances creep resistance.[18] Most of these studies have focused on either room-temperature properties, minimum creep-rate conditions, or stress-rupture properties. However, primary creep resistance is important for practical applications. Gamma-based TiAl exhibits primary creep strains that are typical for metals, where the minimum is reached after about 1 pct strain. In an effort to decrease the creep rate during primary creep deformation, additions of W, Mo, and Si have significantly increased the time to reach 0.5 pct strain. A dynamic precipitation process has been indicated in References 13 and 24. Observations of lamellar refinement during primary creep suggest that a significant amount of mechanical twinning occurs parallel to lamellar interfaces as an easy mode of deformation, and this hardens
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