Supertransus processing of TiAl-Based alloys
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I.
BACKGROUND
THE development of near-g Ti-aluminide alloys with useful engineering properties has been possible due to a better understanding of the effects of alloying additions, thermomechanical processing, and heat treatment on the microstructure and properties of TiAl-based alloys.[1,2,3] The ability to produce hypostoichiometric (e.g., Ti-48Al at. pct; unless otherwise noted, all compositions are in atomic percent) alloys with duplex microstructures resulted in significant improvements in tensile strength and ductility. However, duplex microstructures exhibit relatively low fracture toughness and creep resistance. Duplex microstructures typically contain fine equiaxed g-TiAl grains with low volume fractions (5 to 20 pct) of relatively fine lamellar g-TiAl/a2-Ti3Al grains and/or very fine equiaxed a2-Ti3Al grains and are generated by heat treatment in the a 1 g phase field, typically at temperatures ranging from 1275 7C to 1325 7C. If the alloy contains large amounts of b stabilizers (e.g., Cr, W, etc.), then limited amounts of fine equiaxed grains of b-Ti2AlX, where x 5 Cr, W, etc., are observed. Heat treatment above the a transus results in a fully lamellar microstructure comprised entirely of lamellar ggrains. Typically, fully lamellar TiAl/a2-Ti3Al microstructures have very coarse grain size, due to the rapid grain growth in the a-Ti single-phase field, and exhibit poor tensile strength and poor ductility.[1–4] However, the lamellar microstructure produces very high fracture toughness and creep resistance.[3] In addition, many alloys exhibit lamellar microstructures with serrated grain boundaries, which further improve creep resistance. Based on the superior tensile strength and ductility of the fine-grained duplex microstructure and superior toughness and creep resistance of the coarse-grained lamellar microstructure, a hybrid fine-grained lamellar microstructure would be expected to produce a material with balanced G.E. FUCHS, Advisory Engineer, is with the Advanced Structural Materials Department, Lockheed Martin Company, Schenectady, NY 12301-1072. 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
properties.[3] Three basic methods have been reported to develop the fine-grained lamellar microstructure, sometimes also referred to as refined fully lamellar (RFL) microstructures.[3] The first method was to heat treat the sample for very short times (i.e., on the order of a few minutes) above the a transus and use rapid cooling to form the lamellar microstructure. While at the heat treatment temperature in the a-Ti single-phase field, though, the heat treatment time must be limited to prevent excessive grain growth. However, accurate reproduction of control heating rate, cooling rate, hold times, and temperatures in samples with varying thic
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