Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma ti
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M aluminide alloys containing 45 to 50 at. pct aluminum and small additions of chromium, niobium, manganese, or vanadium have been developed primarily for aerospace and automotive applications. These alloys contain a mixture of fct ␥, ordered hcp ␣2, and, sometimes, a small volume fraction of ordered bcc 2 phase. They may exhibit a variety of microstructures, depending on the specific synthesis, deformation, and heat-treatment procedures used in their manufacture.[1,2,3] These structures range from those composed of fine, equiaxed ␥ grains in a matrix of ␣2 to those consisting of fully lamellar (␥ ⫹ ␣2), polycrystalline aggregates. While lamellar microstructures have been exploited to provide superior service properties such as fracture toughness, tensile and creep strength, and fatigue resistance,[4] the equiaxed microstructures are quite attractive from a processing viewpoint. For example, the success of many secondary processing operations such as precision closed-die forging, sheet rolling, and superplastic forming depends critically on the ease with which fully equiaxed, fine grain structures can be obtained and preserved during processing.[2,3,5] Furthermore, lamellar as well as duplex structures can be obtained readily from equiaxed structures via simple heat treatments. V. SEETHARAMAN, formerly Senior Scientist, UES, Inc., Dayton, OH 45432, is Staff Engineer, Materials & Processes Engineering, Pratt & Whitney, East Hartford, CT 06108. S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Contact e-mail: [email protected] Manuscript submitted March 11, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
Uniform, fine-grain equiaxed microstructures can be developed in titanium aluminide alloys via ingot or powder metallurgy routes. Both involve a series of deformation and heat-treatment steps. In the former approach, cast or cast ⫹ hot isostatically-pressed (hipped) ingots containing predominantly coarse lamellar structures are broken down through isothermal forging, canned extrusion, or canned (conventional) hot forging in the high-temperature alpha-phase field or below the alpha transus (at which ␣ → ␣ ⫹ ␥ ).[1,3] Postdeformation heat treatments in the temperature range from 900 ⬚C to 1200 ⬚C serve to complete or enhance the transformation from a lamellar to an equiaxed microstructure.[1] However, the kinetics of the transformation during primary hot working are strongly influenced by the chemical composition, characteristic features of the lamellar starting structure, and the details of thermomechanical processing. For example, it is well established that the lamellae become increasingly resistant to spheroidization with a decrease in the aluminum content of the alloy.[6] Semiatin et al.[1,3] have described several novel methods to accelerate the dynamic spheroidization as well as refinement of the structure. In spite of these studies, a complete and quantitative description of
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