Microstructure development during conventional and isothermal hot forging of a near-gamma titanium aluminide
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INTRODUCTION
THE expanding interest in intermetallic alloys has generated an acute appreciation of the relationship between processing and microstructure. Although these materials possess attractive high-temperature strength and creep properties, ambient-temperature ductility and toughness are often quite low and typically very sensitive to microstructure. 1~'2~ Prime examples of such behavior are found for alloys based on the face-centered tetragonal gamma phase of the titanium-aluminum system. So-called near-gamma titanium aluminides, which contain small to moderate amounts of the ordered hexagonal close-packed alpha-two phase, may exhibit a variety of microstructures, depending on the specific synthesis, deformation, and heat-treatment methods used in their manufacture. These structures range from those composed of a majority of equiaxed gamma grains to those consisting of fully transformed morphologies of lamellar alpha-two plus gamma phases. The former structure is usually more attractive from a processing standpoint in view of the fact that the latter structure, as well as various duplex (gamma grain-lamellar colony) structures, can be obtained from it via rather simple heat treatments. Uniform, fine gamma grain microstructures in neargamma titanium aluminides can be obtained via ingot metallurgy processing through a series of deformation and heat-treatment steps on cast or cast plus hot isostatic pressed ("hipped") material with a lamellar-starting S . L SEMIATIN, Senior Scientist, is with the Metals and Ceramics Division, WL/MLLN, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433. V. SEETHARAMAN, Senior Scientist, is with UES, Inc., Dayton, OH 45432. V.K. JAIN, Professor, is with the Mechanical and Aerospace Engineering Department, University of Dayton, Dayton, OH 45469. Manuscript submitted September 16, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
microstructure. Typical deformation modes for breakdown of ingots include isothermal forging or canned, conventional hot extrusion. ~ Limited work has also been conducted to determine the feasibility of uncanned and canned conventional hot pancake forging, t3'7-91 Uncanned hot forging (with die temperatures around 260 ~ has been generally unsuccessful, resulting in gross fracture at low reductions, t31 More success has been achieved through canned hot pancake forging of cylindrical ingot mults. I7,8,9J For example, Wurzwallner and his co-workers t7,8,91forged Ti-48A1-2Cr (atomic percent) ingots to approximately 70 pct reduction in a hydraulic press. After decanning, the pancakes revealed "flaky" and sporadically cracked plan surfaces, noticeable dead-metal zones at the center of the pancakes, and very nonuniform cross sections due to flow differences between the can and the titanium aluminide workpiece. Deformation processes often produce partially recrystallized microstructures in cast or cast plus hipped near-gamma titanium aluminide ingot materials with lamellar starting structures. Stored work from such processes is o
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