General aspects of the thermomechanical
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I.
INTRODUCTION
AT present, there is a great interest in the development of high-temperature materials especially for advanced applications in heat protection and propulsion components. Because of its high melting temperature, low density, high elastic modulus, strength up to high temperature, and good creep resistance, y-TiA1 plays a key role in actual material research programs, tq Recent investigations show that the maximum application temperature is basically determined by oxidation resistance (below 900 ~ but a serious hindrance for the application of y-TiAl below this limiting temperature appears to be its poor formability and room-temperature brittleness. The mechanical behaviour of ~/-TiA1 depends strongly on microstructure. Single-phase y-TiA1 suffers from grain growth combined with very poor ductility and fracture toughness, t3,41 A much better ductility at room temperature is found in dual-phase material with an aluminum content of 48 at. pct. [3,4] There, the secondphase a2-Ti3Al is expected to serve as a getter material for oxygen impurities. ]5I Ternary additions, such as Cr, Mn, or V, are often found to improve the ductility. L31 Additions of Nb, Ta, or W improve the oxidation resistance, but unfortunately, they reduce ductilityJ 31 The materials investigated in the present work are binary Ti48A1 and ternary Ti-48A1-2Cr. All compositions are given in atomic percent. The four types of microstructure for these two-phase materials are the dualphase equiaxed, the duplex, the near lamellar, and the fully transformed microstructure.[41 Any of these microstructures can he achieved by thermomechanical treatment (TMT). C. KOEPPE, A. BARTELS, and H. MECKING are with the Technical University of Hamburg-Harburg, Division of Physics and Technology of Materials, D-21071 Hamburg 90, Federal Republic of Germany. J. SEEGER, formerly with the Technical University of Hamburg-Harburg, is with Ohio State University, Department of Materials Science and Engineering, Columbus, OH 43210. Manuscript submitted September 18, 1992. METALLURGICAL TRANSACTIONS A
In general, the TMT can be divided into three major steps: the starting heat treatment, the hot working process, and the subsequent heat treatment. The thermomechanical treatments can follow different paths through these processes and repeating one or several steps can be useful. The aim of TMTs is to transform the microstructure after casting to provide suitable properties for further forming operations and/or application purposes. For instance, best ductility and workability can be achieved with equiaxed microstructures, whereas good creep resistance is found in the fully transformed, i.e., lamellar, microstructure, t41 It is believed that both good ductility and creep resistance can be achieved by the duplex microstructure optimized with respect to grain size and lamellar to globular ratio, t31 The present work deals with equiaxed and near lamellar microstructures and represents an attempt to ductilize y-TiA1 for further forging or rolling processes.
II.
EXPERIMENT
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