Development of a necklace microstructure during isothermal deformation and its properties relative to uniform microstruc
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I.
INTRODUCTION
T H E primary use of powder metallurgy (PM) superalloys has been for rotating disks in modern gas turbine engines. The processing of a PM disk is carefully controlled to achieve a microstructure that provides an appropriate balance in the mechanical properties. The microstructure is an important consideration in determining the processing of the disk. A fine-grained microstructure (-~6 to 8/xm) is most commonly employed to achieve strength, as in the bore section of the rotating disk. A coarse-grained (-~30 to 50 /~m) microstructure is favored in some applications requiring improved crack growth and creep-rupture properties but at the expense of tensile properties. A third variation, the object of this study, is a duplex microstructure consisting of coarse warm-worked grains surrounded by a necklace of fine recrystallized grains. This so-called necklace microstructure is intended as a compromise grain structure to achieve a balance in mechanical properties t1"2"31by combining the high strength of f'me grains with the good creep and crack growth behavior of coarse grains. A necklace microstructure is obtained through control of thermomechanical processing, which is capable of producing a wide range of mixed grain structures. HI Microstructural variations ranging from fully recrystallized (fine grain) to partially recrystallized (duplex grain) to unrecrystallized (coarse grain) microstructures can be achieved depending on the temperature, strain rate, and total strain of the deformation process. For a necklace microstructure, the processing must achieve partial recrystallization and also meet the requirements of preferred grain boundary recrystallization and continued
P.R. BHOWAL, formerly Senior Scientist, Research and Development Department, Cameron Forge Company, is with Textron-Lycoming, Stratford, CT 06497. N.M. BHATHENA, Senior Research Metallurgist, is with the Research and Development Depa~nent of Cameron Forge Company, Houston, TX 77251-1212. Manuscript submitted April 30, t990. METALLURGICAL TRANSACTIONS A
recrystallization to prevent excessive growth of the newly formed grains. This recrystallization process is determined primarily by the distribution and morphology of the 31' precipitates t5'61 which, in turn, are dependent on the starting microstructure and the temperature of the deformation process. Strain rate and strain are other variables which influence the extent of recrystallization at the grain boundary. Since the effects of these variables are not fully understood at this time, one purpose of this study was to investigate how the starting structure and deformation process influence the development of necklace microstructures. The optimization of properties in a necklace microstructure is another area which has received little attention. In one study on a wrought superalloy, t31 a necklace microstructure was shown to possess tensile properties comparable to a fine-grained microstructure while achieving superior sustained peak low-cycle fatigue (649 ~ residual cyclic life (538
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