Microstructural instabilities during superplastic forging of a nickel-base superalloy compact
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R E C E N T L Y , opportunities for improvements in the properties of materials for aerospace applications have arisen from developments in new processing technologies, particularly in the area of powder metallurgy and in the applications of superplastic forming. 1,2These developments concern the high temperature materials for propulsion gas turbines, mainly the nickel-base superalloys, as well as the structural alloys based on titanium. Traditionally, improvements in the properties of these materials have derived from advances in alloy composition and heat treatment2~ In many instances the limits of this approach have been reached? It is becoming increasingly evident, however, that further progress can be achieved by control of the microstructure during thermomechanical processing?,5.6 The combination of powder metallurgy processing and isothermal forging offers design freedoms and opportunities for microstructural control not possible with more conventional forming operations. Control of the microstructure can be exercised both during consolidation of prealloyed powders 7-9 and during the forging of compacts. 3.s-12The factors which influence control of the microstructue during compaction have been extensively examined and are well understood.7,8 However, property control during isothermal forging has not been explored to the same extent, in spite of the potential offered by this approach) 3,14By appropriate control of the forming variables, it should be possible, in principle, to produce forgings with predictable variations in microstructure across the part. This tailoring of structures and properties to design requirements J-P. A. IMMARIGEON, Research Officer, and P. H. FLOYD, Technical Officer, are with the National Aeronautical Establishment, National Research Council Canada, Ottawa. Manuscript submitted October 1, 1979. METALLURGICAL TRANSACTIONS A
remains, however, a relatively unexplored area. This can be attributed to a lack of understanding of the hot working behavior of these types of alloys, which stems in turn from the inherently complex relationship between forming variables and structures developed during working. The success of this development will therefore require a detailed understanding of the physical phenomena involved during hot working of these materials. In this work, the high temperature flow behavior of a nickelbase superalloy compact is examined. The compressive flow curves generated under conditions of constant true strain rate and constant temperature are examined in terms of the microstructures developed during working. The role of grain size during superplastic flow is analyzed, and the behavior of the alloy rationalized in terms of microstructural instabilities. EXPERIMENTAL MATERIAL The compact examined had been prepared previously .5 by hot isostatic pressing from a commercial powder of Inco 713LC, a high strength casting type nickel-base superalloy. In producing this material, attempts had been made to maximize the hot workability of the consolidated product by appropriate cho
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