Processing-structure characterization of rheocast IN-100 superalloy
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I.
INTRODUCTION
THE development of
superalloys reflects a growing demand for materials with superior elevated temperature property response. Since turbine disk operating temperatures are generally as high as 760 ~ (1400 ~ t enhancement in the overall performance of rotating disk engine components requires improvements in high temperature tensile strength and resistance to fatigue failure. These properties may be achieved by alloy design, but improved processing of existing alloys may also be important 2 since solidification processing can dramatically modify the final microstructure. The vast majority of commercially produced castings exhibit a dendritic structure. Control of the cast structure is the underlying objective in superalloy manufacturing processes. Since nickel-base superalloys for turbine disks, such as IN-100, MERL 76, and RENE 95,* contain a large *IN-100, MERL 76, and RENI~ are trademarks of the INCO family of companies, United Technologies Corp.-Pran&Whitney Group, and General Electric Company, respectively.
amount of various alloying elements they freeze over a wide temperature range. Accordingly, these alloys have a tendency to segregate as well as to hot-tear during the "solidification journey" through the mushy zone. The result is a heterogeneous cast structure that makes subsequent forging for grain size control extremely difficult if not impossible. Therefore the width of the mushy zone as well as the amount of time spent in the mushy zone (the local solidification time) are two important parameters which need to be controlled during solidification pi'ocessing.3 One technique is to resort to powder metallurgy (P/M) processing. Since each JUNG-JEN ALLEN CHENG, Research Associate, DIRAN APELIAN, Professor and Department Head, and ROGER D. DOHERTY, Professor, are with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. Manuscript submitted December 11, 1985. METALLURGICALTRANSACTIONS A
powder particle is a "microcasting", macrosegregation is essentially eliminated and due to the fine dendritic spacing, homogeneity is readily attained during sintering. The powders must be consolidated by hot isostatic pressing (HIP), extrusion or forging, e.g., Gatorizing. 4 The drawback is that P/M processing requires powder handling that is both capital and labor intensive. In addition, high operating stress levels and long operating lives require a high level of material cleanliness to reduce inadvertent inclusions in the final component. 5 An alternative approach is to produce a fine grain and more homogeneous microstructure by rheocasting (stircasting). Ingots of fine-grained structure with reduced shrinkage and segregation can be produced by the rheocasting process. 6 These structural characteristics are factors which could improve the properties and performance of the turbine disk. They may also permit the direct processing of ingots to shaped components. In this investigation, the objective was to study the effect of processing variables on the resultant rheocast struc
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