Effect of fluid flow and hafnium content on macrosegregation in the directional solidification of nickel base superalloy
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I.
INTRODUCTION
NICKELbase
superalloy castings are extensively used in gas turbine engine applications. In order to improve the resistance to grain boundary cavitation and thermal fatigue of these cast components, directional solidification processing techniques are currently applied. In directional solidification, the casting is produced with a columnar grain structure in a desirable orientation. The columnar grain structure gives rise to the improved mechanical properties. Directionally solidified MAR-M200 superalloy is one of the best suited alloys for turbine blades, based on its excellent high temperature strength. ~ The equilibrium solidification microstructure of the MAR-M200 alloy (Table I) consists of the primary, face-centered cubic phase, y, containing small amounts of carbides. 2'3 Under nonequilibrium solidification conditions, a small amount of eutectic forms. 1.4 In order to improve the transverse ductility in tension and in creep rupture at low (1033 K) to intermediate (1253 K) temperatures of cast MAR-M200 alloy, hafnium additions have been made to the alloy system.5 However, the directionally solidified turbine blade castings of hafnium modified MAR-M200 alloys (Table I) may exhibit a large variation in the as-cast microstructure, especially in the distribution of nonequilibrium eutectic.4 A consequence of this microstructural variability, if uncompensated, would be undesirable variations in mechanical properties. During dendritic solidification, the solute elements preferentially segregate either to the liquid or to the solid phase. The segregation of solute elements determines the as-cast microstructure. The as-cast microstructure is usually characterized by the presence of dendrites, eutectics, and carbides and by their size, volume percent, and distribution. The segregation behavior is generally classified as microsegregation or macrosegregation depending on the scale of the nonuniformity of the distribution of the solute elements, *MAR-M is a trademark of Martin Marietta Company. R. SELLAMUTHU is Research Associate, Sherman Fairchild Center 161, Lehigh University, Bethlehem, PA 18015. H. D. BRODY is Professor of Materials Engineering, University of Pittsburgh, Pittsburgh, PA 15261. A. E GIAMEI is Principal Scientist, United Technologies Research Center, East Hartford, CT 06108. Manuscript submitted November 23, 1983. METALLURGICALTRANSACTIONS B
i.e., the scale of the dendrite arm spacing or hundreds of times the dendrite arm spacing, respectively. In this work, the macrosegregation behavior (i.e., the distribution of solute elements and eutectic) of the MARM200 and the hafnium-modified MAR-M200 superalloys was investigated. In addition to the application of this research to nickel base superalloys, it is of general interest to investigate the macrosegregation behavior of complex alloys under controlled laboratory conditions and to develop models capable of predicting, at least semiquantitatively, the extent of macrosegregation occurring in complex alloys. The objectives of this study were
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