The influence of cobalt, tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys
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I.
INTRODUCTION
S E V E R A L alloying elements in nickel-base superalloys, such as Co and Ta, have been classified as strategic materials as a result of limitations in their supply. ~ Consequently, the effects of these elements on the microstructure and properties of nickel-base superalloys has received renewed attention. Cobalt in particular has been studied extensively, and reviews of early work can be found in references. ~,2.3These early studies found that additions of 10 to 20 wt pet Co were beneficial for creep-rupture properties in most cases. These property changes were related to the influences of Co on the amount of 3", the amount and type of carbides, stacking fault energy, the formation of grain boundary denuded zones, and the formation of TCP phases. Recent work on more modern polycrystalline superalloys has confirmed many of the earlier conclusions. In studies of superalloys which had a range of 3/ volume fractions, WASPALOY, 4.* UDIMET-700, 5. and MAR-M247, 2'3 it **WASPALOY is a trademark of United Technologies Corporation. 'UDIMET is a trademark of Special Metals Corporation.
was found that Co additions generally increased the creep resistance of the alloys but had only minor effects on tensile properties. However, it did appear that Co levels could be reduced to at least one-half of the standard levels in these alloys. Again, these results were related to 3`' volume fraction and particle size distributions, carbide type and morphology, and stacking fault energy. The present investigation extends the previous work to include single crystal superalloys. These alloys do not require the grain boundary strengtheners C, B, Zr, and Hf. The absence of grain boundary elements can change the roles of the other alloying elements. For example, in polycrystalline MAR-M247, most of the Ta is present in the MC *MAR-M is a trademark of Martin Marietta Company. M. V. NATHAL, Research Metallurgist, is with NASA Lewis Research Center, Cleveland OH, 44135. L.J. EBERT, Professor of Metallurgy and Materials Science, is with Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted November 15, 1984. METALLURGICAL TRANSACTIONS A
carbides; whereas in the single crystal version, C is absent and the Ta is free to partition between the 3` and 3`' phases. Therefore, the influence of Co and Ta on the microstructure of single crystal MAR-M247 was examined. Tungsten was also studied as a substitute for Ta. The influence of these elements on the tensile and creep strength of these alloys is presented in a companion paper. 6
II.
MATERIALS AND PROCEDURES
Eight single crystal compositions were directionally solidified by the withdrawal process. The chemical analyses of these alloys are given in Table I. Alloy G is the standard MAR-M247 stripped of C, B, Zr, and Hf. Cobalt was replaced by Ni to form Alloy E with 5 pet Co and Alloy B with 0 pct Co where all compositions are in weight percent unless otherwise noted. The role of Ta was investigated by replacing Ta with Ni at each Co level, producing Alloys A, D
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