The Influence of Cobalt on the Microstructure of the Nickel-Base Superalloy MAR-M247
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INTRODUCTION
N I C K E L - b a s e superalloys are complex alloys which consist of as many as a dozen elements. Throughout the past several years, an understanding of the role of most of these elements has been attained. However, the role of cobalt in Ni-base alloys is not fully understood. Recent increases in price and limitations in supply ~provide considerable incentive to improve the understanding of the influence of Co, and subsequently, based on this understanding, to minimize the use of Co in Ni-base superallo~s. Nickel-base superalloys consist of a nickel-rich fcc solid solution, 7, an ordered fcc precipitate, 3", and various minor phases including carbides, borides, and topologically close packed (TCP) phases such as o', Ix, and Laves. It has been shown that Co can influence the formation of all of these phases. However, most of the previous studies on the role of Co involved simple binary and ternary alloys, rather than the complex compositions of modern superalloys. The influence of Co on the y phase has been reported by several workers. Because Co is only slightly different in atomic size from Ni, it does not provide much solid solution strengthening. 2'3 However, it has been argued that Co significantly increases the long-time elevated temperature strength by reducing stacking fault energy of 7. 4,5 The presence of Co can also alter the 11' phase formation. Depending on the overall composition, increased Co content can either raise or lower the y' solvus temperature 6 and can also increase the 3" volume fraction. 6,7 In addition, Co, which replaces the Ni lattice sites in the 3", appears to be a weak solid solution hardener of this phase, s Increasing the Co level in Ni-base superalloys reduces the amount of carbide precipitation 6 by increasing the amount of carbon dissolved in the matrix. The type of carbide present may also be a function of Co level. 7'9 Cobalt also affects the formation of TCP phases, such as cr and Ix. Additions of 5
M. V. NATHAL, formerly of Case Western Reserve University, is now a Research Metallurgist, NASA-Lewis Research Center, Cleveland, OH 44135. R.D. MAIER, formerly of Case Western Reserve University, is now Principal Metallurgist, Chase Brass and Copper Company, Solon, OH 44139. L.J. EBERT is Professor of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted October 28,. 1981. METALLURGICALTRANSACTIONS A
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to 10 pct Co prevent o- formation in MAR-M421, but further Co additions do not provide additional benefits.2~ At least 5 pct Co is required to prevent the formation of tx in Ni-base superalloys such as MAR-M247 which are stripped of carbon for single crystal applications.tJ Thus, cobalt has a significant influence on the phases present in nickel-base superalloys. The amounts, compositions, and morphologies of the y , y', carbide, and TCP phases have all been shown to be influenced by the Co level in the alloy. However, the influence of Co varies as a function of the overall composition of the alloy. The pur
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