High refractory, low misfit ru-containing single-crystal superalloys
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demand for developing increased temperature capability and improved creep resistance of nickel-base superalloys has led to alloys with increasing amounts of refractory alloying additions. These additions have dramatically improved the creep resistance of singlecrystal superalloys but have also resulted in an increased propensity for formation of topologically close-packed (TCP) phases.[1,2] The precipitation of these refractory-rich phases is detrimental to the mechanical properties of superalloys due to depletion of the matrix of solid solution strengthening elements and due to the fact that the precipitates serve as damage accumulation sites. Early studies on Ru additions to nickel-base single-crystal superalloys found that Ru additions reduce the tendency for formation of TCP phases,[3–6] increase the liquidus temperature,[3,7] and improve the creep rupture strength at 1100 °C.[5,8] However, the mechanism by which Ru additions improve the hightemperature capability of single-crystal superalloys is still not well understood. Furthermore, the role of Ru in superalloys has not been investigated over a broad range of compositions.[6,8–14] Early work on Ru-containing nickel-base superalloys suggested that Ru additions alter the partitioning of elements between the g and the g9 phases.[4] The observation that Ru additions result in Re partitioning less strongly to the matrix[4] is of particular importance in terms of the g-g9 lattice misfit. There has been considerable disagreement on whether Ru causes ‘‘reverse partitioning’’ of elements. InvesL.J. CARROLL, Lead Engineer, is with GE Aviation. Contact e-mail: [email protected] Q. FENG, formerly Senior Research Fellow, Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109, is Professor, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. J.F. MANSFIELD, Associate Research Scientist, and T.M. POLLOCK, L.H. and F.E. Van Vlack Professor, are with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted January 14, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
tigations on elemental partitioning in Ru-containing singlecrystal superalloys have also been limited and have not considered a broad range of compositions.[6,10–12,14] The Re partitioning ratio, defined as the ratio of the Re (on atomic pct basis) in the g phase compared to the g9, in Ru-containing experimental alloys with approximately 2 to 3 wt pct Cr has been measured to be greater than 10.[10–12,14] Therefore, the observed partitioning behavior is similar to commercial superalloys.[11,15–18] Interestingly, Ofori and co-workers studied platinum group additions to nickel-base superalloys without Cr and found that increasing Ru additions from 0 to 5 at. pct resulted in the g-g9 partitioning ratio of Re decreasing from approximately 6 to 7 to less than 3.[13] To date, this is the only study, besides the initial work, sugges
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