Soldification segregation in ruthenium-containing nickel-base superalloys
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ction materials developments have made substantial contributions to the performance of both aircraft engines and land-based gas turbines. For example, advances in directional solidification technology for Ni-based superalloy blades have provided single-crystal components with higher temperature resistance. As a result of developments in both alloy chemistry and casting technology, the temperature capabilities of high-pressure turbine blades have increased by more than 125 "C in the past approximately 30 years. Recently, a fourth generation of single-crystal alloys with ruthenium as a new alloying addition has been under development.[1,2,3] Ruthenium is one of the four elements (in addition to tungsten, rhenium, and iridium) that increase the liquidus and solidus temperatures of nickel.[4] The expense of iridium limits the practical application of Ir-containing Ni-base superalloys, whereas W and Re are already present in these materials. Ru additions to binary Ni-Al alloys and multicomponent Ni-base superalloys have been shown to increase liquidus temperatures,[3,5] suggesting that Ru is a promising alloying addition for improvement of temperature capability. Ru additions to Ni-base Q. FENG, formerly Senior Research Fellow, with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109, U.S.A., is now a Professor with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] L.J. CARROLL, formerly Ph.D. Candidate, with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109, U.S.A., is now a Lead Engineer with the Materials and Process Engineering Department, GE Aviation, Evendale, OH 45215, U.S.A. T.M. POLLOCK, L.H. and F.E. Van Vlack Professor, is with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109, U.S.A. Manuscript submitted June 16, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
superalloys have also been reported to increase microstructural stability during high temperature exposures and lower the propensity for the formation of topologically closepacked phases, thus improving the stress rupture life.[1] The creep behavior of Ni-base single-crystal superalloys is closely associated with the lattice misfit of coherent g and g# phases, which depends on the chemical compositions of these phases. Ru additions have been found to influence the partitioning behavior of the constituent elements to the g or g# phases compared to Ru-free superalloys.[1,3] Thus, the creep properties of some experimental Ni-base single-crystal superalloys have been reported to improve significantly with Ru additions.[2,6] Ru additions to Ni-base superalloys may also have the advantage of lowering the density. However, to date, investigations on Ru-containing Ni-base superalloys have been conducted over a relatively narrow range of composition. Also, little is known about their tendency to promote or reduce grai
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