Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys
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INTRODUCTION
RECENT trends in the development of high-temperature structural materials for the gas turbine for aerospace applications have focused on the addition of increasingly high levels of refractory elements, such as Re and W, to singlecrystal Ni-base superalloys.[1–5] These elemental additions are extremely potent in enhancing the strength and creep performance of the alloy at low to intermediate temperatures (750 °C to 950 °C). At elevated temperatures (.1000 °C), however, W and Re additions tend to promote microstructural instabilities that lead to the rapid formation of refractory-rich topologically-close-packed (TCP) precipitates.[1,4,6–9] Precipitation of these deleterious phases removes the refractory solid solution strengthening elements from the constituent g-g9 phases. Consequently, creep properties have been shown to degrade rapidly as TCP formation occurs within the microstructure.[10,11] Since single-crystal Ni-base superalloy turbine blades are required to operate under a range of temperatures and stresses, the composition of the alloy has to be carefully engineered to provide a balanced set of properties. In many advanced high refractory content Ni-base superalloys, the microstructural instabilities occurring at elevated temperature severely limit the degree to which W and Re additions can be used to improve the low and intermediate temperature creep properties. Superalloys are engineering materials that often contain in excess of ten alloying additions. Due to the complex chemistries, predicting element interactions and the formation of equilibrium phases in this class of alloys is challengA.C. YEH, formerly Graduate Researcher, Rolls-Royce University Technology Partnership Materials Science and Metallurgy Department, University of Cambridge, Cambridge CB2 3QZ, United Kingdom, is Postdoctoral Researcher, The National Institute of Materials Science, Tskuba 305-0047, Japan. S. TIN, formerly Assistant Director of Research, RollsRoyce University Technology Partnership Materials Science and Metallurgy Department, University of Cambridge, is Associate Professor, Illinois Institute of Technology, Chicago, IL 60616. Contact e-mail: [email protected] Manuscript submitted January 24, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing. Furthermore, due to the low diffusivities of tungsten and rhenium in nickel at elevated temperatures, precipitation reactions involving these elements become sluggish and phases may precipitate as isolated plates or discontinuously with cellular morphologies.[6,7,9,10,12] The variety of TCP phases that may potentially form in Ni-base superalloys (P, m, R, and s) are typically characterized by close-packed layers of atoms forming ‘‘basketweave’’ sheets, which are often aligned with the octahedral planes in the fcc matrix. Similarities in the composition and crystallography of the TCP phases enables these precipitates to develop as mixed structures consisting of a number of different phases.[6,7,8] Although the chemistries of the TCP phases are dependent upon the composition of
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