Oxidation Resistance Improvement of Ni-Base Single-Crystal Superalloy Melted in a CaO Crucible
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INTRODUCTION
NICKEL-BASE single-crystal superalloys are used for high-pressure turbine blades of jet engines and gas turbines, and further improvement of temperature capability is demanded to keep up with the rise of turbine inlet gas temperature. Kawagishi et al. developed the 6th generation Ni-base single-crystal superalloy, TMS-238,[1] which has the highest temperature capability in history and also has comparable oxidation resistance to 2nd generation
TAKUYA SUGIYAMA is with the Department of Applied Mechanics and Aerospace Engineering, Waseda University, Tokyo 169-8555, Japan, and also with the National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan. Contact email: [email protected] SATOSHI UTADA is with the National Institute for Materials Science (NIMS), and with the Safran Aircraft Engines, Rond-Point Rene´ Ravaud - Re´au, 77550 Moissy-Cramayel, France, and also with the Institut Pprime, UPR CNRS 3346, Physics and Mechanics of Materials Department, ISAE-ENSMA BP 40109, 86961 Futuroscope-Chasseneuil Cedex, France. TADAHARU YOKOKAWA, MAKOTO OSAWA, KYOKO KAWAGISHI, and HIROSHI HARADA are with the National Institute for Materials Science (NIMS). SHINSUKE SUZUKI is with the Department of Applied Mechanics and Aerospace Engineering, Waseda University, Tokyo 169-8555, Japan, and also with the Kagami Memorial Institute for Materials Science and Technology, Waseda University, Tokyo 1620051, Japan. Manuscript submitted December 27, 2018. Article published online June 3, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
single-crystal superalloys.[1] The very high creep durability of 4th[2,3] and 5th[4] generation single-crystal superalloys is owed to higher contents of refractory elements, such as Mo, Re, and Ru, which form the oxides with relatively higher vapor pressures, and this disturbs the continuity of the Al2O3 layer on the surface during high-temperature oxidation.[1,3,5] Therefore, it is difficult to optimize the alloy chemistry to improve both the creep strength and the oxidation resistance at the same time. However, by modifying the contents of Cr, Mo, W, and Ta in 4th and 5th generation single-crystal superalloys, TMS-238 successfully retained both creep and oxidation properties at a high level.[1,6] MC-NG is another example of a Ru-containing 4th generation single-crystal superalloy with good oxidation resistance.[7] A possibility of further improvement of creep temperature capability was shown by adding Ir to Ni-base single-crystal superalloys as a new alloying element.[8] Still, a method to improve oxidation resistance while maintaining creep properties is an important issue. In the past, special efforts were made to understand the effect of minor S content on the oxidation properties of Ni-base superalloys, and it is widely known that S can cause severe oxidation resistance degradation.[9–13] According to the research on S distribution, S tends to segregate onto the metal-oxide interface, and therefore, it decreases adherence of oxide scales.[14–17] Two types of approaches were
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