Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfuri
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Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfurization SATOSHI UTADA, YUICHIRO JOH, MAKOTO OSAWA, TADAHARU YOKOKAWA, TAKUYA SUGIYAMA, TOSHIHARU KOBAYASHI, KYOKO KAWAGISHI, SHINSUKE SUZUKI, and HIROSHI HARADA The direct and complete recycling method for Ni-base superalloy is being developed and studied to reduce the material cost for cost-effective operation of gas turbine systems. Understanding the effect of sulfur contamination is important to determine allowable sulfur content after the recycling. However, in the case of single-crystal superalloys, this effect on material properties is not well known except for the detrimental effect on the oxidation resistance. In the present study, creep tests, aging tests, and cyclic oxidation tests have been performed on PWA1484 with varying sulfur content. The increasing sulfur content has been found to correlate with degradation of properties evaluated here. It is observed that the decrease in creep life in PWA1484 due to sulfur doping is primarily due to coarsening of the c/c¢ interfacial dislocation network, increase in precipitation kinetics of topologically closed-packed phase, and decrease in oxidation resistance. For recycling purposes, a CaO crucible was used in the casting process, which successfully decreased the sulfur level in the alloy, and the resulting material showed comparable or even better properties in comparison to the low sulfur content material. https://doi.org/10.1007/s11661-018-4710-4 The Minerals, Metals & Materials Society and ASM International 2018
I.
INTRODUCTION
THE Ni-base single-crystal superalloys have a wide range of applications from turbine blades to vanes of aircraft jet engines and other gas turbine systems due to their superior mechanical properties and oxidation resistance in the high-temperature environment. Ni-base single-crystal superalloys with a very good combination of high-temperature creep resistance and oxidation properties[1] and the alloys with the world’s best high-temperature capability up to 1409 K (1136 C)[2] have been successfully developed in the past. Although they have a huge potential to improve the thermal efficiency of the gas turbines, raising the weight ratio of rare metals, such as Re, Ru, and Ir, causes an increase material cost. As a result, widespread commercial SATOSHI UTADA, YUICHIRO JOH, and TAKUYA SUGIYAMA are with Waseda University, Shinjuku, Tokyo 1698555, Japan, and also with the National Institute for Materials Science (NIMS), Tsukuba Science City, Ibaraki 305-0047, Japan. Contact email: [email protected] MAKOTO OSAWA, TADAHARU YOKOKAWA, TOSHIHARU KOBAYASHI, KYOKO KAWAGISHI, and HIROSHI HARADA are with the National Institute for Materials Science (NIMS). SHINSUKE SUZUKI is with Waseda University. Manuscript submitted March 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
application of such high-performance alloys tends to be avoided. An effective solution to the current material cost issue is the ‘‘direct and complete
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