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INTRODUCTION

NI-BASE superalloys are widely used for the high-temperature components of jet engines such as turbine blades, vanes, and discs, due to their excellent heat resistance. The thermal efficiency of jet engines has been improved by increasing the turbine inlet temperature. As the turbine inlet temperature increases, the creep temperature capability of superalloys for high-pressure turbine discs needs to be improved. A Ni-base turbine disc superalloy, ME501,[1] has the highest creep temperature capability of 1013 K (740 C) for a creep rupture life of 1000 hours at 650 MPa in reported powder metallurgy (P/M) alloys. A Ni-Co-base turbine disc alloy, TMW-4M3,[2,3] developed by the National Institute for Materials Science (NIMS), Japan, has the highest creep temperature capability of 983 K (710 C) among the reported cast and wrought alloys.

YUHI MORI, KYOKO KAWAGISHI, TOSHIO OSADA, HIROSHI HARADA, MICHINARI YUYAMA, YUJI TAKATA, MAKOTO OSAWA, and AYAKO IKEDA are with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 3050047, Japan. Contact e-mail: [email protected] Manuscript submitted October 11, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

In order to improve the creep temperature capability above those of the reported alloys described above, we will assume that Ni-base conventionally cast (CC) superalloys for turbine blades were made into P/M superalloys for high-pressure turbine discs because CC alloys have similar polycrystalline structure as turbine disc alloys and are expected to have higher creep deformation resistance than the existing turbine disc alloys. However, it is difficult to accurately compare the creep deformation resistances of CC and the turbine disc alloys due to significant differences between CC alloys and turbine disc alloys. The main difference is in the grain size. The grain size of CC alloys is much larger than that of the turbine disc alloys. For example, a CC turbine blade has an average grain size of 1.5 mm[4]; however, turbine discs have grain sizes of 3 to 130 lm.[5–7] Meanwhile, grain sizes usually influence the creep property of polycrystalline alloys. Moreover, disc alloys have different grain boundary strengths from CC alloys because the compositions and volumes of grain-boundary-strengthening precipitates such as carbides and borides vary with the alloy compositions. Thus, we will consider an original method of alloy composition screening using the creep property of a single crystal (SC), as it is not influenced by grain size and grain boundary strength. Furthermore, the alloy design program[8] developed by NIMS can be used in the

case of SC. The alloy design program can predict the creep rupture life and physical properties of Ni-base SC superalloys. However, the validity of the screening using SC creep property has not been confirmed yet. Therefore, this study aims to confirm the validity of the alloy composition screening using SC creep property for the design of new Ni-base P/M turbine disc superalloys with better creep resistance.

II.

EXPE