Creep properties of a new Re free single crystal Ni-based superalloy, NKH71

¹ National Defense Academy, Department of Mechanical Systems Engineering, 1-10-20 Hashirimizu, Yokosuka, Japan
² Nagoya University, Department of Materials Science and Engineering, Graduate School of Engineering, Furo-cho, Chikusa, Nagoya, Japan
³ Mitsubishi Hitachi Power Systems Ltd., 3-1-1 Sachi-machi, Hitachi, Japan

^a Corresponding author: kondo@nda.ac.jp

Abstract

The creep of the newly developed Ni-based, Re free, single crystal superalloy NKH71 was examined at 1273 K. The creep properties were compared to those of CMSX-4, a second-generation commercial superalloy containing 3%Re. When the (applied) stress was higher than 160 MPa, the rupture lives of the present alloy were shorter than those of CMSX-4. However, when the stress was below 160 MPa, the rupture lives of the present alloy became longer than those of CMSX-4. It was noted that this lower stress level was more important when Ni-based superalloys were used for engineering land-based gas turbines. TEM and SEM observations showed that NKH71 formed a rafted structure in the early stage of creep. This was because a large number of dislocations existed in the γ/γ′ interfaces before creep. In contrast, CMSX-4 had very few dislocations before creep. This observation explained why NKH71 was inferior to CMSX-4 under high stress. However under lower stress, the γ/γ′ rafted structure changed to a more complex morphology in CMSX-4, while that of NKH71 tended to remain unchanged. This conservation of the rafted structure lead to the good creep resistance of NKH71 under a lower stress. In brief, the new alloy was shown to have good creep resistance even without Re element.