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Effects of Alloying Elements on the Temperature Dependence of Yield Stress in L12-Co3(Al,W) Zhenghao M. T. Chin , Norihiko L. Okamoto , Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japn ABSTRACT The effects of alloying elements (Ni/Ta) on the temperature dependence of yield stress in Co3(Al,W) with the L12 structure have been investigated through compression tests of nearly single-phase polycrystalline alloys in the temperature range between room temperature to 1,473K. Compared with a ternary Co3(Al,W), a Ni/Ta-added Co3(Al,W) alloy exhibits a higher γ΄ solvus temperature and lower onset temperature of the yield stress anomaly (positive temperature dependence of yield stress), suggesting that the CSF energy is increased by Ni/Ta addition. As a consequence, the high-temperature strength in Co3(Al,W) is considerably enhanced. INTRODUCTION Currently, Ni-based superalloys are the most widely used high-temperature structural materials for applications in aircraft engines and power generation systems. These Ni-based superalloys generally possess a two-phase microstructure consisting of the solid-solution based on Ni with the face-centered cubic (fcc) structure (designated as the γ phase) and the ordered intermetallic compound based on Ni3Al with the L12 structure (designated as the γ΄ phase). The γ/γ΄ two-phase microstructure has been believed to lead to the excellent high-temperature mechanical properties of these Ni-based superalloys. However, the operating temperature has already been increased almost above the melting temperature. Recently, Sato et al. [1] discovered a stable L12 phase that can coexist with a fcc solid-solution based on Co in the Co-Al-W ternary system although there is no stable L12 phase in the Co-Al binary system. They also reported that γ/γ΄ two-phase microstructures that resemble those in Ni-base superalloys form in ternary and some quaternary alloys. This discovery has opened up a pathway to the development of a new class of high-temperature structural materials based on cobalt, “Co-based superalloys”. Mechanical and physical properties of the Co-Al-W based alloys with the γ/γ΄ two-phase microstructure [2-5] as well as the constituent L12 phase [6-9] have been investigated in recent years. It has been found that a polycrystalline

L12-Co3(Al,W) alloy (Co-12Al-11W (at.%)) exhibits a positive temperature dependence of yield stress in a very narrow temperature range between 950 and 1100K as shown in figure 1[9]. The onset temperature (Tonset) of the yield stress anomaly (YSA) in the L12-Co3(Al,W) phase is much higher (~950K) than that in Ni3Al, whose Tonset is below room temperature. On the other hand, the temperature at which the yield stress reaches the peak value is just above Tonset. This is because the γ΄ solvus temperature (Tsolvus) of Co3(Al,W) is rather low and the L12 phase transforms to the fcc phase [9]. These considerably high Tonset and relatively low γ΄ solvus temperature limit the high-temperature strength of the L12-Co3(