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Co-based alloys are used to produce the components for gas turbine engine and power generation system due to their inherent advantage of outstanding hot corrosion resistance1,2). However, the application of Co-based alloys has been signi cantly restricted, since a shortage of valid strengthening methods at high temperature. In general, conventional Cobased superalloys are strengthened by solid solution strengthening and carbide precipitation strengthening, but both of these strengthening methods would be ineffective due to dissolution or coarsening of carbides at the temperature above 800°C3). Even though a γ’-Co3(Al, W) precipitate has been discovered latterly for strengthening the Co-based alloys4), the γ’ phase is unstable at the temperature above 900°C5). It’s signi cant to increase the service temperature of Co-based alloys, since it enables to remarkably increase the thermal efciency of the industrial gas turbine. In order to further improve their applicable temperature and simultaneously retain the strength at elevated temperature, the advanced strengthening approaches for developing Co-based alloys are necessary. Meanwhile, with increasing temperature, a favorable oxidation resistance also should be key issue to be considered. The best high-temperature oxidation resistance could be achieved through developing outer/single Al2O3 scale because of its excellent stability at elevated temperature, but this alumina former has high-demanding on alloys composition with high Al content. The single Al2O3 formation has been proved to be inapplicable for the conventional carbides and γ’ strengthened Co-based alloys because of their narrow composition range. Based on the superiority of oxide dispersion strengthening (ODS) with stable oxide particles up to 1200°C and exibly controlled composition6), developing Co-based ODS superalloys for extremely high temperature applications has been focused, in which the strength and oxidation resistance are expected to be guaranteed by a reliable oxides pinning effect and the single Al2O3 former at elevated temperature, respectively. In fact, the Ni-based and Fe-based ODS alloys have been developed and already partially used as industrial materials6,7). In addition to their superior creep resistance and favorable high-temperature strength, ODS superalloys show better oxidation resistance than the ODS-free alloys with the same basic composition, which has been frequently proved through the research of Feand Ni-based ODS superalloys8). However, the Co-based ODS system has received much less attention than Niand Fe-based ODS superalloys, and even no reports on the high-temperature oxidation of Co-based ODS superalloys can be referenced. Since Hf was claimed to be fairly effective to re ne dispersoids through forming small Y2Hf2O7 particles9), a Co-3Al1.5Y2O3-1.2Hf ODS alloy was discovered by our previous work. Takezawa et al.10) studied the composition and found that the Y2Hf2O7 oxide particles are extremely stable at the temperature above 1000°C, suggesting that the high temperature strength of this Co-based ODS superalloy can be enhanced dramatically as compared with the conventional Co based superalloys. Besides, Sasaki et al.11) also announced that the above Co-based ODS alloys were able to maintain high hardness with an outstanding microstructure stability after annealing at 1000°C for 240 h, owing to the pinning of grain boundaries by the dispersed oxide particles. To develop advanced Co-based ODS superalloys for extremely high temperature applications with favorable strength and superior oxidation resistance at elevated temperature, in addition to the dispersion of ne Y2Hf2O7 dispersoids, the formation of a single alumina scale is another key target. Therefore, a higher Al content was added in the present Cobased ODS system, and chromium was also introduced to promote the formation of alumina scale according to the reports claimed by G.N. Irving et al. that the addition of Cr to Co-Al base alloys markedly reduced the level of Al required in the ternary alloy below that required in the binary to form a continuous external scale of Al2O3 12). Aimed at discovering alumina-forming Co-based ODS su* Corresponding author, E-mail: qqyu1111@163.com Materials Transactions, Vol. 59, No. 4 (2018) pp. 563 to 567 Special Issue on Advanced Structural Materials for Extremely High Temperature Applications in Future ©2017 The Japan Institute of Metals and Materials

Effect of Cr on the Oxidation Resistance of Co-Based Oxide Dispersion Strengthened Superalloys