Effect of Co on microstructural stability of the third generation Ni-based single crystal superalloys
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e effect of Co on element segregation and microstructure is investigated in the third generation Ni-based single crystal superalloys with 4, 8.5, and 11.5 wt% Co addition. The results show that the increase of Co content leads to a severe element segregation in as-cast microstructure. After heat treatment, the size of c9 phase is slightly reduced with Co content increase. During the thermal exposure, the c9 phase coarsens gradually but its coarsening rate decreases with increasing Co content. In addition, some acicular and blocky topologically close-packed (TCP) phases are precipitated in 4% Co and 8.5% Co alloys. However, no TCP phase can be found in 11.5% Co alloy. Finally, it may be concluded that although a higher Co content is harmful for the element segregation, it is beneficial to maintain the cuboidal morphology of c9 phase, decrease its coarsening rate, and impede the precipitation of TCP phase.
I. INTRODUCTION
Ni-based single crystal superalloys are widely used in the manufacture of aero engines turbine blades because of their superior high-temperature capability and good high temperature performance.1–3 In the 1980s, the first generation single crystal superalloys were produced by the directional solidification technology. Currently, the commercial single crystal superalloys have been progressed to the fifth generation. The first three generation alloys contain 0% Re, 3% Re, and 6% Re, respectively. Afterward, 3% Ru and 6% Ru are introduced into the third generation superalloys to form the fourth and the fifth generation alloys to further increase the temperature capability with approximately 30 °C step.4–8 Microstructural stability during thermal exposure plays a very important role in controlling the mechanical property of Ni-based single crystal superalloys, which includes the precipitation of topologically close-packed (TCP) phase and the stability of c9 phase. In the third generation superalloys, the total amount of refractory elements reaches up to 20%. Therefore, they are prone to enrich in the local area of c matrix, resulting in the precipitation of TCP phase and further decreasing in the high temperature performance.9–15 For example, although the amounts of refractory elements were 20 and 19.4% in René N6 and CMSX-10 as the typical third single crystal superalloys, respectively, they presented a different precipitated tendency of TCP phase. The biggest difference
in alloy compositions was that they contained 12% and 3% Co contents, respectively. Therefore, there are lots of controversies about the effect of Co on the microstructural stability for a long time. Although Walston believed that high Co content could improve the microstructural stability.12,16 Erickson believed that low Co content could reduce the precipitated tendency of TCP phase,17,18 and Rae and Reed believed that the alloy with high Co content exhibited an excellent short-term stability and it was easy to form some coarse l phases after long-term thermal exposure.9 Overall, the mechanism of Co element on TCP phase precipitation is st
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