On the oxidation of the third-generation single-crystal superalloy CMSX-10
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TRODUCTION
THE turbine blading required for a modern gas turbine engine is now very often cast in single-crystal form, to avoid the grain boundaries that re deleterious to the creep and fatigue properties. There continues to be significant evolution in the compositions of the alloys designed for these applications, with the latest ones containing significant quantities of the transition metals, rhenium and ruthenium, which markedly improve the mechanical properties. However, the operating temperatures of the gas stream continue to rise such that the degradation of the blading during service, by oxidation or corrosion, is a strong possibility, as will be demonstrated in this article. For this reason, it is usual to protect the blading using coating technology, e.g., either by aluminization or by the application of a ceramic thermal barrier layer with a metallic bond coat. Unfortunately, for various reasons, it is not always possible to apply coatings to the superalloys. Therefore, the mechanisms of oxidation and corrosion are of the greatest importance, not only for the coating, but also for the bare superalloys themselves; consequently, much work continues to be reported on this topic. For example, the oxidation characteristics have been examined in recent years for the aluminized[1–7] and uncoated[1,2,3,7–12] second-generation single-crystal superalloys. However, the third-generation superalloy, CMSX*-10, contains refractory metals (W *CMSX is a trademark of Cannon-Muskegon Co., Muskegon, MI.
A. AKHTAR, Adjunct Professor, is with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada, V6t 1Z4. Contact e-mail: [email protected] M.S. HOOK, formerly Graduate Student, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom, is Investment Banker, Deutsche Asset Management, 1 Appold Street, London EC2A 2UU, UK. R.C. REED, Professor, is with Department of Materials, Imperial College, London, South Kensington Campus, London SW7 2AZ, United Kingdom. Manuscript submitted July 13, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Mo Re Ta) at a total content of 19.4 wt pct and a Cr concentration of 2.0 wt pct, as compared with 16.1 and 6.5 wt pct, respectively, for its second-generation counterpart, CMSX-4.[13] In common with other third-generation superalloys such as RENÉ* N6, the low chromium content *RENÉ is a trademark of General Electric Co., Fairfield, CT.
of CMSX-10 raises concerns about whether the resistance to oxidation will be adequate in the environment under which the blade is to operate.[7] Of note here is the oxidation of the third-generation superalloy RENE` N6. That alloy, which has a (W Mo Re Ta) content of 19.7 wt pct, a value similar to that of CMSX-10 but with a higher Cr content of 4.5 wt pct, was oxidized in the laboratory at 1200 °C.[14] It was found that reducing the sulfur content to a level below 0.1 ppm prevented spalling of the scale, which results from thermal cycling. In another recent
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