Effects of Alloying Elements on Thermal Expansion Behavior of Ni-Fe-Cr-Based Superalloys
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ELECTRIC generation by coal combustion is one of the most crucial energies for economic development even though the wind, solar, and nuclear energies are developing rapidly. Unfortunately, coal combustion can lead to severe environmental pollution. Therefore, it is necessary to increase the steam parameters (temperature and pressure) of coal-fired power plants in order to increase the energy efficiency and reduce the pollution emission.[1–3] During the past years, the steam parameters have been upgraded from subcritical to supercritical, ultra-supercritical (USC), and presently advanced ultra-supercritical (A-USC). For the A-USC technology, the required mechanical properties of some key materials such as reheater and superheater tubes are beyond the temperature capacity of current ferritic or austenitic steels.[4,5] Accordingly, Ni-based superalloys with excellent high-temperature properties are considered as the appropriate candidates for the hottest boiler and turbine applications.[6–10] Cost constraint drives design engineers to minimize the employment of Ni-base alloys and make use of ferritic steels in the
YUNSHENG WU is with the Institute of Metal Research, Chinese Academy of Sciences, No. 72, Wenhua Road, Shenhe District, Shenyang 110016, China and also with the School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China. XUEZHI QIN and LANZHANG ZHOU are with the Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted July 20, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
low-temperature components as much as possible, meaning that the joint or weld between the superalloys and ferritic steels are inevitable. However, Ni-base superalloys have a much higher TEC than the ferritic steels, which leads to a significant TEC mismatch between them. During service, the TEC mismatch can probably cause the catastrophic stress concentration in or between the components and accelerate their failure.[11] The TEC of alloys is closely related to their chemical compositions. The effects of alloying elements on the TEC in Ni-Fe-Co-based superalloys have been widely investigated.[12–17] For example, K. Sato et al.[18] thought that the increase of Co/(Ni + Co) ratio, as well as a lower content of Ni + Co, decreased the TECs of 903 and 909 alloys. Plentiful experiments[14,19–24] revealed that an appropriate quantity of Fe reduced the TEC, while Cr intensively increased the TEC. In addition, much work has been done on the thermal expansion behaviors of Ni-Cr-based superalloys. Mo, Ti, W, Al, and (Nb + Ta) can decrease the TEC of alloys, whereas Fe and Mn increased it.[13,14,23] The effect of Cr on the TEC was controversial. Yamamoto et al.[14] considered that Cr increased the TEC, but Hull et al.[23] found that Cr first increased the TEC and then decreased it with the increase of the content of Cr. The precipitation of a-Cr phase with a low TEC from the matrix was probably the main reason why the TEC dropped when the Cr con
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