Computational and Experimental Design of Fe-Based Superalloys for Elevated-Temperature Applications, (Final Report).

摘要

Analogous to nickel-based superalloys, Fe-based superalloys, which are strengthened by coherent B2-type precipitates are proposed for elevated-temperature applications. During the period of this project, a series of ferritic superalloys have been designed and fabricated by methods of vacuum-arc melting and vacuum-induction melting. Nano-scale precipitates were characterized by atom-probe tomography, ultra small-angle X-ray scattering, and transmission-electron microscopy. A duplex distribution of precipitates was found. It seems that ferritic superalloys are susceptible to brittle fracture. Systematic endeavors have been devoted to understanding and resolving the problem. Factors, such as hot rolling, precipitate volume fractions, alloy compositions, precipitate sizes and inter-particle spacings, and hyperfine cooling precipitates, have been investigated. In order to understand the underlying relationship between the microstructure and creep behavior offerric alloys at elevated temperatures, in-situ neutron studies have been carried out. Based on the current result, it seems that the major role of I2I with a 16%-volume fraction in strengthening ferritic alloys is not load sharing but interactions with dislocations. The oxidation behavior of one ferritic alloy, FBB8 (Fe-6.5Al-10Ni-10Cr- 3.4Mo-0.25Zr-0.005B, weight percent), was studied indry air. It is found that it possesses superior oxidation resistance at 1,023 and 1,123 K, compared with other creep-resistant ferritic steels (T91 (modified 9Cr-1Mo, weight percent) and P92 (9Cr-1.8W-0.5Mo, weight percent)). At the same time, the calculation of the interfacial energiesbetween the -iron and B2-type intermetallics (CoAl, FeAl, and NiAl) has been conducted.