"Analyzing the Competitive Precipitation Behavior in Alumina-Forming Austenitic Steels and Its Impact on High Temperature Creep" 

Khaled Ayman ElToukhy, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

Alumina Forming Austinites (AFAs), an Fe-based superalloy, can serve as a less financially demanding alternative to Ni-based superalloys in high-temperature applications. AFAs offer a combination of high-temperature creep resistance and corrosion/oxidation resistance due to the formation of alumina scale and a wide variety of precipitates, such as γ′-Ni3Al and NbC. γ′-Ni3Al and NbC offer the highest degree of strengthening in high-temperature creep through dislocation/precipitate interaction. However, some precipitates can degrade these properties, such as χ, σ, and δ, while others, such as Cr23C6, Laves-Fe2Nb, and β-NiAl, can offer lower or conditional strengthening. The primary challenge in AFA alloy design is to engineer an alloy that can maintain a single austenitic matrix throughout operation, provide high-temperature strengthening, and form an external alumina scale instead of internal oxides. This can only be achieved by a proper understanding of the competitive precipitation behavior within the alloy, which is crucial for achieving the necessary balance required to fulfill all three requirements. Precipitation of the unwanted phases offers no additional benefits, as it depletes the matrix of the solutes necessary to maintain the austenitic matrix. Furthermore, through solute depletion, the alloy fails to form an external alumina scale, instead forming discontinuous internal oxides, which compromise both the mechanical and chemical properties of the alloy. In addition, a high volume fraction of precipitates can promote crack formation in crack-sensitive precipitate-free zones. γ′-Ni3Al, Laves-Fe2Nb, β-NiAl phase, NbC, and Cr23C6

precipitation behavior are studied with emphasis on their stability during creep and their precipitation conditions. These precipitates share common elements, making it essential to understand the competitive preferential precipitation behavior and its contribution to the final microstructure and mechanical properties