Elementary mechanisms during the early stages of scale formation on single crystalline Co- and Ni-base superalloys at high temperatures

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Excellent resistance against oxide scale formation is crucial for applying newly designed alloys at high temperatures, making a detailed understanding of oxidation processes essential for successful material design. This thesis investigates oxide scale formation on single-crystalline, γ′-strengthened Co- and Ni-base model alloys. Classical thermogravimetry and sequential exposure to ¹⁶/¹⁸O₂ were complemented by advanced surface analytical techniques. High-resolution transmission electron microscopy revealed the impact of the two-phase microstructure during the early stages of scale formation in the ternary Co-Al-W system. Kinetic processes during high-temperature oxidation of these alloys were studied based on varying W content between 800 and 900 °C. The influence of base elements (Co or Ni) was further explored through another series of single-crystalline model alloys. The formation of diffusion-limiting barrier layers and unwanted phases correlated directly with the Co/Ni ratio in the alloy. Two-stage tracer exchange experiments in ¹⁶/¹⁸O₂ atmospheres examined the transport of reactants through growing oxide scales. This approach confirmed not only the counter-current transport of cations and anions along different paths but also the development of pores and microchannels within the oxide scales.