Elementary Mechanisms During the Early Stages of Scale Formation on Single Crystalline Co- and Ni-base Superalloys at High Temperatures

Abstract

Excellent resistance against oxide scale formation is essential for the application of newly designed alloys at high temperatures. Therefore, the detailed understanding of oxidation processes is crucial for the successful design of new materials for application. In the course of the present thesis, oxide scale formation on single-crystalline, γ′-strengthened Co- and Ni-base model alloys was investigated. Results derived by classical thermogravimetry and sequential exposure to ¹⁶/¹⁸O₂ were supplemented by various sophisticated, surface analytical techniques. The impact of the two-phase microstructure during the early stages of scale formation could be demonstrated by high-resolution transmission electron microscopy for the ternary Co-Al-W system. Kinetic processes during high-temperature oxidation of ternary model alloys were investigated depending on the W content between 800 and 900 °C. Furthermore, the role of base elements (Co or Ni) was elucidated with the help of another single-crystalline model alloy series. The development of diffusion-limiting barrier layers as well as the formation of unwanted phases could be directly correlated to the Co/Ni ratio in the alloy. Two-stage tracer exchange experiments in ¹⁶/¹⁸O₂-containing atmospheres were conducted to investigate the transport of reactants through growing oxide scales. Besides counter-current transport of cations and anions along different paths also the development of pores and microchannels was confirmed by the selected experimental approach.