Abstract
Elementary processes during the oxidation of single crystal Co/Ni-base model alloys between 800 °C and 900 °C were investigated in the present study. The influence of the base element (Co or Ni) on the oxidation rate was elucidated by continuous thermogravimetry in synthetic air for 100 h. After exposure, the multilayered oxide scales were characterized by scanning electron microscopy combined with electron probe micro analysis. The substitution of Co by Ni caused reduced oxidation rates at 900 °C. At lower temperatures an inverse trend with Co-rich alloys demonstrating lower mass gains was observed. The development of diffusion-limiting scale sections that lead to overall thinner oxide layers could be proved for the Co-base superalloy without Ni at 800 °C and 850 °C. The growth of AlN as well as the unwanted phase transformation of the γ/γ′ microstructure into Co3W persisted in alloys with high Co levels for all investigated temperatures. Material transport at 900 °C was elucidated by two-stage oxidation experiments in normal oxygen and 18O-enriched atmospheres. Transport of O2 via microchannels could be concluded for all compositions. The provided insights contributed to deeper understanding of essential processes during the formation of barrier layers in the scale.
Highlights
Until today, the resistance against internal oxidation is a major drawback of γ′-strengthened Co-base alloys
Since detailed considerations of the oxidation behavior at 900 °C are included in a previous publication,[14] only the most important findings are briefly summarized in the following
The oxidation behavior of 5 single crystal Co/Ni-base model alloys during 100 h exposure in synthetic air was systematically investigated between 800 °C and 900 °C
Summary
The resistance against internal oxidation is a major drawback of γ′-strengthened Co-base alloys. Since the continuous mass gain per sample area of NC00sx exceeds that of all other compositions at during the entire duration of exposure 850 °C (Fig. 1b), the decrease of oxidation temperature most has a significant impact on the development and the stability of barrier layers on the interface between d2 and d3.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
