Oxidation of Hf- and Zr-modified Al10Co25Cr8Fe15Ni36Ti6 alloys at temperatures above 750°C: mechanisms and optimization by coatings

• Applicants: Dr. Anna Manzoni; Professorin Dr.-Ing. Anke Silvia Ulrich
• Subject Area: Coating and Surface Technology ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551561837

During the last years research on high entropy alloys or compositionally complex alloys (HEA/CCA) has developed into different paths. In the present case, the focus was the development of two alloys, i.e. Al9.5Co25Cr8Fe15Ni36Ti6Hf0.5 (at.%) and Al9.5Co25Cr8Fe15Ni36Ti6Zr0.5. Both alloys show good mechanical properties in a temperature range around 700°C, even surpassing the commercial alloys Alloy 800 H and Inconel 617, which served as initially chosen references. However, until now little is known about the alloys’ oxidation behaviors, especially in terms of quantitative data such as mass changes or oxide scale thicknesses. First qualitative investigations show the formation of a protective oxide scale and an up to 400 micrometer thick microstructurally modified subsurface zone. In this project, the oxidation behavior will be investigated in more detail to understand its mechanism. Two different (alpha-)Al2O3-forming coatings will be applied and tested to further improve the oxidation resistance: An Al-diffusion coating applied using the slurry technique and minimal surface modification via the halogen effect using an F-containing polymer. The oxidation atmosphere in this study is dry and wet (10 % H2O) synthetic air. In addition, the scale adhesion to the bulk material under mechanical load is investigated during creep tests. The formed scales are compared to the scales formed under isothermal exposure. A combination of optical microscopy, scanning, and transmission electron microscopy, as well as x-ray diffraction and Raman spectroscopy will be used to investigate the coatings and the oxidation products. Thermodynamic calculations by ThermoCalc and FactSage will help in understanding the phase formation and the growth of the oxide products during the heat treatments. The depth of the surface depletion zone, which forms during exposure due to outward diffusion of oxide scale formers, is calculated additionally. Throughout the project the material will be compared to IN718. The project allows for new insights onto two high temperature alloys of the Co-Cr-Fe-Ni-system, which were mostly developed for their mechanical properties. These alloys will now be investigated and further optimized with a new focus onto their oxidation resistance.

• DFG Programme: Research Grants