Formation and growth mechanisms of CrTaO4 scales and improvement their properties on relevant high temperature materials

• Applicants: Privatdozent Dr.-Ing. Mathias Galetz; Professorin Dr.-Ing. Bronislava Gorr
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Coating and Surface Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467750555

The oxidation resistance of high temperature materials relies on the formation of oxide scales which separate the metallic substrate from the atmosphere. While the properties of Cr2O3, Al2O3 and SiO2 - which are protective in many applications – have been intensively investigated for decades, the protective potential of complex oxides such as CrTaO4 is widely unknown. The unexpectedly high oxidation protectiveness of several novel high entropy alloys is attributed to the formation of dense CrTaO4 scales. The oxidation rates of some Ni-based alloys also decrease due to the reported formation of CrTaO4 scales.The main objective of this project is to gain comprehensive knowledge about CrTaO4 scales. To reach this objective, we target experimental investigations in following research areas: (i) mechanisms of formation and growth of CrTaO4 scales and their resistance to evaporation and nitrogen diffusion, (ii) doping effect aiming at decreasing the diffusion rates through CrTaO4 and (iii) transfer of the doping effect to other relevant alloy systems. The first research issue mentioned above will be studied using model alloys Cr-20Ta, Ni-20Cr-20Ta and equiatomic Ta-Cr-Ti-Al. The alloy Cr-20Ta represents the simplest CrTaO4-forming alloy, while Ni-20Cr-20Ta und Ta-Cr-Ti-Al will serve as references for CrTaO4-forming Ni-based and high entropy alloys. Conducting oxidation studies in air as well as in nitrogen-free atmospheres at 900, 1000 and 1100°C, the impact of the alloy’s chemical composition and surrounding conditions on the properties of CrTaO4 scales should be determined. Further, it will be clarified whether oxygen vacancies represent the major point defects in CrTaO4 lattice. The doping effect will be studied by additions of 3 At.% Ag, Cu, Re und W to the equiatomic alloy Ta-Cr-Ti-Al. It is assumed that the additions of Ag and Cu will enhance the defect concentration of oxygen vacancies leading to higher oxidation rates, while additions of Re and W should result in the opposite effect on the concentration of oxygen vacancies and consequently on the oxidation resistance. In order to proof this hypotheses, extensive oxidation studies in atmospheres with different oxygen partial pressures and at different temperatures are planned. The identification of the CrTaO4 characteristics will be performed by comprehensive microstructural investigations including high-resolution transmission electron microscopy. Finally, the doping effect should be transferred to other relevant alloy systems such as widely used Ni-based alloys as well as novel high entropy alloys.

• DFG Programme: Research Grants