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

High-entropy alloys (HEAs) have emerged as promising candidates for applications in extreme environments due to their exceptional structural stability. Among them, refractory HEAs show great potential for high-temperature use, especially when protected by stable oxide layers. Recent observations have revealed the formation of complex CrTaO₄ oxides on some HEAs and Ni-based alloys, which exhibit outstanding oxidation resistance at temperatures up to 1500°C. These findings highlight the potential of Cr-Ta-based oxides as protective layers in high temperature material systems.

This project focuses on two key material systems to better understand the formation and behavior of Cr-Ta-based oxides. The alloy Cr-20Ta is studied as a clean, binary model system that forms CrTaO₄ without interference from additional alloying elements, representing the fundamental Cr-Ta-oxide-forming behavior. In contrast, the equimolar CrTaTiAlMo alloy serves as a representative model for refractory high-entropy alloys, allowing us to study CrTaO₄ formation in a more complex chemical environment. Together, these systems provide a framework for understanding how alloy composition influences oxide formation, structure, and stability.

The goal of this research is to enhance the oxidation resistance of Cr-Ta-oxide-forming alloys through targeted doping. By introducing small amounts of Ag, Cu, Re, and W into the CrTaTiAlMo and Cr-20Ta alloys, we aim to investigate how these elements influence the defect chemistry of CrTaO₄, particularly the concentration and behavior of oxygen vacancies by altering the dominant defect types in CrTaO₄ and thereby modifying the oxidation kinetics by influencing oxygen diffusion. The insights gained from this study will help the design of more oxidation-resistant materials based on Cr-Ta-oxide protection for use in high-temperature environments.