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

高温材料的抗氧化性依赖于将金属基材与大气分离的氧化鳞片的形成。虽然Cr2O3， Al2O3和SiO2在许多应用中具有保护作用，但其特性已经被深入研究了几十年，但复合氧化物（如CrTaO4）的保护潜力却鲜为人知。几种新型高熵合金出人意料的高抗氧化性是由于形成了致密的CrTaO4鳞片。一些镍基合金的氧化速率也由于CrTaO4鳞片的形成而降低。本项目的主要目标是获得关于CrTaO4尺度的全面知识。为了实现这一目标，我们将在以下几个研究领域进行实验研究：(1)CrTaO4鳞片的形成和生长机制及其对蒸发和氮扩散的抗性；(2)旨在降低CrTaO4扩散速率的掺杂效应；(3)将掺杂效应转移到其他相关合金体系。上述第一个研究课题将采用模型合金Cr-20Ta、Ni-20Cr-20Ta和等原子Ta-Cr-Ti-Al进行研究。Cr-20Ta合金是最简单的crtao4成形合金，而Ni-20Cr-20Ta和Ta-Cr-Ti-Al合金将为crtao4成形ni基和高熵合金提供参考。在900、1000和1100℃的空气和无氮气氛中进行氧化研究，应确定合金的化学成分和周围条件对CrTaO4鳞片性能的影响。进一步阐明氧空位是否为CrTaO4晶格中的主要点缺陷。将通过添加3at来研究掺杂效应。% Ag, Cu， Re和W到等原子合金Ta-Cr-Ti-Al。假设Ag和Cu的加入会增加氧空位的缺陷浓度，从而提高氧化速率，而Re和W的加入则会对氧空位的浓度产生相反的影响，从而提高抗氧化性。为了证明这一假设，计划在不同氧分压和不同温度的大气中进行广泛的氧化研究。鉴定CrTaO4的特征将通过包括高分辨率透射电子显微镜在内的综合显微结构研究来完成。最后，掺杂效应应转移到其他相关合金体系，如广泛使用的镍基合金和新型高熵合金。

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.