High-Temperature Oxidation Behaviour of Co-Re-Cr-based Alloys: Limitations and Ways to Improve

Ni-based alloys are the most widely used alloy system in high-temperature applications. However, the use of Ni-based alloys is limited to temperatures below 1100 degrees C. The experimental Co-Re-Cr-based alloys are promising for high-temperature applications for service temperatures beyond 1200 degrees C. A complete miscibility in the Co-Re system allows to steadily elevate the melting point of the system with the rhenium content. In addition, rhenium takes the role as solid solution strengthening element. In the case of Co-based alloys, the oxidation resistance at high temperature is mainly based on the formation of a protective Cr2O3 scale. The purpose of the present investigations is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. Earlier investigations of the authors showed a rather poor oxidation resistance during exposure to laboratory air. Oxidation at 1000 degrees C in air yielded an oxide scale that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. As a consequence of the lacking protectiveness of the oxide layer the vaporization of rhenium oxide takes place and hence leads to a rapid loss of Re. The aim of recent investigations is to study the effect of Si on the high-temperature oxidation behaviour of Co-Re-Cr alloys by means of kinetic and microstructural examinations. It was found that Si stabilizes the Cr2O3 scale, enhancing the oxidation resistance significantly. Hence, the synergetic effect of chromium with silicon could be considered as an encouraging perspective to improve the oxidation resistance of Co-Re-Cr alloys. Apart from that, other concepts to enhance the oxidation resistance of this class of materials are discussed, such as the formation of a borosilicate layer or protective Al2O3 scale on the substrate surface.