Erforschung des Einflusses von Nickel, Silizium und Yttrium auf die Mikrostruktur und das Hochtemperaturoxidationsverhalten von Co-Re-Cr-Legierungen

The development of new materials for jet engines with higher operation temperatures is driven by the demand to enhance thermal efficiency and reduce emissions. Refractory metal-based alloys, such as Co-Re-based alloys, are regarded to be suitable candidates as they exhibit an increased melting temperature and a remarkable combination of strength and ductility required for high-temperature applications.
The aim of the work is to develop alloys based on the system Co-Re-Cr-Ni-Si-Y with an improved property profile including microstructural stability and oxidation resistance. In preliminary studies, a positive effect of Ni on the oxidation behaviour was observed, whereby the underlying mechanisms are unexplored. Thus, the focus of this work is to gain a fundamental understanding of the positive effect of Ni on the oxidation behaviour as well as the identification of synergetic effects between Ni, Cr, Si and Y. Another focus of this work lies on the microstructure (evolution) and its interaction with the oxidation behaviour. A correction of the commercially acquired Co-Re-Cr thermodynamic dataset of the software FactSage and its extension by the elements Ni and Si enables thermodynamic calculations of phase stabilities and phase fractions allowing the identification of driving forces and relevant mechanisms. Kinetic data from thermogravimetry combined with microstructural analysis, such as X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM) build the basis to characterize the microstructure and oxidation behaviour. The microstructure studies reveal that the face-centered cubic (fcc) matrix phase and the σ phase stabilize with increasing Ni and combined Cr + Ni additions. In terms of the oxidation behaviour, increased Ni concentrations up to 25 at.% combined with a Cr concentration of 23 at.% cause a steady improvement in the oxidation resistance as the formation of a protective Cr₂O₃ layer is accelerated. This phenomenon is explained by an enhanced Cr diffusion in the matrix with increasing Ni concentration. A reduction of the Cr content to 18 at.% deteriorates the oxidation resistance and leads to enhanced mass loss due to evaporating Re-oxides. A Si addition of 2 at.% to the alloy Co-17Re-18Cr-15Ni slightly improves the oxidation behaviour as the Si addition simultaneously facilitates the formation of the σ phase which diminishes the positive effect of Si on the oxidation behaviour. Because of the low solubility of Y in Co-Re alloys, only the low Y concentration of 0,1 at.% improves both the isothermal and cyclic oxidation behaviour of the alloy Co-17Re-18Cr-15Ni-2Si, while Y additions of 2 at.% cause unfavourable microstructural changes leading to enhanced mass loss as a result of evaporation of Re-oxides. With the help of the knowledge gained, it is possible to determine an optimum composition range based on Co-Re-Cr-Ni-Si-Y, which best meets the requirements for microstructural stability and oxidation resistance.