Comparison of Ni-based and Co-based superalloys
Single crystal superalloys based on Ni are commonly used as turbine blades in industrial gas turbines for aero engines and power plants. The observation of a stable Co3(Al,W) phase with L12 structure showed that single crystal alloys based on Co can also be fabricated. Such Co-based systems exhibit similar hardening mechanisms at a potentially higher melting point. In this project, we compare the stability of topologically close-packed (TCP) phases in Co- and Ni-based superalloys from an atomistic point of view. The precipitation of TCP phases reduces the γ/ γ'-hardening effect of superalloys.An atomistic understanding of the formation, properties and stabillity is crucial to evaluate how sensitive a new casted alloy is to the formation of such unwanted precipitates. Therefore, we compute phase stabilities of binary TCP phases using a high-throughput density functional theory environment. We also investigate the elastic properties of TCP phase and the influence of stress on the phase stabilities. Besides these ab initio calculations, we also develop and apply coarse-grained semi-empirical approaches (bond order potentials and tight binding) which provide a computationally efficient approximation to the electronic structure for different band-fillings. Furthermore we use a recently derived empirical structure map to investigate trends in the stability of TCP phases and characterize these phases according to their composition. We could already show that these structure map predictions are in-line with ab initio results and are (in combination with experimental and other theoretical studies) a powerful tool for the identification of TCP phases. Our results are used as input for ab initio CALPHAD calculations and for the interpretation of experimental measurements. This project is carried out as project C1 in the collaborative research centre SFB/Transregio 103 (http://www.sfb-transregio103.de/) in close collaboration with several theoretical and experimental groups in Bochum and Erlangen.