Ab initio based calculation of the stability of selected TCP precipitates in steels: Temperature and interface effects

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The mechanical properties and long-term stability of materials for high-temperature energy and transportation applications are strongly influenced by the precipitation of secondary phases. In particular, topologically closed-packed (TCP) precipitates play a decisive role in many technologically relevant alloys like the Fe-Nb system. While these phases are often clearly identified in experiments, their simulation is either limited to empirical models or to the stability of separate phases at T=0K. Therefore, a reliable prediction under which circumstances the precipitate phases form in the material is often not possible. Within the present project, we will apply ab initio based simulation techniques in order to understand, calculate and optimise the thermodynamic and microstructural conditions for the formation of complex precipitate phases. On the one hand, all entropy contributions for the free energy of formation up to the melting point will be determined with high accuracy. On the other hand the interfaces and the correlation of precipitates and microstructure of the matrix material will be considered. In contrast to previous approaches, the interplay of these two aspects will be mapped realistically in the simulation of precipitate formation.The concept of the project requires the use of a hierarchy of methods, which is achieved in a joint effort of two groups at the MPIE Düsseldorf and ICAMS Bochum. Density functional theory is needed for the accurate consideration of excitation processes at finite temperatures, whereas the microstructural description is achieved by a coarsening of the electronic structure towards a simplified description. The application of bond-order potentials makes the simulation of the combined microstructure of precipitate and matrix material accessible. The investigations of the present project will be focussed on the formation of Laves phases within the Fe-Nb system. This material system has a high potential for applications, is experimentally well characterized, but suffers from a limited understanding of precipitate formation. With the help of the project the competition between different TCP phases, the impact of further alloying elements on their stability, the expected morphology of the precipitates and the role of grain boundaries will be clarified. In addition to the direct interpretation of recent experimental measurements for Laves phases in Fe-Nb, the investigations will form the ground to an understanding of TCP precipitates in various material systems.