Ab inito calculation of the formation energy of the sigma phases at 1000 Kelvin : With a five sublattice model for the ternary Iron-Chromium-Nickel system

Sammanfattning: This project investigates the σ-phase formation energy at 1000 Kelvin using Exact Muffin TIn Orbital (EMTO) method and considers the disordered local moment approach to handle the paramagnetism in a self-consistent manner. The sigma phase is detrimental and causes embrittlement when it occurs primarily in stainless steel and superalloys with Iron (Fe) and Chromium (Cr), which is why it is necessary to understand it better and therefore avoid it. The lattice of σ contains 5 nonequivalent sites which will be modeled through a 5 sub lattice model for the intermetallic phase. The method is based on Density Functional Theory and generates the formation energy for different configurations with different compositions of Iron, Chromium and Nickel(Ni) in the 5 sub lattice model with different Wigner Seitz radii. That made it possible to calculate properties as energy related to other phases and bulk modulus which can be used to calculate other properties. Comparing the results to values of other metals and experimental values of the sigma phase, the results were reasonable but need to consider what simplifications were made. With continued efforts with the results it is a possibility to use it for Calphad modeling which was not possible due to time. The results did not achieve the goal which was to use it for Calphad modeling, however showed that the results were reasonable and that EMTO can be used in the future for simulations of the sigma phase. Other things such as site occupancy and the vibrational contribution to the free energy is reserved for future work.