Modelling of Fe-Cr alloys: A Multiscale approach

Abstract

With an aim of building next generation ssion nuclear reactors with better reliability and e ciency, scienti c community has shortlisted candidate struc- tural materials which shall be tested exhaustively before they are taken up in the power industry. The research studies have established that Ferritic and Martensitic Fe-Cr alloy steels are the front-line candidate materials for the most promising of future reactor designs. These reactors would be serving in o aggressive working conditions, with temperature range 300-900 C and neutron dosage of 15-150 dpa. These Fe-Cr alloys bring a number of advantages like void swelling resistance and relatively good corrosion and creep resistance in the pro le of reactor steels. However, if we pursue them as it is, they still pose major technological hurdles for such extreme working conditions, and could be potentially disastrous. A real-time experimental approach for these irradiation studies prove to be very expensive, and at times even inaccessible. Hence, modeling these struc- tural materials through suitable approaches has been widely used to augment our understanding. In this thesis, we used a hierarchy of modeling techniques, which span di erent length and time scales of the underlying phenomena in these alloys. At the lowest order of this multiscale approach we investigated through density-functional theory calculations using plane-wave basis within VASP package, followed by nudged elastic band method for estimating the migration barriers of various activated processes, under the assumptions of Harmonic Transition State theory. Our results of these energetics is consis- tent with reported ab initio and experimental studies. These energetics are further taken to micro- and milli- length and time scales by kinetic monte carlo (KMC) simulations, whose code was developed by me for this work. This is o tested using Einstein's equation of di usivity over the range of 300-1500 C for vacancy migration in ferritic (bcc) Fe. This approach of studying the energet- ics of the dominant atomistic migrations which lead to di usional transport, and later suitably accommodating these in the KMC code, could reveal the key mechanisms in the responses of material in reactor-like conditions. This would be very crucial to understand the underlying phenomena and design resistant structural materials to endure radiation damage.