BCC Fe under Stress from First Principles

Anna Nikiforova, Massachusetts Institute of Technology

Nuclear energy progress depends on the development of advanced materials. However, materials development requires fundamental understanding of materials behavior in extreme environments. Stress corrosion cracking (SCC), a sudden failure of normally ductile metals, is one of the main causes of degradation of materials subjected to a tensile stress in a corrosive environment. The objective of this research is to determine the atomistic relation of the reactivity and microstructure of interfaces to the initiation of SCC of Fe-Cr.

Fe-Cr is a structural alloy known for its ability to form a protective surface oxide layer which helps to slow down corrosion rates. The past research on the Fe-Cr alloys has fallen short of elucidating the formation, physical nature, and stability of such surface oxide film. Atomistic simulation can be used to complement the experiments, but the electronic nature of Fe-Cr alloy poses a nontrivial modeling challenge. Important examples of experimentally known facts which lack explanation include: 1) connection of process of oxide film formation with a fundamental charge transport mechanism, and 2) no Cr solubility for Fe-Cr alloy with low Cr concentration according to existing phase diagrams.

In this work, Fe-Cr alloy in SCC conditions is studied using electronic and atomistic level modeling and simulation. The near-term objectives are to investigate the behavior of BCC Fe under uniaxial tensile load. The response to the load together with the change in reactivity can be tied to the SCC initiation mechanism. The stress-related changes in density of electronic states (DOS) and Fermi surface can be linked to bonding characteristics of Fe and Cr. We found that DOS of Fe and Cr changed significantly with tensile strain. The hypothesis for how strain influences the reactivity of the metal surface to oxygen and the mechanism of initiation of the oxidation is also discussed in the poster.

Abstract Author(s): Anna Nikiforova and Bilge Yildiz