Hydrogen Effects on the Point Defect Spectrum in Fe-C Alloys

As part of a multi-scale approach for modeling hydrogen embrittlement in hardened steels we have investigated, employing density functional theory methods, the stability and concentrations of the point defect clusters present in metastable Fe-C-H alloys with vacancies. The defect spectrum is found to be strongly dependent on the local vacancy concentration, and for low hydrogen levels sharp highly non-linear changes in the defect cluster population are observed at critical vacancy concentrations. This critical-like behavior suggests an energy activation mechanism which can be characterized by an effective defect-cluster formation energy barrier. By analogy with similar activated processes such as the liquid-to-glass transition in super-cooled liquids, we postulate that this criticality is associated with the presence of deep wells in the energy landscape where chemical composition plays the role of generalized coordinate. Increases in the hydrogen content have the qualitative effect of reducing the slopes in the defect concentrations. The drastic sensitivity of the defect cluster spectrum to local changes in vacancy and impurity concentrations implies that in proximity of surfaces and extended defects multiple defect clusters become statistically significant and migration dependent phenomena, such as creep-relevant to hydrogen embrittlement-and super-diffusion, should be controlled by multiple activation barriers. Thesis Supervisor: Prof. Sidney Yip