DFT energetics of hydrogen binding to point defects in iron and steels

It is well known that hydrogen degrades the properties of iron and steel. One proposed mechanism of hydrogen attack is the concept of ``hydrogen traps'', which are generally microstructural material defects that bind hydrogen atoms. The stability and energetics of a number of potential traps (like vacancies, interstitials, and substitutional alloying elements) in bcc and fcc iron are investigated using density functional theory (DFT). We find that hydrogen is very sensitive to its local environment. For example, inIn a perfect bcc iron lattice, hydrogen prefers to sit in the tetrahedral site rather than the octahedral site or a substitutional site, and there is a repulsive interaction between carbon and hydrogen. In the presence of a vacancy, the energy barrier between the octahedral and tetrahedral sites disappears and hydrogen only resides in the octahedral sites. The hydrogen-vacancy binding energy changes as more hydrogen atoms bind to the vacancy, but the binding energy does not significantly change in the presence of carbon. We also find that H$_{2}$ molecules are not stable inside a vacancy: they dissociate spontaneously (without a barrier) into octahedral positions. Details about vacancy and other potential traps are presented and their roles in hydrogen embrittlement are discussed.