High-Strength Steel Hydrogen Embrittlement. Structural Optimization and Molecular Dynamics by Ab-Initio Calculations

Abstract

Hydrogen embrittlement is believed to be one of the main reasons for cracking of metals under stress. High strength steels in the structures often include a ferritic core made of alpha-iron (body centered cubic lattice). Previous work was concerned with the interaction of atomic hydrogen with iron using first principles calculations. We studied the effect of interstitial hydrogen in the iron lattice and the stress induced by the interstitial hydrogen in the host lattice. In this paper we also show the dynamical behaviour of hydrogen inside the iron lattice. Using ab-initio Molecular Dynamics we obtain diffusion coefficients by taking statistical averages of hydrogen diffusing inside iron.
Depending on temperature and hydrogen concentration, the diffusion path preferably involves going through tetrahedral or octahedral sites, respectively. Simulations where a number of hydrogens occasionally cluster in a particular region have been performed to elucidate the effect of interactions
between hydrogens. From simulated diffusion paths, the diffusion coefficient is calculated using Einstein´s equation. We also find that the Fe-Fe interaction weakens due to the interstitial hydrogen.
Iron Debye temperature decreases for increasing concentration of interstitial hydrogen, proving that iron-iron interatomic potential is significantly weakened in the presence of a large number of diffusing hydrogen atoms.