Hydrogen Trapping in bcc Iron

Abstract

Fundamental understanding of H localization in steel is an important step towards theoretical descriptions of hydrogen embrittlement mechanisms at the atomic level. In this paper, we investigate the interaction between atomic H and defects in ferromagnetic body-centered cubic (bcc) iron using density functional theory (DFT) calculations. Hydrogen trapping profiles in the bulk lattice, at vacancies, dislocations and grain boundaries (GBs) are calculated and used to evaluate the concentrations of H at these defects as a function of temperature. The results on H-trapping at GBs enable further investigating H-enhanced decohesion at GBs in Fe. A hierarchy map of trapping energies associated with the most common crystal lattice defects is presented and the most attractive H-trapping sites are identified.

The Potential of the Internal Friction Technique to Evaluate the Role of Vacancies and Dislocations in the Hydrogen Embrittlement of Steels

“…[ 42,43 ] Alternatively, using Hirth's interpretation, the binding of hydrogen to a screw dislocation could be estimated to be 0.2–0.3 eV based on the shift of the γ‐peak toward lower temperatures, [ 61 ] which is in correspondence to binding energies obtained by DFT calculations. [ 41–44 ] But although the H‐CW peak in bcc iron has been investigated quite extensively, [ 42,65,66,69–78 ] only limited studies have been made linking this peak to the hydrogen trapping capacity and HE. Kikuta et al [ 79 ] observed a correlation with the height of the H‐CW peak and the decrease of the notch tensile strength for pure iron and a high‐strength steel.…”

“…For example, using the Schoeck model, the binding energy of hydrogen to mixed dislocations can be determined to be 0.3 eV, [ 69 ] which is rather similar to the values obtained by TDS [ 17,21,22,61 ] and DFT results. [ 42,43 ] Alternatively, using Hirth's interpretation, the binding of hydrogen to a screw dislocation could be estimated to be 0.2–0.3 eV based on the shift of the γ‐peak toward lower temperatures, [ 61 ] which is in correspondence to binding energies obtained by DFT calculations. [ 41–44 ] But although the H‐CW peak in bcc iron has been investigated quite extensively, [ 42,65,66,69–78 ] only limited studies have been made linking this peak to the hydrogen trapping capacity and HE.…”

“…Many DFT calculations indicate a strong interaction between vacancies and hydrogen in bcc iron. [34][35][36][37][38][39]43,[84][85][86][87][88][89][90] In general, it is found that the binding energy of a hydrogen atom to a monovacancy is %0.6 eV. Moreover, also clusters of multiple hydrogen atoms around one vacancy can be formed and one vacancy can accommodate up to six hydrogen atoms.…”

“…Furthermore, the binding energy of hydrogen to dislocations is shown to be strongly dependent on the position of hydrogen at the dislocation [40][41][42] and the dislocation type. [43,44] Typically, the maximum binding energy at an edge dislocation core is %0.4-0.5 eV, [44] whereas the maximum binding energy to a screw dislocation is considerably lower, i.e., %0.2-0.3 eV. [41][42][43][44] But when the screw core configuration is altered from the easy core to the hard core configuration, the binding energy of hydrogen is increased to %0.4 eV.…”

“…[43,44] Typically, the maximum binding energy at an edge dislocation core is %0.4-0.5 eV, [44] whereas the maximum binding energy to a screw dislocation is considerably lower, i.e., %0.2-0.3 eV. [41][42][43][44] But when the screw core configuration is altered from the easy core to the hard core configuration, the binding energy of hydrogen is increased to %0.4 eV. [41] However, often discrepancies exist between the calculated results present in literature results due to different computational details as well as different assumptions, e.g., regarding the initial configuration.…”