The Hydrogen Cold Work Peak in BCC Iron: Revisited, with First Principles Calculations and Implications for Hydrogen Embrittlement

Abstract

We examine experimental and theoretical results on the cold-work (Snoek-Köster) peak in bcc Fe due to H using density functional theory (DFT). We reaffirm that Seeger's interpretation of the H cold-work peak (Hcwp), involving motion of H with kinks on non-screw dislocations associated with the intrinsic-dislocation α peak, has experimental backing. Use of the solute-dragging theory of Schoeck suggests a H-mixed dislocation binding energy of 0.3 eV. The theory of Hirth, that the Hcwp involves H-screw dislocation interaction manifested as the temperature-reduced intrinsic-dislocation γ peak by the presence of H, has merit in that our DFT calculations disclose a similar magnitude, 0.2 eV, of H-screw dislocation binding. This result offers support for models of H-enhanced localized plasticity of H embrittlement. We also explore possible roles of H-vacancy binding, shown by DFT to be characterized by a binding energy of 0.6 eV, in H trapping and H embrittlement and lesser effects of H-solute binding involving small binding energies of ~ 0.1 eV.

Site stability and pipe diffusion of hydrogen under localised shear in aluminium

“…where u i and r i are the polar coordinates defined in [41] with the extra plane of the edge dislocation above the glide plane and b = (mb e /3p)((1 + n)/(1 − n)Dv) where μ and ν are the elastic constants, b e is the norm of the Burgers vector and Dv represents a local volume change related to the insertion of the H atom at position (u i , r i ). With Dv equal to 20% of the atomic volume, the segregation energy at (u i = −p/2, r i = b e ) is −0.13 eV, a reasonable value for searching an upper bound of the elastic trapping effect (for DFT calculations on H at a screw dislocation in Fe, see [44]). Furthermore, stress also affects the energy in the saddle configuration, also in a linear way [43].…”

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.…”

“…[ 62 ] Nevertheless, a combined action of both the Hirth and Seeger mechanisms was proposed by Gibala. [ 42 ] In this case, the Hirth and Seeger mechanisms are considered to be operating more or less simultaneously and the H‐CW peak temperature is determined by the extent at which the H‐edge/mixed dislocations and the H‐screw interactions meet in IF space and allow for Schoeck‐like bowing. Furthermore, the H‐CW peak has also been mentioned to consist of two peaks, one at 120 K and one at 150 K. One way of interpretation is that the first peak is related to the interaction of hydrogen with the nonscrew dislocations and the second peak is related to the interaction with screw dislocations.…”

“…Various authors [ 34–39 ] have reported DFT calculations indicating that hydrogen can lower the vacancy formation energy and, as such, enhance vacancy formation, which is one of the cornerstones of the HESIV mechanism. Furthermore, the binding energy of hydrogen to dislocations is shown to be strongly dependent on the position of hydrogen at the dislocation [ 40–42 ] 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.…”