Hydrogen effects on the mobility of edge dislocations in BCC Fe and ferrite

Abstract As hydrogen becomes one of the most promising clean energy technologies, understanding the effects of this element in metallic materials becomes increasingly important. The hydrogen enhanced localised plasticity (HELP) model, one of the most commonly used to explain hydrogen embrittlement phenomena, is based on observations of increased dislocation mobility in the presence of hydrogen. However, the mechanism by which hydrogen increases mobility remains unclear. In order to elucidade HELP phenomena, molecular dynamics simulations were used to investigate the effects of hydrogen on the dynamics of an edge dislocation gliding on the 1 2 ⟨ 1 1 1 ⟩ { 1 1 0 } slip system in BCC Fe and ferrite (the solid solution of carbon in BCC iron) at temperatures between 100 K and 600 K under applied stresses between 20 MPa and 800 MPa. Simulation results show that hydrogen atoms reduced dislocation mobility which followed viscuous drag dynamics in pure iron and solute drag in presence of sufficient hydrogen solutes. When a C interstitial was present in the absence of hydrogen, the dislocation motion was dominated by the contact times between the C atoms and the dislocation. No dislocation motion was seen when hydrogen was added, suggesting a strong pinning effect of the C and H interstitials. The simulations suggest that hydrogen is unlikely to promote the motion of edge dislocations under the conditions used in our simulations in Fe crystals and in presence of C interstitials. This finding conflicts with the HELP theory but enhanced dislocation mobility could be possible in different scenarios not reviewed for in this work (such as screw dislocations, other obstacles such as grain boundaries or different temperatures).