Hydrogen Effect on the Mobility of Edge Dislocation in α-Iron: A Long-Timescale Molecular Dynamics Simulation

Abstract

Explaining the hydrogen effect on dislocation mobility is crucial to revealing the mechanisms of hydrogen-related fracture phenomena. According to the general perspective, reducing the speed of dislocation can give enough time to hydrogen to catch up with the dislocation migration. In this research, we conducted molecular dynamics (MD) simulations to investigate the impact of hydrogen on the edge-dislocation motion in α-iron at various dislocation speeds and temperatures. It was discovered that, for all hydrogen concentrations evaluated in this paper, the hydrogen effect on dislocation transition from pinning to dragging occurs at a dislocation speed of around 0.1 m/s at 300 K. When the dislocation velocity is reduced to 0.01 m/s employing long timescale MD simulations over 1 µs, it is observed that hydrogen follows dislocation motion with small jumps in the dislocation core. The required stress to migrate the edge dislocation at a speed of 0.01 m/s was discovered to be 400 MPa, even at a lower hydrogen concentration, which was achieved in a gaseous hydrogen environment with lower pressure than atmospheric pressure. Although the dislocation still traps hydrogen at 500 K, as temperature increases, the impact of hydrogen on the shear stress required for dislocation glide becomes negligibly small. The required shear stress at lower dislocation speeds was predicted by employing the stress-dependent thermal activation model assuming the hydrogen diffusion rate-determining. The finding demonstrated that the edge dislocation should slow down until 1 mm/s order or less in the presence of hydrogen and suitable stress for α-iron.