Origins of large activation volume for dragging, serrated flow stress, and localized plasticity: Analyses based on atomistic simulations of hydrogen–edge dislocation interactions in α-iron

Herein, we performed comprehensive molecular dynamics (MD) simulations, including long-term submicrosecond calculations, to analyze the interaction between hydrogen atoms and an {112}<111> edge dislocation in α-iron. We confirmed three interactions: (1) dragging, (2) cycle of pinning and depinning (pinning/depinning), and (3) twinning initiation. Dragging was typically observed at low dislocation speeds ( ≤ 0.1 m/s) and high temperatures. The drag stress was proportional to trapped-hydrogen concentration ( C H ) and dislocation speed and sharply decreased with increasing temperature. Twinning was caused by the strong pinning effect on the involved dislocation by hydrogen atoms and was typically observed at low temperatures and high C H values. Under intermediate conditions, pinning/depinning was observed. Based on atomistic mechanisms revealed via MD simulations, we developed theoretical models for these interactions and verified their predictability. We also clarified that dragging and partial/temporal depinning occurred simultaneously at wide ranges of strain rates and temperatures. Subsequently, we predicted the shear stress–dislocation speed relation for long dislocation lines and the boundaries of dragging-dominant, depinning-dominant, and twining regions at realistic strain rates; these predictions were beyond the spatiotemporal reach of direct atomistic simulations. We also clarified the origins of the experimentally measured large activation volume for dragging as well as the modification of stress–strain curves and deformation behavior by hydrogen atoms. Finally, we proposed mechanisms that accounted for the coexistence of high flow stress and localized plasticity. The developed theoretical models and identified mechanisms can be applied to solute–dislocation interactions across various slip systems, material–solute combinations, and crystallographic structures.