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Using large-scale molecular dynamics powered by a density-functional-theory-trained deep neural network potential, this study investigates the unexplained impact of hydrogen on phase transformation and deformation twinning in bcc iron. The simulations reveal that 10% hydrogen lowers yield stress while actively suppressing detwinning, which stabilizes a persistent, interconnected {332}–{10 9 3} multimodal twin network that induces post-yield hardening. These findings establish a pathway-based framework showing that hydrogen governs the kinetic survival of high-index twin modes rather than solely modifying dislocation-mediated plasticity.
Counterintuitively, dissolved hydrogen suppresses detwinning in bcc iron to lock in complex, high-index multimodal twin networks that drive unexpected post-yield strain hardening.
Hydrogen significantly affects plasticity in bcc iron, but its role is commonly attributed to dislocation-mediated mechanisms, leaving the influence of hydrogen on phase transformation and deformation twinning poorly understood. We use a density-functional-theory-trained deep-neural-network interatomic potential and large-scale molecular dynamics to study pure bcc Fe and Fe containing 10% hydrogen. It is found that hydrogen lowers the yield stress but, more importantly, increases the persistence of {112} twin variants and suppresses detwinning. This effect stabilizes an interconnected {332}-{10 9 3} multimodal twin network and produces pronounced post-yield hardening. Higher temperature promotes the initial transformation but weakens the persistence of the high-index multimodal twin network. Moreover, twinning follows distinct loading-dependent pathways: compression generates {332} and {10 9 3} boundaries through co-zone and non-co-zone twin-twin interactions, whereas tension produces {7 4 1} boundaries through non-co-zone interactions. These results establish a pathway-based picture in which the intermediate phase determines the accessible twin modes and variant crystallography, while hydrogen and temperature control their kinetic survival and the emergence of high-index twin networks.