Molecular dynamics studies of the grain-size dependent hydrogen diffusion coefficient of nanograined Fe.

Nanograined materials have much denser grain boundary (GB) networks than their coarse-grained counterparts, thereby the hydrogen (H) diffusion and trapping behaviors in nanograined materials, which are strongly influenced by GBs, may differ greatly from those in coarse-grained materials. In the present research, the grain-size dependent hydrogen diffusion coefficient in nanograined Fe is studied by theoretical analysis and molecular dynamics (MD) simulations. A theoretical model based on thermodynamics is developed. The GB-related material parameters required by the model are then obtained by fitting the MD simulation results. Finally, the grain-size dependent diffusion coefficients are compared with model predictions to evaluate the validity of the model. It is found that the trapping effect of triple junctions that usually ignored in coarse-grained materials becomes increasing important as grain size and temperature decrease. Due to the strong trapping effect of GBs in nanograined Fe, H diffusion is slowed down by the GBs. • A theoretical model describing the effect of GBs on H diffusion is proposed. • GB-related intrinsic properties are obtained from MD simulation results. • GBs are strong traps for H atoms in nanograined Fe. • Triple junctions of GBs are found to be important trapping sites. [ABSTRACT FROM AUTHOR]