Atomistic modelling of the kinetics of diffusion and segregation of light elements in steel. Application to H in Fe
Hydrogen impurities in iron and steels can significantly influence the mechanical properties of the material. An important aspect is the segregation of hydrogen to defects such as grain boundaries (GBs), leading to an enhanced local concentration of hydrogen. At GB interfaces such a high concentration of hydrogen can weaken the bonding strength between Fe atoms across the GB interfaces, leading to a degradation of the mechanical properties of the material.
In our work, we study the interaction of H interstitials with representative GBs in bcc Fe, and investigate migration processes of H atoms within these GBs, using density functional theory (DFT). We find that the investigated bcc Fe GBs provide energy traps for H interstitials, and that these trapped H interstitials may facilitate the crack growth along the GB interfaces.
In our work, we study the interaction of H interstitials with representative GBs in bcc Fe, and investigate migration processes of H atoms within these GBs, using density functional theory (DFT). We find that the investigated bcc Fe GBs provide energy traps for H interstitials, and that these trapped H interstitials may facilitate the crack growth along the GB interfaces.
Our results show that diffusion barriers for hydrogen interstitials are considerably higher within the grain boundary interfaces as compared to diffusion barriers in bcc Fe bulk. In open grain boundary structures H atoms can still diffuse with a lower diffusivity, whereas H interstitials that are trapped at closely-packed Fe GBs become practically immobile.
To study the diffusion behavior we setup a lattice kinetic Monte Carlo model of hydrogen within an idealized grain structure in bcc Fe. The needed hydrogen migration barriers are obtained from our DFT calculations. Within our model we can then study the dependence of hydrogen diffusivity on grain sizes, temperature, and hydrogen concentration.
