Effects of lattice strain on hydrogen diffusion, trapping and escape in bcc iron from ab-initio calculations

Lattice strain potentially alters hydrogen (H) behaviors in structural materials and thus H -induced damages. Herein, we computationally investigate effects of lattice strain on H diffusion in the bulk region, and trapping by vacancy defects and escape in body-centered cubic (bcc) iron (Fe) using ab-initio calculations and statistical mechanics. The anisotropy of strain effect on H diffusion in bcc Fe is found in contrast with fcc systems, which essen-tially determines the alteration of H diffusion coefficient. The hydrostatic tensile strain attenuates H trapping, while the hydrostatic compressive strain inhibits H escape. The strong anisotropy of strain effect on H escape is confirmed, leading to low-barrier escape channels for H under the given anisotropic strain and facilitating H escape. This strong anisotropy is also reflected in the hopping of solute atoms He, C and O within {100} crystal planes. Strain effects on H trapping and escape become progressively more evident with decreasing temperature as shown by the escape rate. The obtained strain effects are in accordance with previous experimental observations on H in iron and steels under loading. Furthermore, the low-barrier channels of H escape from vacancy defects under strain are found to be the pathways where the density of electron gas is lower and the H-induced lattice distortion is weaker. The above results indicate a possibility of strain-promoted H -induced degradation of materials: strain-accelerated H transport from defects with low trapping depths for H to those with high trapping depths for H. This work also provides significant insights towards better understanding of H-isotope retention under strain in fusion reactors.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

In this study, the effects of lattice strain on hydrogen diffusion, trapping, and escape in body-centered cubic iron were investigated computationally using ab-initio calculations and statistical mechanics. The results showed that the anisotropy of strain effect on hydrogen diffusion in body-centered cubic iron is opposite to that in face-centered cubic systems, altering the diffusion coefficient. Hydrostatic tensile strain weakens hydrogen trapping, while hydrostatic compressive strain inhibits hydrogen escape. The strong anisotropy of strain effect on hydrogen escape leads to low-barrier escape channels for hydrogen under the given anisotropic strain, facilitating its escape.