Proceedings of the Third International Conference on Metals & Hydrogen P05

# Diffusion and segregation of hydrogen in nickel single and bi crystals: stress-strain field contributions

## J. Li (1)1 , M. Hallil (1)1 , A. Oudriss (1)1 , A. Metsue (1)1 , J. Bouhattate (1)1 , X. Feaugas (1)1

• (1) 1

LaSIE, CNRS-UMR 7356, Université de la Rochelle, France, xfeaugas@univ-lr.fr

## Abstract

Hydrogen Embrittlement (HE) is one of the causes mainly evoked in premature rupture of industrial components exposed to aggressive environment. Many studies have been conducted in order to understand the mechanisms involved during this degradation. However, the effects of the grain boundaries (GBs), triple junction (TJs) and several defects (dislocations, vacancies …) in FCC materials and their interactions with hydrogen on the mechanisms of metal damages remain a controversial. Actually, several works suggest that in FCC materials the grain boundaries represent preferential paths for hydrogen diffusion (short-circuit diffusion), and this kind of hydrogen diffusion along GBs is faster than the interstitial diffusion. However, GBs contain different defects, particularly, dislocations and vacancies. These defects are able to trap hydrogen affecting the diffusion mechanisms. Although, many theories have been proposed to describe the role of GBs for hydrogen diffusion and segregation, none of them is able to give an exact answer. In addition, there is little suitable experimental data available.

Therefore, in the present study, we have studied at the first time some nickel single crystals1. We evaluate the hydrogen diffusion and trapping mechanisms using the electrochemical permeation (EP) coupled to the thermal desorption spectroscopy (TDS). The analysis of coupling stress state/solubility in a thermodynamic framework has shown the importance of the stress state induced by the presence of the solute on the diffusion. Two major effects emerge: the apparent diffusion coefficient is a function of hydrogen concentration and self-stress is the origin of the anisotropy of hydrogen diffusion in nickel, which is also a function of the concentration of hydrogen in relation with the formation of vacancies.

Later, we propose to screen several bi-crystals of pure nickel with different grain boundaries ($\dpi{80} \tiny \sum$11-50°30<110>{311}, $\dpi{80} \tiny \sum$11-129°30<110>{332}, $\dpi{80} \tiny \sum$3-70°30<110>{111} and $\dpi{80} \tiny \sum$5-37°<100>{310})2. For each bi-crystal, therefore, for each grain boundary, the EP and TDS testing were performed to study the diffusion and segregation of hydrogen. In addition, Molecular Dynamics (MD) simulations have become a useful method to comprehend the behaviour of hydrogen in these types of GBs. The results allow us to associate the short-circuit diffusion and trapping phenomena to the grain boundaries character (excess volume, defects density and distribution …) in relation with the local stress-strain field.