Hydrogen influence on diffusion in nickel from first-principles calculations

Abstract

We propose a method to evaluate the diffusion coefficient of vacancy-hydrogen clusters (VH n ) in metals. The key is a good separation of time scales between H diffusion and the metal-vacancy exchange. The Ni-H system is investigated in details, using ab initio calculations, but the arguments can be transposed to other systems. It is shown that cluster diffusion can be treated as an uncorrelated random walk and that H is always in equilibrium before the vacancy-metal exchange. Then, the diffusion coefficient is a sum over jump paths of the equilibrium probability of being in a specific VH n configuration times the corresponding activation terms. The influence of H on the energy barrier is well reproduced by effective pair interactions between the jumping Ni and the H atoms inside the vacancy. This model is motivated by an analysis of the electronic charge redistribution in key saddle configurations. The interaction is repulsive and decreases with distance. The model is used to find easy jump path, reduce the number of saddle searches, and provide an estimate of the error expected from this reduction. The application to the Ni-H system shows that vacancies are drastically slowed down by H. The effects of temperature and bulk H concentration are explored and the origin of the non-Arrhenius behavior is explained. At equilibrium, VH n clusters always induce a speedup of metal diffusion. The implications concerning H induced damage, in particular in regards to Ni-Cr oxidation, are discussed.

Study of vacancy-(H,B,C,N,O) clusters in Al using DFT and statistical approaches: Consequences on solubility of solutes

“…The maximal effect is at low T (300 K), where full filled vacancies { VH 6 ) have large concentration for usual H concentra tions. We clearly see that even if VHm should move slowly like in nickel [33], the abundance of vacancies will affects the diffusion of species in aluminum.…”

Effects of grain size and deformation temperature on hydrogen-enhanced vacancy formation in Ni alloys

“…It is assumed here that hydrogen charging enhances generation of thermally activated vacancies primarily at grain boundaries and specimen free surfaces. Using the average time for a vacancy diffusion hop in Ni at 600 K (0.6  10 -4 s) [35] the upper value of the characteristic diffusion path for a monovacancy under the applied charging conditions is estimated to be 33 m. This implies the SC and 1-mm grain size samples will Grain size effects on vacancy generation and clustering are assessed as a function of material condition.…”

Impact of the clusterization on the solubility of oxygen and vacancy concentration in nickel: A multi-scale approach

“…Thus, it has been seen that a large segregation of H atoms in the vacancies increases significantly the concentration of vacancies, especially at low and intermediate temperatures (below 600 K). It has also been shown that in nickel, hydrogen-vacancy interactions enhance the diffusion mechanisms of solutes or self-interstitial atoms at low temperatures [3].…”