Hydrogen-Enhanced Vacancy Diffusion in Metals

Abstract

Vacancy diffusion is fundamental to materials science. Hydrogen atoms bind strongly to vacancies and are often believed to retard vacancy diffusion. Here, we use a potential-of-mean-force method to study the diffusion of vacancies in Cu and Pd. We find H atoms, instead of dragging, enhance the diffusivity of vacancies due to a positive hydrogen Gibbs excess at the saddle-point: that is, the migration saddle attracts more H than the vacancy ground state, characterized by an activation excess ΓH m ≈ 1 H, together with also-positive migration activation volume Ωm and activation entropy S m. Thus, according to the Gibbs adsorption isotherm generalized to the activation path, a higher μH significantly lowers the migration free-energy barrier. This is verified by ab initio grand canonical Monte Carlo simulations and direct molecular dynamics simulations. This trend is believed to be generic for migrating dislocations, grain boundaries, and so on that also have a higher capacity for attracting H atoms due to a positive activation volume at the migration saddles.

Accelerating off-lattice kinetic Monte Carlo simulations to predict hydrogen vacancy-cluster interactions in $α$-Fe

“…It has also been found that the interaction between H and vacancy-clusters has not followed the pattern if extrapolating from V 1 H; perhaps the most interesting behaviour is the reduction in diffusion barrier, upon introduction of H into the larger clusters V 4 H and V 5 H. A similar phenomenon, dubbed the hydrogen lubrication effect, has been explored computationally in FCC metals [80]. If this trend continues and the gap continues to increase, it could help to explain the experimentally observed H-induced nano-void migration [81] and could have implications for HE by contributing towards vacancy agglomeration during the HESIV mechanism [14,82,83].…”

Dislocation‐Mediated Hydride Precipitation in Zirconium

“…Furthermore, the vacancy carrying hydrogens can even move faster and is therefore easier to gather together. [ 47 ] This facilitates the nucleation of hydride embryos, which requires a large number of highly concentrated hydrogen. Figure S4, Supporting Information, shows one example of a nano hydride nucleation at the subsurface of the sample and inward growth into the Zr matrix (see Movie S6, Supporting Information).…”

Influence of hydrogen vacancy interactions on natural and artificial ageing of an AlMgSi alloy

“…Ab initio calculations have shown that interactions between hydrogen atoms and vacancies are attractive in aluminium (interaction energy between -0.25 eV [29] and -0.33 eV [28]), indicating that hydrogen atoms most probably stand in the vicinity of vacancies. But, ab initio calculations have also shown that hydrogen can affect the vacancy mobility both ways (increases in Ni [39] and decreases in Cu or Pd [40]). Since H atoms may be located on octahedral or tetrahedral sites in FCC metals [29], their exact position in the lattice may influences diffusion mechanisms.…”