Hydrogen Induced Vacancy Clustering and Void Formation Mechanisms at Grain Boundaries in Palladium

Abstract

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at 3 and 5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H 2 , and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

Vacancy diffusion in palladium hydrides

“…For instance, vacancy clusters with 1-2 hydrogen atoms were found to predominate above approximately 160°C in palladium. 12 Page 5 of 14 Physical Chemistry Chemical Physics…”

“…13 Accordingly, void formation at grain boundaries may be caused by segregation of hydrogen and vacancy-hydrogen clusters in these regions. 8,12 Hydrogen can additionally affect the microstructural features of palladium such as increased dislocation densities, 6 presumably influenced by chemical expansion and associated strain. While void formation may be further facilitated by microstrain, abundant monovacancies and clusters, 11,12 the present work implies that enhanced vacancy diffusion is not a major factor in void formation.…”

“…9 The presence of hydrogen further leads to increased concentrations of metal vacancies due to favorable interactions with hydrogen interstitials through formation of vacancy-hydrogen clusters. [10][11][12] Despite their abundance, monovacancies do not spontaneously coalesce into larger voids due to the significant loss of configurational entropy that would be associated with clustering. 12 Void formation therefore appears to be caused by other factors such as decohesion related to socalled hydrogen-enhanced local plasticity.…”

“…[10][11][12] Despite their abundance, monovacancies do not spontaneously coalesce into larger voids due to the significant loss of configurational entropy that would be associated with clustering. 12 Void formation therefore appears to be caused by other factors such as decohesion related to socalled hydrogen-enhanced local plasticity. 13 Self-diffusion of palladium is central to most of the afore-mentioned degradation processes and can govern the degradation rate and lifetime of the material or device 8,14 Dissolution of hydrogen has been reported to result in hydrogen-induced lattice migration and diffusion creep in palladium.…”

Hydrogen solubility and diffusivity at Σ3 grain boundary of PdCu

“…It should be pointed out that the hydrogen defect complexes would possibly arise in the GB of PdCu and have some effects on H diffusivities and solubilities. 60…”