Hydrogen induced vacancy clustering and void formation mechanisms at grain boundaries in palladium

Jonathan M Polfus,Ole Martin Løvvik,Rune Bredesen,Thijs Peters

doi:10.1016/j.actamat.2020.06.007

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 Σ3 and Σ5 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 H2, 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.

Highlights

The role of hydrogen on the structural and functional properties of metallic materials has long been a subject of interest for a broad range of applications
Nazarov et al performed a comprehensive study of hydrogen interacting with mono- and divacancies in twelve different fcc metals including Pd; the results consistently showed energetically favorable trapping of multiple hydrogen interstitials in the vacancies [33]
We further investigate vacancy and void formation mechanisms in the bulk and along grain boundaries of palladium in the presence of hydrogen by means of density functional theory (DFT) calculations

Summary

Introduction

The role of hydrogen on the structural and functional properties of metallic materials has long been a subject of interest for a broad range of applications. Metals are in many cases directly exposed to hydrogen-containing environments, for instance when used as sensors [1], hydrogen-selective membranes [2], catalysts [3] and hydrogen plasma facing materials in nuclear fusion reactors [4]. Hydrogen is a typical product of corrosion and otherwise omnipresent as the most abundant element in the known universe. Exposure to hydrogen can lead to structural damage and deterioration of the mechanical integrity of the material. Combined with a range of microstructural changes, one of the primary mech-

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