Anisotropy of hydrogen diffusion in nickel single crystals: the effects of self-stress and hydrogen concentration on diffusion

Abstract

Hydrogen diffusion has an important role in solute-dependent hydrogen embrittlement in metals and metallic alloys. In spite of extensive studies, the complexity of hydrogen diffusion in solids remains a phenomenon that needs to be clarified. In this paper, we investigate the anisotropy of hydrogen diffusion in pure nickel single crystals using both an experimental approach and a thermodynamic development. As a first approximation, experimental data from electrochemical permeation and thermal desorption spectroscopy are described using the classical Fick’s laws and an apparent diffusion tensor. Within a thermodynamic framework, the diffusion equation can be derived from Fick’s laws with an apparent diffusion coefficient which contains an added solute content dependent term β. This term is due to the elastic strain field associated with the insertion of solute atoms. For nickel crystals, the dependence of β on the crystallographic orientation arises from the elastic anisotropy. Additionally, our results elucidate the discrepancies between the thermodynamic model and experimental observations of the effect of the solute concentration on the diffusion process. Moreover, this highlights the importance of the impact of hydrogen on vacancy formation and the subsequent consequences on the anisotropy of the apparent diffusion coefficient.

Effects of Incorporated Iron or Cobalt on the Ethanol Oxidation Activity of Nickel (Oxy)Hydroxides in Alkaline Media

“…These facts would agree with the direct involvement of Ni(III) in the EOR and with Ni being the active site in these catalysts. It is worth to mention that the catalytic activity and/or Ni(II) oxidation of bimetallic materials may also be influenced by other processes such as the adsorption energies of the reactants/products or parameters influencing the diffusion processes within the (oxy)hydroxide structure such as the crystal lattice constant, presence of counterions [60], availability of vacant sites [61], or local stress field [62]. The combined effect of those properties would probably explain the different activity of the catalyst at different metallic ratios.…”

Multiscale analysis of hydrogen-induced softening in f.c.c. nickel single crystals oriented for multiple-slips: elastic screening effect

“…Vacancy formation and clustering induce a long-range internal strain which reduces apparent elastic stiffness coefficients to a greater extent than hydrogen self-stress. Consequently, the softening behavior observed for short-range interactions in dislocations can be directly related to the point defects and clusters of vacancies produced during the initial incorporation of hydrogen, as confirmed by TEM observations 11,45 and our present measurement (around 3.8 10 −4 V/Ni for the unstrained sample after a hydrogen pre-charging to 7 wppm compared with 2.8 10 −24 V/Ni at P H2 = 1 bar and 300 K for pure nickel single crystals 46 ). This interpretation is mainly based on the hypothesis that vacancy concentration does not change during strengthening, which is not the case, as previously shown 47,48 .…”

“…These analyses were performed with a Jobin Yvon Horiba EMGA-621W hydrogen analyzer composed of an impulsion furnace system coupled with a thermal conductivity detector. Initial or after-straining conditions, vacancy concentration induced by hydrogen incorporation (SuperAbundant Vacancies SAV 46,52 ) and/or tensile strain were measured using Differential Scanning Calorimetry (DSC) 11,45,51 . After the introduction into the Q100 TA Instruments DSC test bench, the sample was heated from room temperature up to 673 K at increments of 10 K/min.…”

Three‐dimensional immersed finite‐element method for anisotropic magnetostatic/electrostatic interface problems with nonhomogeneous flux jump

“…Anisotropy is a common phenomenon in many materials and media due to the varying composition of their constituents. Accurate description of their properties in different directions is important for understanding their physical characteristics in applications . Also, in anisotropic media, the electric/magnetic field usually presents anisotropy .…”