[Paper Review] Hydrogen clustering in bcc metals: atomic origin and strong stress anisotropy

Jie Hou, Xiang-Shan Kong+2 more|arXiv (Cornell University)|May 14, 2020

Nuclear Materials and Properties70 references4 citations

TL;DR

This study reveals that hydrogen clustering in body-centered cubic (bcc) metals like W, Fe, Mo, and Cr is strongly driven by anisotropic stress fields, particularly tensile stress along <001> crystallographic directions. Using first-principles and atomistic simulations, the authors demonstrate that <001> edge dislocations maximize this tensile component, enabling thermodynamically favorable nanohydride formation at low H concentrations, with platelet-like hydride morphologies aligning to maximize <001> tension.

ABSTRACT

Hydrogen (H) induced damage in metals has been a long-standing woe for many industrial applications. One form of such damage is linked to H clustering, for which the atomic origin remains contended, particularly for non-hydride forming metals. In this work, we systematically studied H clustering behavior in bcc metals represented by W, Fe, Mo, and Cr, combining first-principles calculations, atomistic and Monte Carlo simulations. H clustering has been shown to be energetically favorable, and can be strongly facilitated by anisotropic stress field, dominated by the tensile component along one of the &lt;001&gt; crystalline directions. We showed that the stress effect can be well predicted by the continuum model based on H formation volume tensor, and that H clustering is thermodynamically possible at edge dislocations, evidenced by nanohydride formation at rather low levels of H concentration. Moreover, anisotropy in the stress effect is well reflected in nanohydride morphology around dislocations, with nanohydride growth occurring in the form of thin platelet structures that maximize one &lt;001&gt; tension. In particular, the &lt;001&gt; type edge dislocation, with the &lt;001&gt; tensile component maximized, has been shown to be highly effective in facilitating H aggregation, thus expected to play an important role in H clustering in bcc metals, in close agreement with recent experimental observations. This work explicitly and quantitatively clarifies the anisotropic nature of stress effect on H energetics and H clustering behaviors, offering mechanistic insights critical towards understanding H-induced damages in metals.

Motivation & Objective

• To resolve the atomic origin of hydrogen clustering in non-hydride-forming bcc metals, where the mechanism remains controversial.
• To investigate the role of stress fields—particularly anisotropic components—in facilitating hydrogen aggregation.
• To determine the thermodynamic feasibility of hydrogen clustering at dislocation sites, especially edge dislocations.
• To correlate stress anisotropy with observed nanohydride morphology in bcc metals.
• To validate the predictive power of continuum models based on hydrogen formation volume tensor in describing clustering behavior.

Proposed method

• Employed first-principles density functional theory (DFT) calculations to compute hydrogen formation energies and formation volume tensors in W, Fe, Mo, and Cr.
• Conducted atomistic simulations to model hydrogen clustering behavior under various stress conditions.
• Applied Monte Carlo simulations to assess thermodynamic stability of hydrogen clusters under different stress anisotropies.
• Used a continuum model based on the hydrogen formation volume tensor to predict stress-dependent hydrogen energetics.
• Analyzed the interaction between hydrogen and <001> edge dislocations, focusing on stress field components and their influence on clustering.
• Correlated simulated stress fields with observed nanohydride morphologies, particularly platelet structures aligned with <001> tensile directions.

Experimental results

Research questions

• RQ1 What is the atomic-scale origin of hydrogen clustering in non-hydride-forming bcc metals?
• RQ2 How does anisotropic stress, particularly along <001> directions, influence hydrogen clustering energetics?
• RQ3 Can the stress effect on hydrogen clustering be quantitatively predicted using a continuum model based on formation volume tensors?
• RQ4 Is hydrogen clustering thermodynamically favorable at edge dislocations, even at low hydrogen concentrations?
• RQ5 How does stress anisotropy manifest in the morphology of nanohydrides around dislocations?

Key findings

• Hydrogen clustering is energetically favorable in bcc metals and is strongly promoted by tensile stress along <001> crystallographic directions.
• The <001> edge dislocation exhibits the highest tensile component along <001>, making it the most effective site for hydrogen aggregation.
• Nanohydride formation is thermodynamically feasible at low hydrogen concentrations due to stress-driven stabilization at dislocation cores.
• The stress effect on hydrogen clustering is well predicted by the continuum model using the hydrogen formation volume tensor.
• Nanohydrides grow in thin platelet structures that maximize the <001> tensile component, reflecting strong morphological anisotropy.
• The findings are in close agreement with recent experimental observations of hydride formation at dislocation sites in bcc metals.