Is Hydrogen Diffusion along Grain Boundaries Fast or Slow? Atomistic Origin and Mechanistic Modeling

Area of Science:

• Materials Science
• Computational Materials Science
• Physical Chemistry

Background:

• Grain boundaries (GBs) significantly influence hydrogen (H) diffusion in metals, impacting material properties and performance.
• Understanding H diffusion mechanisms at GBs is crucial for predicting hydrogen embrittlement and optimizing material behavior.

Purpose of the Study:

• To elucidate the distinct roles of grain boundaries (GBs) in affecting hydrogen (H) diffusion in face-centered cubic nickel (Ni).
• To demonstrate the transition between slow and fast H diffusion along GBs and identify the underlying mechanisms.

Main Methods:

• Comprehensive first-principles calculations.
• Kinetic Monte Carlo simulations.
• Dislocation description of GBs and the Frank-Bilby model.

Main Results:

• A transition from slow to fast H diffusion along GBs was observed, characterized by an abrupt change in H diffusivity.
• Low-angle GBs act as high-barrier trapping regions inhibiting H diffusion, accurately described by the classical trapping model.
• High-angle GBs provide interconnected low-barrier channels facilitating H transport.
• The slow-fast diffusion transition was identified to result from dislocation core overlapping and was accurately predicted.

Conclusions:

• GBs play distinct roles in H diffusion in Ni, with low-angle GBs impeding and high-angle GBs promoting H transport.
• The study provides mechanistic insights for interpreting experimental H diffusion studies and for predictive modeling of hydrogen embrittlement.
• This work enhances the understanding of H and other interstitial impurity kinetics within metal microstructures.