Dynamic Formation of Preferentially Lattice Oriented, Self Trapped Hydrogen Clusters

TL;DR

This study combines MD and DFT simulations with a machine-learned interatomic potential to reveal how hydrogen self-clusters into stable platelets in tungsten, which may explain high hydrogen retention and blistering.

Contribution

It introduces a new machine-learned interatomic potential to simulate hydrogen clustering in tungsten and demonstrates the formation and stability of hydrogen platelets.

Findings

Hydrogen platelets form along low Miller index orientations.

Platelets larger than six hydrogen atoms are stable at high temperatures.

Hydrogen atoms interact over longer distances within platelets than previously thought.

Abstract

A series of MD and DFT simulations were performed to investigate hydrogen self-clustering and retention in tungsten. Using a newly develop machine learned interatomic potential, spontaneous formation of hydrogen platelets was observed after implanting low-energy hydrogen into tungsten at high fluxes and temperatures. The platelets formed along low miller index orientations and neighboring tetrahedral and octahedral sites and could grow to over 50 atoms in size. High temperatures above 600 K and high hydrogen concentrations were needed to observe significant platelet formation. A critical platelet size of six hydrogen atoms was needed for long term stability. Platelets smaller than this were found to be thermally unstable within a few nanoseconds. To verify these observations, characteristic platelets from the MD simulations were simulated using large-scale DFT. DFT corroborated the MD results in that large platelets were also found to be dynamically stable for five or more hydrogen atoms. The LDOS from the DFT simulated platelets indicated that hydrogen atoms, particularly at the periphery of the platelet, were found to be at least as stable as hydrogen atoms in bulk tungsten. In addition, electrons were found to be localized around hydrogen atoms in the platelet itself and that hydrogen atoms up to 4.2 Angstrom away within the platelet were found to be bonded suggesting that the hydrogen atoms are interacting across longer distances than previously suggested. These results reveal a self-clustering mechanisms for hydrogen within tungsten in the absence of radiation induced or microstructural defects that could be a precursor to blistering and potentially explain the experimentally observed high hydrogen retention particularly in the near surface region.