Changes in Dislocation Punching Behavior Due to Hydrogen-Seeded Helium Bubble Growth in Tungsten

TL;DR

This study uses molecular dynamics to explore how combined helium and hydrogen gas bubbles affect dislocation behavior in tungsten, revealing a thermodynamic switch that could influence surface damage and hydrogen trapping.

Contribution

It provides the first atomistic insight into the combined effects of He and H bubbles on dislocation mechanisms in tungsten, highlighting a thermodynamic crossover in dislocation types.

Findings

H addition changes dislocation loop characteristics in He bubbles.

High H concentrations favor sessile $<$100$>$ dislocations over glissile ones.

The thermodynamic switch may reduce surface morphology changes in tungsten.

Abstract

The accumulation of gas atoms in tungsten is a topic of long-standing interest to the plasma-facing materials community due the metal's use as a divertor material in some tokamak fusion reactors. The nucleation and growth of He/H gas bubbles (along with their isotopes) can result from impinging fluxes of these gases which give rise to damage at the W divertor surface. The inclusion of He or H in W has been studied extensively by the community, finding that He bubbles modify the surface through periodic dislocation punching and bursting mechanisms while H bubbles impact the metal through plastic-strain induced material failure. However, the mechanisms which are present during the combined flux of both He and H is not well-studied atomistically. Motivated by this, an atomistic modeling study is conducted using molecular dynamics to assess the behavior of mixed concentration He:H bubbles in W. We find that the introduction of H into a growing He bubble results in a dramatic change in the nature and presence of dislocation loops which are typically generated via dislocation punching in over-pressurized He bubbles. Most notably, at high H concentrations, there is a switchover in energetic favorability from glissile 1/2$<$111$>$ dislocations to sessile $<$100$>$ dislocations. This thermodynamic crossover could imply a significant reduction in W surface morphology changes than with pure He bubbles and, additionally, we show this to have implications on the trapping of H in the bubbles and their associated dislocations.