Strain Effects on the Diffusion Properties of Near-Surface Self-Interstitial Atoms and Adatoms in Tungsten

Bochuan Sun,Enrique Martínez,Brian D Wirth,Dimitrios Maroudas

doi:10.3389/fmats.2021.678858

Abstract

Tungsten (W) is a candidate for the plasma-facing components and divertor in future fusion applications. The material will be subject to a large particle influx (mainly helium and hydrogenic species) that will form bubbles. As bubbles grow, they compress the material, adding to thermal stresses, and eject self-interstitial atoms (SIAs—isolated or in clusters) to release internal pressure. These SIAs diffuse towards the surface in large stress/strain fields and on the surface are thought to act as precursors for nanotendril formation (also known as fuzz) that develops on the material surface modifying its morphology. In this work we analyze the effect of strain on the diffusion properties of both SIAs and adatoms. Relying on atomistic simulations, we compute the average time that a SIA created in the center of a tungsten slab takes to reach a (110) surface for different strains and temperatures. This time relates to the SIA diffusivity and allows to compute the activation energy and dipole tensor including surface effects. We observe a large strain effect that significantly modifies the propensity for SIAs to reach the surface and, hence, to cluster to form dislocation loops in the bulk crystal. Strain also alters the diffusivity of the adatom although to a lesser extent. Finally, we report on the resulting surface roughness evolution and its dependence on strain.

Highlights

In fusion environments, plasma-facing components (PFCs) are exposed to stringent conditions as imposed by the plasma, required for the nuclear reaction to occur
The results presented here intend to convey a general picture of the effects of strain on the transport of self-interstitial atoms (SIAs) and its effect on the morphological evolution of the free surface, in conjunction with providing activation parameters that can be readily used in largerscale models, such as in cluster dynamics or reduced-order frameworks. (Blondel et al, 2018; Dasgupta et al, 2019)
We have analyzed the effect of equibiaxial strain on the transport of self-interstitial atoms towards a (110) surface and of an adatom at this surface in tungsten relying on a molecular dynamics methodology

Summary

Introduction

Plasma-facing components (PFCs) are exposed to stringent conditions as imposed by the plasma, required for the nuclear reaction to occur. Certain drawbacks have been observed, such as the formation of a fuzz-like nanostructure in the W surface due to the helium (He) ash implantation, which leads to the formation of high-pressure bubbles that eject self-interstitial atoms (SIAs) to relax such high stresses, Strain Effects on Tungsten Transport generating vacancies in the bubbles The diffusion of these SIAs, either individually or in clusters, towards the surface in the presence of stresses generated thermally and by the bubble distribution, seems to be an important mechanism contributing to the observed surface morphological evolution and nanotendril formation (Takamura et al, 2006; Baldwin et al, 2009; Yang et al, 2015; Wang et al, 2017). Fuzz forms under certain plasma conditions, when W is exposed to He and He-deuterium (D) mixed plasmas but not for pure D plasmas (Baldwin et al, 2009), at exposure temperatures in the range of 900–2000 K and incident ion energies around 20–100 eV with incoming fluxes on the order of 1024 m−2 s−1. (Kajita et al, 2009) Note that these incident energies are insufficient to generate radiation damage in the form of vacancies and selfinterstitials, but lead to the formation of over-pressurized bubbles and subsequent trap mutation

Objectives

Methods

Results

Discussion

Conclusion