Molecular dynamics studies of hydrogen diffusion in tungsten at elevated temperature: Concentration dependence and defect effects
Introduction
Due to its deleterious effects on metallic and alloy materials, hydrogen diffusion plays one of the most important factors in many technological challenges such as steel embrittlement [[1], [2], [3], [4], [5], [6], [7], [8]] and hydrogen assisted cracking [[9], [10], [11], [12], [13], [14], [15], [16], [17]]. In particular, in fusion reactor environment, plasma facing materials (PFMs) experience high fluence plasma irradiation and hydrogen isotopes could penetrate through the surface and congregate in the bulk of PFMs [[18], [19], [20], [21], [22]]. This will result in issues of hydrogen isotopes retention and PFMs surface blistering, which eventually affects the long-term operations and safety of the reactors [[23], [24], [25], [26]]. To fully address these problems, understanding hydrogen diffusion in tungsten, which is one of the most promising candidates of PFMs, is essential. Numerous studies have been performed to investigate the mechanism and character of hydrogen diffusion in tungsten [[27], [28], [29], [30], [31], [32]], and most of these studies are performed within pristine tungsten bulk. In practical fusion environment, however, collisional cascades fired by neutron irradiations will result in massive structure damages including mono-vacancies, vacancy clusters, interstitials and dislocation loops in tungsten bulk. These defects could interact with hydrogen isotopes and further affect hydrogen diffusivity and retention, which leads to a synergistic effect of neutron and plasma irradiation on tungsten. There have been several experiments that observed a significant decrease of hydrogen diffusivity in damaged tungsten [[33], [34], [35]], and various trapping effects of irradiation defects on hydrogen isotopes have been widely confirmed by numerous thermal desorption spectroscopy (TDS) measurements [[34], [35], [36], [37], [38], [39], [40], [41], [42]]. Further, the influence of irradiation defects on hydrogen behaviors has been found not only determined by defect types [43], concentrations [31,44],distribution [37], but also related to temperatures and annealing time [33,35,36]. Unfortunately, the micro-mechanisms of diffusion and trapping of hydrogen atoms in neutron-irradiated tungsten at elevated temperatures are still unclear [34]. Underling physics of hydrogen isotopes diffusion with trapping effects, and careful characterization of microstructures and defects evolution are required to improve understanding of effective hydrogen isotopes diffusivity [45].
Due to resolution limitation, experiments are usually difficult to study hydrogen diffusion processes at atomic-scale. Molecular dynamics simulation, instead, provides a powerful means. By employing empirical interatomic potentials, molecular dynamics could directly model hydrogen diffusion including trapping and detrapping processes at different temperatures, and shed light on micro-structure evolution of hydrogen with irradiation defects in tungsten. Therefore, in this paper, we employ molecular dynamics to directly model hydrogen diffusion in tungsten and systematically investigate the influences of hydrogen concentration and structural defects introduced by neutron irradiation. We will first discuss how hydrogen concentration affects its diffusion in tungsten in Concentration effects. Thereafter, neutron irradiation defects are considered including self-interstitials, vacancies and vacancy clusters in Defect effects. Results of these investigations are supposed to be not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for the long-term simulation methods such as kinetic Monte Carlo and rate theory models.
Section snippets
Methods
A new embedded atom method (EAM) potential developed by Wang et al. for hydrogen-tungsten system is employed [46]. This potential takes the tungsten-tungsten interactions from Marinica et al. [47], and fits the hydrogen-tungsten and hydrogen-hydrogen interactions to density functional theory data. It exhibits good performance on simulating hydrogen diffusion in tungsten, and it accurately reproduces the interaction among hydrogen atoms and various defects in tungsten.
Molecular dynamics
Concentration effects
Even though hydrogen is an endothermic impurity and its equilibrium concentration is low in tungsten, due to the large flux of hydrogen irradiation in the nuclear fusion reactor, the actual local concentration of hydrogen could exceed the equilibrium value, and be high enough to strongly affects the properties of tungsten [54]. Thus, the hydrogen concentration itself is a non-negligible factor of its diffusion. It has been speculated that the diffusivity of hydrogen will decrease when
Conclusion
By carrying out molecular dynamics simulations, we systematically investigate the concentration and irradiation defect effects on hydrogen diffusion in tungsten. It is found the increase of concentrations of hydrogen, vacancy or SIAs could significantly reduces hydrogen diffusivity. In particular, at high hydrogen concentration, platelet-like hydrogen clusters where hydrogen atoms occupy the octahedral interstitial sties could be spontaneously formed in pristine tungsten, which leads to a
Acknowledgments
This work is supported by Science Challenge Project under Grants No. TZ2016001 and TZ2018002, National MCF Energy R&D Program under Grant No. 2018YFE0308100, and the the National Natural Science Foundation of China with Grant No. 51871007 and No. 51720105006.
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