One of the key requirements of potential plasma-facing material PFMs is low hydrogen retention. A possible candidate material is tungsten, which features a very low solubility for hydrogen among other favorable properties such as a low physical sputtering yield and a high melting temperature. However, the actual amount of hydrogen that can be permanently retained in tungsten is determined by the defect in the material. Solute atoms, whether they are voluntary added to tailor the properties of materials, left from the elaboration process, or introduced during the component lifetime as a result of radiation damage, can immediately impact the hydrogen retention in materials as one type of defect and also indirectly affect the hydrogen behavior by modifying other defect properties. Recent experiments clearly showed that the solute atoms can significantly affect the hydrogen retention in tungsten, which shows a peculiar flux effect-dependence. (Nuclear Fusion 53, 23021; Nuclear Fusion 53, 13013; Nuclear Fusion 54, 23013).

In order to reveal the influence of the solute atoms on the hydrogen retention in tungsten, a research team led by Prof. LIU Changsong in Institute of Solid State Physics, Hefei Institutes of Physical Sciences, recently investigated the interaction of transitional metal (TM) solute with hydrogen in tungsten by first-principles calculations, which reveals the physical mechanism of the solute-H interaction in metals, provides a sound explanation for the recent experimental phenomena of hydrogen retention in tungsten alloy, and recommends a suitable W–Re–Ta ternary alloy for possible PFMs. This study was published in Nuclear Fusion entitled First-principles calculations of transition metal solute interactions with hydrogen in tungsten.

In the study, they performed systematic first-principles calculations to predict the interaction between TM solutes and hydrogen in the interstitial site as well as the vacancy in tungsten. They found the solute-H interactions are mostly attractive except for Re, which can be well understood in terms of the competition between the chemical and elastic interactions (Fig. 1).The chemical interaction dominates the solute-H interaction for the TM solutes with a large atomic volume and small electronegativity compared to tungsten (the early elements), while the elastic interaction is primarily responsible for the solute-H interaction for the TM solutes with a small atomic volume and large electronegativity relative to tungsten (the latter elements).

Therefore, the solute-H binding energies show a negative correlation to the atomic volume for the latter elements and exhibit a negative correlation to electronegativity for the early elements. This correlation of the solute-H binding energies with the solute atomic size and electronegativity can be applied in other metal-H systems.

Based on the first-principles calculations results, they estimated the influence of some potential alloying elements, including Re, Ta, Ti and V, on hydrogen dissolution and diffusion in tungsten.

On the one hand, these solute atoms, as pre-existing defects, can trap multiple hydrogen atoms to form interstitial hydrogen clusters, impede the hydrogen diffusion, and thus increase the hydrogen retention in tungsten. On the other hand, these solutes exert a restraining effect on the growth of the radiation-induced defects and decrease the concentration of defects, particularly the vacancies and vacancy clusters, and finally decrease the retention of hydrogen in tungsten.

Thus, the flux effect-dependence of the influence of solute atoms on hydrogen retention in tungsten can be interpreted, namely pre-existing micro-structural traps at low-flux exposure and plasma-induced ones at high-flux exposure.

The research was financially supported by the National Magnetic Confinement Fusion Energy Research Program, the National Natural Science Foundation of China and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

Fig. 1 (a) The solute atomic volume Ω, and the volume change of the supercell caused by the solutes, ΔVSol, and the solute-H pair, ΔVSol-H. (b) The electronegativity of the solute elements and hydrogen, χP, and the Bader charge of the solute, QSol, and the hydrogen, QH. (c) The relation of the binding energy with the atomic volume and the electronegativity. The red dot line denotes the values of these physical quantities in pure tungsten. The blue line indicates the electronegativity of the hydrogen. In the case of the QSol, the hollow and solid icons correspond to the situation of the solute atom with and without hydrogen nearby, respectively.(Image by KONG Xiangshan)

Contact:

LIU Changsong

Institute of Solid State Physics (http://english.issp.ac.cn/)

Hefei, Anhui 230031, China.

Tel: 0086-551-65591062

E-mail: csliu@issp.ac.cn