Diffusion behavior of di-interstitials with different configurations in tungsten
Authors: Ran Qin , Wang Huan , Zhong Rui , Wu Jian-Chun , Zou Yu , Wang Jun ,   Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China
Published: December 2020

Tungsten, due to its desirable properties (high melting point, low sputtering coefficient, good irradiation resistance etc.), is considered as a promising candidate for the plasma facing materials in future nuclear fusion reactors. Therefore, it will work in extremely harsh environments because it is subjected to the bombadement of high-flux plasma particles and the irradiation of high energy neutrons, resulting in vacancies and interstitials. The migration behavior of self-interstitial atoms is one of the most important factors determining the microstructure evolution in irradiated metals because it will greatly affect the mechanical properties of materials. The study of the diffusion behavior of di-interstitials with different configurations contributes to a better understanding of the self-interstitial atom behavior in tungsten. Despite the inherent difficulty in experimental approaches, atomistic simulation provides an effective means of investigating the defect evolution in materials. In this paper, based on the newly developed interatomic potential for W-W interaction, the diffusion behavior of self-interstitial atoms in tungsten is studied by molecular dynamics simulation. This work focuses on the investigation of the diffusion behavior of di-interstitials with different configurations at different temperatures. The obtained results show that the di-interstitials with the first nearest neighbor configuration presents the one-dimensional migration in the <inline-formula><tex-math id="Z-20190530101816-19">\begin{document}$\left\langle 111 \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101816-19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101816-19.png"/></alternatives></inline-formula> direction at temperatures below 1400 K. As the temperature increases, it makes rotations from one <inline-formula><tex-math id="Z-20190530101818-20">\begin{document}$ \left\langle 111 \right\rangle$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101818-20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101818-20.png"/></alternatives></inline-formula>- to other <inline-formula><tex-math id="Z-20190530101823-21">\begin{document}$\left\langle 111 \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101823-21.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101823-21.png"/></alternatives></inline-formula>-directions. Thus migration of di-interstitial atoms with the first nearest neighbor configuration exhibits a change in mechanism from one-dimensional to three-dimensional migration, keeping the stable <inline-formula><tex-math id="Z-20190530101828-22">\begin{document}$\left\langle 111 \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101828-22.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530101828-22.png"/></alternatives></inline-formula> configuration in the whole investigated temperature range. The migration of di-interstitial atoms with the second nearest neighbor configuration is one-dimensional along the <inline-formula><tex-math id="Z-20190530102029-23">\begin{document}$\left\langle 111 \right\rangle$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530102029-23.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190310_Z-20190530102029-23.png"/></alternatives></inline-formula> direction within a certain temperature range. When the temperature is above 600 K, the di-interstitial atoms will dissociate into two individual self-interstitial atoms and move independently. However, the migration of di-interstitial atoms with the third nearest neighbor configuration dissociates at a temperature just above 300 K. The non-parallel self-interstitial atoms form a sessile configuration within a certain temperature range. Once the sessile cluster is formed it can hardly move. Interestingly, it will transform into mobile defect when the temperature is higher than 1000 K. By comparing the migration energy values of these configurations obtained by nudged elastic band method with those of the Arrhenius fits, we find that the diffusivity for each of single- and di-interstitial atoms in tungsten is a linear function of temperature rather than Arrhenius as usually assumed.