The nucleation and growth of Helium hubbles at grain boundaries of bcc tungsten: a molecular dynamics simulation

Abstract

Tungsten (W) is a potential candidate for plasma facing materials (PFMs) of fusion reactor. The helium (He) produced in fusion reaction is insoluble and easy to gather and form to He bubbles in W, resulting in embrittlement and degradation of the performance of the W matrix. In this paper, based on molecular dynamics, the nucleation and growth of helium bubbles in the bulk and at ∑3[211](110) and ∑9[110](411) grain boundaries of W was studied. As a result, the growth mechanism of Helium bubbles at grain boundary of W was different from in bulk. Helium bubbles in bulk W grow up by extruding dislocation rings. The growth mechanism of helium bubbles at ∑3[211](110) grain boundary was as follows: Firstly, a small amount of W interstitial atoms were extruded and emitted. And then the 1/2<inline-formula><tex-math id="Z-20200121175109">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175109.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175109.png"/></alternatives></inline-formula> dislocation line was extruded. Finally, the 1/2<inline-formula><tex-math id="Z-20200121175126">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175126.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191069_Z-20200121175126.png"/></alternatives></inline-formula> dislocation line would migrate along the direction of [111] of the grain boundary interface. Moreover, the emission of W interstitial atoms and dislocation extrusion of the helium bubble were not observed in our simulated time scale at the ∑9[110](411) grain boundary. Then we used the NEB method to calculate the diffusion barrier of self-gap atoms in the bulk and at ∑3[211](110) and ∑9[110](411) grain boundaries of W, which explained the simulation results. The migration energy barrier of W self-gap atoms in the bulk and at ∑3[211](110) grain boundary was only a few to a few millielectron volts. So as long as W self-gap atoms dissociated from the surface of the He bubble in the thermal relaxation process, they can be easily migrated out. However, The migration energy of the W self-gap atom at the ∑9[110](411) grain boundary can be from a few tenths to a few electron volts. Even during the thermal relaxation process, the W self-gap atoms dissociated from the surface of the He bubble. It was difficult for the W self-gap atoms migrated out. Finally, the correlation between He bubble size and stress released was given. Either in bulk or at ∑3[211](110) and ∑9[110](411) grain boundaries of W, after the pressure of the helium bubble becomes stable with time, the radius of the helium bubble would increase rapidly whenever the pressure dropped sharply. So there was a small step on the curve of the evolution of the radius of the helium bubble with time. Thus, helium bubbles in W could promote growth by releasing pressure intermittently.