The microstructural evolution of tungsten (W) nanofibers has been observed during the W fuzz growth due to low-energy He+ irradiations at an elevated temperature. In this study, we find that the frequent fracture and merging of W nanofibers occur during the W fuzz growth. Our annealing experiments also show the fracture and instability of W nanofibers at an elevated temperature. Detailed analysis reveals the tensile-stress driven thermodynamic instability of W nanofibers containing plenty of helium (He) bubbles at an elevated temperature. The mergence of W nanofibers coming into contact with each other is due to the deposition of W adatoms diffusing over W nanofibers onto the mergence locations, which decreases the surface free energy of W nanofibers. The fracture of W nanofibers is attributed to the tensile stress induced cracking of W nanofibers containing He bubbles, and the subsequent W adatom surface diffusion over W nanofibers.
The effect of fusion-relevant He ion flux on the evolution of He nano-bubbles in W