Effect of He on the hardness and incipient plasticity of W films during nanoindentation

In future fusion reactors, the production and accumulation of helium (He) will lead to a dramatic mechanical failure for the material. Tungsten (W) is considered as one of the most promising plasma facing materials. In order to study the effect of He on mechanical properties of W, the novel He-charged W films with a homogeneous He dose profile were fabricated by the magnetron sputtering device. The prepared samples in this study can well avoid the problems like damage gradient effect, indentation size effect, and soft substrate effect during the nanoindentation test, which will appear in the samples obtained by conventional high energy He+ implantation. Nanoindentation results showed that the hardness of the films increased with the increasing He content, and the hardness-depth relationship is in good agreement with the Nix-Gao model. The pop-in events can be observed in the pure W film, which were absent in the He-charged films. Results of molecular dynamics (MD) simulations proved that, the He bubble not only promotes the nucleation and propagation of dislocations, but also hinders the dislocation movement, which leads to a smoother transition from elastic to plastic deformation in the indentation process.