First-principles study on the solute-induced low diffusion and self-trapping of helium in fcc iron

The addition of alloying elements plays an essential role in the behaviors of helium (He) produced by transmutation in metal alloys. The effects of solutes (Ni, Cr, Ti, P, Si, and C) on the behavior of He and He–He pairs in face-centered cubic (fcc) iron were investigated via first-principles calculations based on density functional theory (DFT). For the interactions between solutes and He, we found that Ti, P, Si, and C were more potent in attracting He than Ni and Cr in fcc iron. We determined the most stable configuration for the He–He pair, which is the substitutional–tetrahedral He pair with a binding energy of 1.60 eV. In considering the effect of solutes on the stability of the He–He pair, we proposed a unique definition of binding energy. By applying this definition, we suggest that Ti and P can weaken He self-trapping, Cr and C are beneficial for He self-trapping, and Ni is similar to the matrix Fe itself. For the diffusion of He, which is the necessary process of forming the He bubble, we determined that the most stable interstitial (int) He is in a tetrahedral (tetra) site and can migrate with the energy barrier of 0.16 eV in pure fcc iron. We also found that Ti and Si can increase the barrier to 0.18 and 0.20 eV; on the contrary, Cr and P decrease the barrier to 0.10 and 0.06 eV, respectively. Summarizing the calculations, we conclude that Ti decreases while Cr increases the diffusion and self-trapping of He in fcc iron.