Density functional theory calculation of helium segregation and decohesion effect in W110/Ni111 interphase boundary

Abstract

Density functional theory is employed to study the preferential distribution and decohesion effect of He in a W–NiFe composite consisting of W particles embedded in an Ni-based solid solution matrix. A slab containing {110}⟨100⟩W//{111}⟨110⟩Ni interface is used as a surrogate model for the W–NiFe system. First, the fracture energy of the W/Ni interphase boundary (IB) (4.37 J/m2) is higher than the cleavage energy of Ni{111} (3.82 J/m2) and lower than the cleavage energy of W{110} (6.60 J/m2). The comparison shows that the cohesion of the IB is stronger than the Ni{111} planes that are away from the IB. However, the cohesion between the Ni{111} planes adjacent to the IB is found to be the weakest in this system, with a cleavage energy of 3.11 J/m2. Subsequently, the formation energy of He is calculated in the Ni slab, W slab, and various interstitial sites in the IB. The calculations show that He is significantly more stable in Ni than in W by about 1.75 eV. Interestingly, He does not prefer to segregate at the IB as compared to bulk Ni. Nevertheless, it prefers to segregate to the region between the Ni{111} planes adjacent to the IB and decreases the cohesion of the already weakest region. Based on an estimated amount of He gas production in 5 years under first wall neutron irradiation (neutron flux of 1.04 × 1015 n/cm2/s), He will decrease the cleavage energy of the weakest region by 21.2%, 15.4%, and 12.2% at 800, 1000, and 1200 °C, respectively.