Basic physical behavior of impurity carbon in molybdenum for nuclear material: A systematical first-principles simulation

Abstract

• Systematic DFT calculations for basic physical behavior of impurity carbon in Mo. • Defect vacancy can easily capture impurity carbon atoms to form C n V clusters. • The concentration distributions of impurity carbon and C n V clusters are calculated. • Synergistic diffusion effect between impurity carbon and C n V clusters. Based on first-principles simulations, we have systematically studied physical behavior of impurity carbon in molybdenum. A single carbon atom is preferable to occupy the octahedral interstitial position (oip) rather than the tetrahedral interstitial position (tip) in molybdenum. Two carbon atoms are inclined to bind each other at two neighbor oips along the 〈2 1 0〉 direction. A mono-vacancy captures up to four carbon atoms to form C n V ( n = 1–4) clusters. The C 2 V cluster is the most stable cluster owing to its most favourable capturing energy of −1.71 eV. The vacancy concentration in the form of C n V clusters dramatically raises owing to powerfully exothermic reactions of C n V clusters, in according with the experimental results. The diffusion activation energy of carbon/vacancy are 1.22/1.17 eV, agreeing with the experimental value of 1.20/1.35 eV. The C n V formations are ascribed to vacancy capturing mechanism. The C n V clusters are nearly immobile at lower temperature regime. When the temperature achieves a critical point ~1700 K, carbon atoms and vacancy are separating from C n V cluster to produce the isolated interstitial carbon atoms and an individual vacancy, respectively. All C n V clusters do not exist anymore when the temperature exceeds ~1700 K. These C n V clusters do not nearly have the effect on interstitial carbon diffusion.