Interactions between helium, hydrogen and intrinsic point defects in TaC crystal

Abstract

Tantalum carbide is not only an important precipitate in reduced-activation steels, but also an ultrahigh-temperature ceramic useful for protective coating. Under 14 MeV neutron irradiation, He and H atoms are produced from the (n, α) and (n, p) transmutation reactions, respectively, along with certain amount point defects created by cascades in the materials. In order to understand the role of TaC precipitates on the clustering of He/H in RAFM steels under irradiation, the energetics and stability of helium (He) atoms, hydrogen (H) atoms and intrinsic point defects inside TaC crystal have been systemically investigate by first-principles calculations. From energetic point of view, the C vacancy and interstitial defects are more stable than those of Ta. H impurity has two stable interstitial sites in TaC, while other atoms (He, C, or Ta) has only one stable site. With a pre-existing vacancy, H prefers to occupy an interstitial site in the space of the monovacancy rather than the vacancy center, while He sits the vacancy center. Both He and H exhibit high diffusion barriers (0.73 eV for He and 0.31–0.44 eV for H) between two interstitial sites. He-He pair has stronger attraction than H-H, while the binding of C/Ta/Fe-He pair is stronger than C/Ta/Fe-H pair. These results suggest that He atoms are easier to form cluster with C/Ta/Fe interstitial atom. We further discuss the stability of small HexHy, Hex-vacancy and Hy-vacancy clusters. The formation energy of mixed HexHy cluster increases linearly with the total number of He and H atoms, while the pure Hy clusters is more stable than the Hex clusters.