Interaction of primary precipitates in reduced -activation ferritic/martensitic steel F82H with hydrogen atoms: Atomistic calculation based on the density functional theory

Abstract

• We used DFT to perform first-principles calculations to understand the fundamental aspects of the interactions between hydrogen isotopes and VC and Cr 23 C 6 crystals in F82H steel. • The single H atom in the carbon vacancy in VC crystal is situated at the center of the vacancy, and the trapping energy is 1.99 eV. • Each H atom and C atom are bound to each other with strong covalent forces, and up to six H atoms can be trapped in the vanadium vacancy in VC crystal. • The trapping energy of H atom in the Cr(8c) vacancy is the strongest among the five types of vacancies in Cr 23 C 6 crystal. To understand the interaction of hydrogen isotopes with VC and Cr 23 C 6 precipitates, in reduced-activation ferritic/martensitic steel (F82H), we have performed first-principles calculations based on density functional theory. Energy calculations and electron density analysis were performed with a focus on the hydrogen retention by vacancies in precipitates. The H atoms in the C vacancy in the VC crystal are bound to the surrounding Cr atoms by relatively weak covalent forces and Coulombic attraction, and up to four H atoms are trapped. In the case of a Cr vacancy, H atoms are strongly covalently bonded to neighboring C atoms as well as to an interstitial H atom, capturing up to six H atoms. H atoms in vacancies in Cr 23 C 6 have a weak bonding force with the Cr atom but a strong repulsive force with the C atom. As a result, H atom is not trapped in the Cr(4a) vacancy, and H atoms are trapped only at a distance from the C atom in the Cr(48H) and Cr(32f) vacancies. The Cr(8c) and C vacancies are relatively far from the C atoms and have higher hydrogen trapping energies. The Cr 23 C 6 precipitate containing vacancies may be a dominant trapping site in the temperature range when H atoms immediately dissociate from vacancies. In addition, the VC precipitate may be the only trapping site for hydrogen isotopes at higher temperatures when Cr 23 C 6 precipitates cannot trap them.