Hydrogen trapping in mixed carbonitrides

Transition metal carbides with NaCl-structure have long been in focus for the design of hydrogen embrittlement (HE) resistant high strength steels. While previous studies extensively investigated the pure transition metal precipitates, modern steels often contain multiple carbide forming elements at once, leading to the formation of mixed carbonitride precipitates. Since only little is known about the influence of precipitate chemistry on hydrogen trapping, we performed systematic density functional theory (DFT) calculations of disordered structures in order to investigate hydrogen trapping within the (Ti x V 1-x )(C y N 1-y ) -precipitate system. Addressing possible trapping positions at bulk and interface non-metal vacancies, the coherent interface between (Ti x V 1-x )(C y N 1-y ) and bcc iron, the strained iron lattice and misfit dislocations, we found that carbon vacancies in TiC, both at the interface and in the bulk, are the deepest hydrogen traps with segregation energies exceeding −1.1 eV. For all investigated positions, a dependence of the hydrogen trapping capability on the precipitate composition has been found, with non-metal vacancies being strongly affected, whereas misfit dislocations and the coherent interface exhibit comparably smaller changes to their hydrogen trapping behavior. With few exceptions, TiC offers the strongest hydrogen traps, while alloying with V or N decreases the hydrogen trapping capability of (Ti x V 1-x )(C y N 1-y ) -precipitates. This study provides a detailed investigation of a wide range of hydrogen trapping sites related to mixed carbonitride precipitates within the (Ti x V 1-x )(C y N 1-y ) -system. Our results extend the existing knowledge on hydrogen trapping at pure stoichiometric carbides and nitrides towards alloyed precipitates that help to improve current steel design strategies focusing on the hydrogen trap design.