Abstract
A common approach to tackle the phenomenon of hydrogen embrittlement in high strength steels is the introduction of nano-sized precipitates to trap the mobile hydrogen [1]. Numerous experimental studies have shown the positive effect of Ti [2, 3, 4] carbides on the resistance to hydrogen embrittlement. Their hydrogen trapping behavior has been investigated experimentally by means of thermal desorption spectroscopy (TDS) [3, 4, 5, 6] as well as theoretically via density functional theory (DFT) [7, 8, 9, 10].
However, modern steels typically contain multiple carbide forming elements that lead to the formation of so-called mixed-metal carbides and carbonitrides, consisting of a solid solution on both metal and non-metal sublattices [11, 12, 13]. This fact is often neglected when assessing hydrogen trapping from DFT where precipitates are often considered as stoichiometric compounds and not as alloys [14, 15]. The current study focuses on a systematic investigation of the effect of precipitate composition on the trapping behavior of hydrogen. A special quasirandom structure (SQS) [16, 17] approach is used within the framework of DFT to model the alloying of TiC with Mo. Hydrogen segregation energies are calculated for various possible trapping sites (including vacancies, interface misfit dislocations and their intersections) within the precipitates and at the interface to the bcc Fe matrix. The results indicate that the segregation energies follow a linear trend with respect to the local chemical environment, where changes in the metal sublattice composition have a stronger effect compared to alloying on the non-metal sublattice.
The current study evaluates the effect of Mo on the hydrogen trapping behavior of TiC and provides a comprehensive list of possible trapping sites and energies for comparison with experimental data. An accurate description of the effect of alloying on the hydrogen trapping capability is an important step towards deeper understanding of the relation of MC carbides to the hydrogen embrittlement in ferritic steels and interpretation of experimental TDS data on hydrogen trapping.