Hydrogen trapping efficiency of carbide nano-precipitates and their effect on hydrogen embrittlement susceptibility of single phase ferritic AHSS

This work studies three compositions of single-phase ferritic steel with a high concentration of finely dispersed TiC, NbC and VC carbide nano-precipitates, specifically designed for this research. The steels are heat treated in N₂ and H₂, which is used to simultaneously charge the precipitate bulk with H. Slow strain rate tensile testing is performed to evaluate the effect of H trapping by the precipitates on embrittlement of the steels. Simultaneously, Density Functional Theory (DFT) - based methods are employed to obtain information on the trapping characteristics of H inside precipitate bulk and on the precipitate/matrix coherent interface. C vacancies in bulk TiC are found to be the strongest H traps with a solution enthalpy of -1.09 eV, while VC and NbC show stronger trapping at the interfacial C vacancies as compared to bulk C vacancies, with solution enthalpy being close to ~0.5 eV. The H₂ charging is found to have no discernible effect on the mechanical behaviour of the TiC alloy, while the NbC and VC alloys are embrittled by 7% and 10%, respectively.