Hydrogen trapping and diffusion in sub-stoichiometric vanadium and niobium carbide precipitates in high-strength steels

High-strength steel is a structural metal crucial for load-bearing components yet is known to be highly susceptible to hydrogen embrittlement (HE). Vanadium (V) and niobium (Nb) containing precipitated carbides introduce strong hydrogen traps to immobilize hydrogen, thus mitigating HE. However, variations in intrinsic vacancy concentrations in these carbides affect hydrogen thermodynamics and kinetics but remain poorly understood. Employing first-principles calculations, hydrogen trapping and diffusion in V/Nb carbides were investigated. Hydrogen dissolution energies are composition-dependent, revealing a transition from reversible to irreversible trapping with increasing carbon vacancy content, prescribed by the strength of covalent bonds with neighboring V/Nb atoms. Meanwhile, the diffusion energy barrier decreases with increasing carbon vacancy content, attributed to changes in vacancy patterns within carbides. The findings contribute new and critical knowledge for understanding hydrogen trapping and diffusion in sub-stoichiometric V/Nb carbides, providing valuable guidance for process and composition innovation of high-strength alloy steels for better HE resistance. • VC x /NbC x precipitates show distinct hydrogen embrittlement resistance in steels. • H–V/Nb bonds strengthen H trapping with rising carbon vacancy content. • H activation barriers vary with different carbon vacancy arrangements in VC x /NbC x . • 1NN vacancies ease H diffusion, making M 4 C 3 /M 3 C 2 effective against embrittlement.