A study of hydrogen trapping behaviors in Nb and V carbides in α-Fe matrix by first-principles calculations

The introduction of nanosized carbides (NbC, TiC, VC, etc.) into a matrix is one of the most efficient approaches for improving the hydrogen embrittlement resistance of traditional high strength steels. In the present work, first-principles calculations were used to investigate the characteristics of hydrogen trapping at NbC/bcc-Fe and VC/bcc-Fe interfaces. Hydrogen atoms prefer to occupy the tri2-site in bulk NbC and VC lattices and perfect NbC and VC lattices cannot trap H atoms. H atoms can segregate at perfect NbC/Fe and VC/Fe interfaces, in which the solution energies of the H atoms at the NbC/Fe interfaces are lower. The carbon vacancies at the carbide/Fe interfaces can act as relatively deep hydrogen traps but are unfavorable for formation owing to their high formation energies. Other interstitial sites at interfaces containing carbon vacancies can trap H atoms more strongly than perfect interfaces. Compared with the NbC/Fe interface, it is more probable for H atoms to be trapped by high-density vacancies in the interior of VC carbides once H atoms obtain sufficient energy under certain conditions, eg. at high temperatures.