Carbide particles act as effective hydrogen traps that can mitigate hydrogen embrittlement in steels, with their trapping efficacy governed by the binding energy of hydrogen at atomic-scale sites within the carbide and at the carbide/matrix interface. Within the carbide, carbon vacancies are established trap sites, while at the carbide/matrix interface, multiple distinct trap sites may form depending upon the interface character. State-of-the-art experimental methods cannot attribute binding energies to individual sites unambiguously, particularly at the carbide/matrix interface, where distinct trap sites may exhibit similar binding energies. Density functional theory (DFT) calculations have partially addressed this limitation, but their computational cost prohibits accurate treatment of semicoherent and incoherent interfaces, for which hydrogen binding energies remain poorly characterised. In this work, we develop a DFT-accurate machine-learning Fe–TiC–H interatomic potential that enables simulations of hydrogen energetics and kinetics at various TiC/Fe interfaces in the Baker–Nutting orientation. Predictions of structural, dynamic and energetic properties of both bulk phases (i.e., bcc Fe and rocksalt TiC) and the coherent interface with and without hydrogen are in good agreement with DFT. We apply the potential to examine the atomic structure and hydrogen trapping at semicoherent and incoherent interfaces. We find that the binding energy of hydrogen at misfit dislocations is similar to that at coherent interfaces, with a difference of no more than 0.1 eV. However, the misfit strain field reduces carbon vacancy formation energies at the interface, where hydrogen can be trapped with similar binding energies as that in a bulk C vacancy (approximately –1 eV). Lower vacancy formation energies are also found at the incoherent interface, where hydrogen binding energies span a broader range, depending sensitively on the local atomic environment. Overall, the results enable interpretation of experimental observations of hydrogen segregation around semicoherent and incoherent TiC interfaces.

Rapid developments in the field of hydrogen energy have prompted the need for safe and efficient hydrogen transportation and storage. Steels form the backbone of the current energy infrastructure and thus offer a fast and cost-effective solution. Their excellent mechanical properties are attributed to the underlying microstructure which comprises of finely dispersed nano-precipitates. However, one major factor restricting their application is their susceptibility to Hydrogen Embrittlement (HE). In the past decade, experimental and theoretical works have been carried out to understand if the nano-sized carbides can aid in reducing the susceptibility to HE along with providing strengthening. Within this ab-inito study, we investigated the effectiveness of fully coherent nano-carbides (i.e. TiC, VC and NbC) to limit the diffusible hydrogen content in bcc Fe. Our study revealed that the interplay between hydrogen and carbon vacancies, local atomic environment at interface as well as elastic strain fields at the interface can lead to significantly increased hydrogen solubilities. While in TiC, the deepest traps were found to be in the bulk of carbides, in VC and NbC, the elastic strain fields around the interface led to the strongest trapping. Further, the formation of a two-hydrogen-vacancy complex was found to be favourable in VC. Finally, the migration barriers for hydrogen trapping in bulk TiC as well as across the Fe/TiC coherent interface indicate that these deep traps in the form of carbon vacancies are fairly accessible.

Controlling the detrimental effect of hydrogen on the mechanical behaviour of advanced high strength steels is decisive for their application. Precipitates in steels can be useful in irreversibly trapping the hydrogen atoms, thereby preventing their diffusion to critical regions in the microstructure where they can be most detrimental. In this work, the capability of precipitates of transition metals in limiting the amount of diffusible hydrogen has been examined. A combined ab initio–experimental approach was used to study the hydrogen sorption in two DP800 steel grades with different concentrations of titanium and vanadium using cyclic voltammetry. Under the same charging conditions, diffusible hydrogen concentration was found to be higher in the vanadium grade as compared to the titanium grade. Scanning electron microscope characterisation revealed a more compact layer of oxide on the vanadium grade which contributed to more hydrogen absorption on the surface. Density Functional Theory calculations were performed to determine the trapping strength of precipitates of titanium and vanadium. C vacancy in titanium carbide was found to be the strongest hydrogen trap, but the C vacancy formation energy was much lower in vanadium carbide. At finite temperatures, however, both precipitates are experimentally known to be off-stoichiometric. Our DFT-based finding of the titanium grade being irreversible hydrogen trap is thus in agreement with the experimental results.