Atomic-scale investigation of deep hydrogen trapping in NbC/α-Fe semi-coherent interfaces

The precipitation of niobium carbide (NbC) is a superior approach to mitigating hydrogen embrittlement (HE). The role of the semi-coherent interface between NbC and α-Fe on hydrogen trapping and HE resistance in high-strength tempered martensitic steel was investigated in this study. High-resolution transmission electron microscopy observations are performed to reveal the atomic-scale crystallographic orientation relationship, atomic arrangements, and associated crystalline defects in the NbC/α-Fe semi-coherent interface. We observed the Kurdjumov–Sachs orientation relationship with (11¯1¯)_NbC//(101)_α−Fe and [01¯1]_NbC//[1¯11]_α−Fe between the NbC and α-Fe phases. Noticeably, two sets of misfit dislocations with Burgers vectors of b⁽¹⁾=a_b/2[111] on (011¯) α-Fe planes and b⁽²⁾=a_b/2[11¯1] on (110) α-Fe planes (a_b is the lattice constant of α-Fe), which would be the deep hydrogen trapping sites, were characterized in the NbC/α-Fe semi-coherent diffuse interface. In addition, density functional theory-based first-principles calculations revealed that the deep binding energy between the NbC/α-Fe semi-coherent interface and hydrogen is 0.80 eV, which well matches the hydrogen desorption activation energy of 81.8 kJ/mol determined via thermal desorption spectroscopy experiments. These demonstrate that the nature of the deep hydrogen trapping sites of the NbC/α-Fe semi-coherent interface is the misfit dislocation core. Distinguished HE resistance was obtained and ascribed to the deep hydrogen trapping of uniformly dispersed NbC nanoprecipitates with an average diameter of 10.0 ± 3.3 nm. The strategy of deep hydrogen trapping in the NbC/α-Fe semi-coherent interface is beneficial for designing HE-resistant steels.