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 alpha-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/alpha-Fe semi-coherent interface. We observed the Kurdjumov-Sachs orientation relationship with (1 (1) over bar(1) over bar)(Nbc) // (101)(alpha-Fe) and [0 (1) over bar1](Nbc) // [(1) over bar 11](alpha-Fe) between the NbC and alpha-Fe phases. Noticeably, two sets of misfit dislocations with Burgers vectors of b((1)) = a(b)/2[111] on alpha-Fe planes and b(2) = a(b)/2[1 (1) over bar1] on (110) alpha-Fe planes (a(b) is the lattice constant of alpha-Fe), which would be the deep hydrogen trapping sites, were characterized in the NbC/alpha-Fe semi-coherent diffuse interface. In addition, density functional theory-based first-principles calculations revealed that the deep binding energy between the NbC/alpha-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/alpha-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/alpha-Fe semi-coherent interface is beneficial for designing HE-resistant steels. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.