Lattice diffusion of hydrogen in the precipitate Cr7C3

Chromium alloyed steels are widely used due to their high yield strength and wear resistance in tool steels or their corrosion resistance in stainless steels. However, high-strength steels, such as bearing steels, may be prone to hydrogen embrittlement (HE). A key issue to address is the potentially deleterious effects associated with diffusional hydrogen. To control the reversibility of potential trap sites and diffusion paths in the microstructure thus becomes essential. There is a need to find new possibilities to design more HE resistant steels, and here DFT could be used to calculate trapping energies and mapping diffusion paths. A wide range of microstructural features may act as trap sites and influence diffusion paths, e.g., precipitates, such as carbides. There are three different carbides common in chromium containing steels: carbide cementites M₃C, M₇C₃ and M₂₃C₆, where M can be Fe or Cr. The latter two carbides are mainly present in alloyed steels like tool steels or stainless steels. In the present work, we study Cr₇C₃, which has an orthorhombic Pnma structure with the lattice parameters a = 4.506 Å, b = 6.945 Å, and c = 12.038 Å. In order to find simple ways for the evaluation of hydrogen diffusion in structures with a more complex geometry we have investigated possible hydrogen diffusion paths in chromium carbide Cr₇C₃ by use of spin-polarized DFT using the PBE functional. Nudged elastic bands were used to identify minimum energy paths in the short a- and the long c-direction of the lattice. Sixteen images were created along pathways between possible hydrogen sites and optimized with the constrain of equal distancing between each other. An energy barrier of 1 eV was found for diffusion in a-direction and of 3.2 eV for diffusion in c-direction. Diffusion in a-direction takes place through octahedral holes, whereas diffusion in c-direction is even through tetrahedral holes. Carbon atoms hamper hydrogen diffusion in c-direction. The presented results are part of a systematic study with the goal to shed some more light on the role of carbides for hydrogen diffusion through alloyed metals.