Interstitial hydrogen diffusion in M₇C₃ (M = Cr, Mn, Fe),

To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensiticsteels, we have performed a density functional theory study on the solution energies and energy barriers forhydrogen diffusion in orthorhombic M₇C₃ (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice andcause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogenat different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03eV/H-atom for Cr₇C₃, Mn₇C₃, and Fe₇C₃, respectively. Energy barriers for the diffusion of hydrogen atoms havebeen probed with the nudged elastic band method, which shows comparably low barriers for transport viainterstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation betweenlow solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that thepresence of carbon tends to increase the energy barrier. Our results can explain previous experimental findingsof hydrogen located in the bulk of Cr₇C₃ precipitates and provide a solid basis for future design efforts of steelswith high strength and commensurable ductility