Atomistic Study on Diffusion and Trapping of Hydrogen in Nanocrystalline Steel

The local equilibrium of hydrogen atoms in nanocrystalline steel was analyzed using molecular dynamics simulations with a Modified Embedded Atom Method potential developed for the Fe-C-H system from experimental data and first-principle calculations. We examined hydrogen diffusion and trapping in a nanocrystalline structure composed of lath sub-grain boundaries, microvoids, solution carbon, and carbides. The nanocrystalline structure of similar to 1 million atoms is constructed with ten wt. ppm (0.1 at.%) hydrogen atoms and 0.3 wt% carbons to represent tempered martensitic steel. At 800 K, when hydrogen has higher mobility, hydrogen diffuses toward irregularities of those various trapping sites. The results show that within one ns, approximately 80% of total hydrogen atoms are trapped in the vicinity of grain boundaries and carbide interfaces, while 10% of the atoms reside adjacent to solution carbon and 10% remain as free hydrogen in bulk. This work provides comprehensive measures of high local concentrations of hydrogen at grain boundaries and interfaces that could lead to intergranular fracture.