Molecular dynamics simulation of the tensile properties of polycrystalline α-Fe: Effects of hydrogen concentration, temperature, and grain size

Abstract

Hydrogen embrittlement (HE) significantly degrades the mechanical properties of steel materials. Under varying hydrogen concentrations and temperatures, the HE behavior of materials with smaller grain sizes and a higher proportion of grain boundaries (GB) differs markedly from that of polycrystalline materials with larger grain sizes. Molecular dynamics simulations were employed to study the uniaxial tensile behavior and deformation mechanisms of polycrystalline α-Fe models under different hydrogen concentrations, temperatures, and grain sizes. The results show that as the hydrogen concentration increases from 0 % to 1 %, the ultimate tensile strength (UTS) and fracture strain (ε_f) of the steel decrease significantly. Beyond 1 % hydrogen concentration, the UTS of models with smaller grain sizes continues to decrease. The saturation hydrogen concentration for models with grain sizes of 2.84 and 2.5 nm is 1 %, while for the 2.14 nm grain size model, the hydrogen concentration does not reach saturation. As the grain size decreases, the proportion of grain boundaries increases, leading to a higher corresponding saturation hydrogen concentration. The peak stress of each model decreases with increasing temperature. The hydrogen diffusion coefficient decreases with decreasing grain size and also decreases with lower temperatures. Models with smaller grain sizes can weaken the suppression of local energy release by hydrogen atoms, thereby reducing their promoting effect on crack initiation and propagation. Therefore, the smaller the grain size, the better the material's resistance to HE. This study elucidates the rules of hydrogen-grain size-temperature interactions on mechanical properties on an atomic scale, providing a theoretical basis for understanding hydrogen embrittlement behavior in steel.