Influence of Ni Contents and Local Hydrogen Concentration on Crack Propagation in FCC Fe-Ni Alloy Models: A Molecular Dynamics Study

This study investigates the atomic-scale effects of hydrogen concentration and Ni content on crack propagation in Fe-Ni alloy models using molecular dynamics methods. A Mode I crack model with a (001)[100] orientation was constructed, and hydrogen atoms were locally introduced at the crack tip with concentrations of 5.3 at.% and 14.3 at.%. Fe-Ni alloy models with 5%, 10%, 15%, and 20% Ni were compared in terms of crack growth, dislocation evolution, stacking fault energy, and hydrogen diffusion. The results show that local hydrogen introduction has a limited effect on the peak stress–strain response, while hydrogen clearly accelerates crack propagation in the middle stage, especially at high concentrations. For the 10% Ni model, the middle-stage crack growth rate increases to 0.36 Å/ps under 14.3 at.% crack-tip hydrogen. Crack growth in all models shows three stages. The 15% Ni model exhibits a clear plateau in the second stage and the shortest final crack length. Further analysis shows that Ni content regulates dislocation behavior through stacking fault energy. At 15% Ni, sustained dislocation entanglement and high-density dislocation multiplication occur near the crack tip, which helps dissipate local stress. Hydrogen diffusion analysis indicates that hydrogen mobility is lower in the 15% Ni model, which may be related to hydrogen retention near dislocation-rich regions. A normalized comparison based on hydrogen diffusion and middle-stage crack growth rate further identifies 15% Ni as the lowest crack propagation tendency composition among the studied models. These results provide atomic-scale data for Ni-content optimization in hydrogen-resistant alloys, although the direct engineering transfer of the findings is limited by the length and time scales of molecular dynamics simulations.