Grain Boundary Segregation of Interstitial and Substitutional Impurity Atoms in Alpha-Iron

The macroscopic behavior of polycrystalline materials is influenced by the local variation of properties caused by the presence of impurities and defects. The effect of these impurities at the atomic scale can either embrittle or strengthen grain boundaries (GBs) within. Thus, it is imperative to understand the energetics associated with segregation to design materials with desirable properties. In this study, molecular statics simulations were employed to analyze the energetics associated with the segregation of various elements (helium, hydrogen, carbon, phosphorous, and vanadium) to four 〈100〉 (Σ5 and Σ13 GBs) and six 〈110〉 (Σ3, Σ9, and Σ11 GBs) symmetric tilt grain boundaries in α-Fe. This knowledge is important for designing stable interfaces in harsh environments. Simulation results show that the local atomic arrangements within the GB region and the resulting structural units have a significant influence on the magnitude of binding energies of the impurity (interstitial and substitutional) atoms. These data also suggest that the site-to-site variation of energies within a boundary is substantial. Comparing the binding energies of all 10 boundaries shows that the Σ3(112) boundary possesses a much smaller binding energy for all interstitial and substitutional impurity atoms among the boundaries examined in this study. Additionally, based on the Rice–Wang model, our total energy calculations show that V has a significant beneficial effect on the Fe grain boundary cohesion, while P has a detrimental effect on grain boundary cohesion, much weaker than H and He. This is significant for applications where extreme environmental damage generates lattice defects and grain boundaries act as sinks for both interstitial and substitutional impurity atoms. This methodology provides us with a tool to effectively identify the local as well as the global segregation behavior that can influence the GB cohesion.