Atomic and electronic structures of a grain boundary in iron with impurity segregation

We present a short review on our current investigations of the atomic and electronic structures of a grain boundary in iron. Atomic structures of grain boundaries were simulated and the local electronic densities of states were calculated in the simulated structure. When phosphorus impurity atoms segregated at the grain boundaries in iron, trigonal prismatic FeP clusters were formed. Segregated boron atoms tended to stay at the central site of polyhedra constructed by host atoms in the grain boundaries. The non-bonding states of the iron atom at the grain boundary disappear by forming a strong bonding orbital with the orbital of the segregated impurity atom. This bonding orbital is formed in a Fe3d host band in the case of a boron impurity. On the other hand, the bonding orbital is formed at lower energies for the phosphorus impurity and is less-mixed with the Fe3d host band. Non-bonding states are formed around the Fe₉P clusters. These can give a qualitative explanation for the embrittlement of the impurity segregated grain boundary. Finally, we can explain from the viewpoint of the electronic structure why the interstitial impurity is the only cohesive enhancer.