Quantifying the energetics and length scales of carbon segregation to α;-Fe symmetric tilt grain boundaries using atomistic simulations

Segregation of impurities to grain boundaries (GBs) plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to GBs. Molecular statics simulations are used to calculate the segregation energies for carbon within multiple substitutional and interstitial GB sites over a database of 125 symmetric tilt GBs in Fe. The simulation results show that there are two energetically favorable GB segregation processes: (1) an octahedral C atom in the lattice segregating to an interstitial GB site and (2) an octahedral C atom and a vacancy in the lattice segregating to a grain boundary substitutional site. In both cases, lower segregation energies than appear in the bulk lattice were calculated. Moreover, based on segregation energies approaching bulk values, the length scale of interaction is larger for interstitial C than for substitutional C in the GB (≈5 Å compared to ≈3 Å from center of the GB). A subsequent data reduction and statistical representation of this dataset provides critical information about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of alloying elements and impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of GB character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation to substitutional and interstitial sites in a specific class of GBs in α-Fe.