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Role of strain on the stability of B, C, N, and O in Iron

The preference for the occupation of solute atoms like B, C, N, and O at various sites in iron is generally explained by the size of the solute and the volume available for the solute atoms to occupy. Such an explanation based on the size of solute atoms and available space at the occupation site assumes that distortion alone dictates the stability of solute atoms. Using first-principles density functional theory (DFT), we separately calculate the distortion energy (DE) and electronic binding energy (EBE) of solute atoms in iron. We show that electronic binding dictates the relative stability of O rather than distortion. In contrast, the relative stability of B, C, and N is dictated by the distortion it exerts on iron atoms. Contribution to the relative stability of B atoms is dictated mostly by distortion. It suggests that B could occupy a large volume region like grain boundaries. The same agrees with experiments indicating B segregates at grain boundaries and planar defects. Such conclusions could not have been drawn from the formation energy calculation, which shows that B is stable at the substitution site.