First-principles calculations of the grain-boundary cohesive energy. Embrittling or strengthening effect of solute segregation in a bcc FeΣ3(111) grain boundary

The microscopic mechanism of the grain boundary embrittlement in metals by solute segregation has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy (2γint) of bcc iron Σ3 (111) symmetrical tilt grain boundary is reduced by the segregation of sulfur (S) or phosphorus (P), while it is increased by the segregation of boron (B) or carbon (C). The rate of the decrease/increase in the 2γint was excellently proportional to the experimental ductile-to-brittle transition temperature (DBTT) shift with increasing segregation; this demonstrates that the grain boundary embrittlement is governed by the change in the 2γint. (author)