First-Principles Study of B Segregation at Austenite Grain Boundary and Its Effect on the Hardenability of Low-Alloy Steels

Addition of B is beneficial for the hardenability of low-alloy steels and the effect is further improved when combined with the addition of Mo. While experiments demonstrated that Mo reduces the M23(C,B)6 precipitation and indicated an interaction between the alloying elements at the grain boundary, the underlying mechanism remains unclear. In the present study, the segregation behavior of B and its interaction with C and Mo at an austenite grain boundary were investigated using first-principles calculations. It was demonstrated that B has a strong tendency to segregate to the grain boundary and leads to a remarkable reduction in grain boundary energy, which is considered to be responsible for the improvement in hardenability. A strong attractive interaction between B and Mo was revealed, consistent with the experimentally observed co-segregation. The partitioning energies of Mo and B from grain boundary into borocarbide were calculated, and it was found that Mo can suppress the precipitation by increasing the partitioning energy of B and destabilizing the M23(C,B)6 phase.