Thermodynamic Analysis of the Fe-B-C Ternary System and an Evaluation of the Grain Boundary Segregation Behavior of B and C

Thermodynamic analysis of the Fe-B-C ternary system was performed using the CALPAHD approach coupled with first principles calculations, and then based on the evaluated thermodynamic parameters, the amounts of segregated B and C in the grain boundary were calculated. The calculated phase diagrams and thermodynamic properties agreed with the experimental data as well as the results of the first principles calculations, and thus highly accurate parameters for this ternary system were evaluated. In using the obtained thermodynamic parameters, the grain boundary segregation behavior of B and C was analyzed by means of the parallel tangent scheme. The Gibbs free energy of the liquid phase obtained in the present work was adopted for that of the grain boundary phase. According to the model, it was confirmed that the amount of segregated B content in the grain boundary of gamma -iron decreased the addition of C. Thus, B and C atoms show tendencies to compete for a finite number of segregation sites. When equilibrium precipitates are formed in a matrix phase, the amount of B segregation further decreases due to a solution of B in the borocarbide phases, such as Fe23(B,C)6, Fe3(B,C) Fe2(B,C), and Fe(B,C) phases. Therefore, irrespective of the presence or absence of precipitates, the effect of hardenability decreases with the presence of C in steel due to the decreasing segregated B content in the grain boundary.