Influence of boron grain boundary segregation on the results of thermodynamic calculations in the Fe-C-B-N system

The influence of boron alloying at concentrations of some tenths of weight parts per million in microalloyed steel is investigated with a focus on the role of grain boundaries, on thermodynamic descriptions. The strong segregation tendency of B atoms to grain boundaries impacts the formation and stability of the boron nitride phase, associated with the significant Fe-B-C-N phase diagram. This study employs experimental evidence via advanced characterization techniques on four microalloyed steels with B fractions up to 65 wt.-ppm, produced in a vacuum-induction laboratory furnace, including differential scanning calorimetry, wave-length-dispersive spectroscopy, as well as atom probe tomography, and a critical thermodynamic assessment of the system Fe-B-C-N in the Fe-rich corner. The influence of boron on the solvus temperature of BN, and on the liquidus and solidus temperatures of the system, is determined. Constrained thermodynamic modeling is used to elucidate the effect of B segregation to grain boundaries on the phase stabilities. The findings reveal that the consideration of trapping of B at grain boundaries, especially in systems with minute additions of B, is crucial for a consistent description of the system thermodynamics. This study explains the prevailing discrepancy between the proposed thermodynamic and solubility product descriptions of the BN phase.