Boron segregation at austenite grain boundaries: An equilibrium phenomenon

High-resolution secondary ion mass spectrometry imaging, atom probe tomography and transmission electron microscopy have been employed to investigate boron segregation at austenite grain boundaries (γGB) in a high-strength steel. Boron content in solid solution, interfacial excess at γGB and precipitate number density were quantified after heat treatments specifically designed to promote non-equilibrium segregation of boron. The samples were first soaked at 1100°C, followed by rapid cooling to different isothermal holding temperatures, and then quenched to room temperature. We found that boron distribution does not evolve after holding at 460°C, even after bainitic transformation, suggesting a low mobility of boron at this temperature. In contrast, after holding at high temperatures (780°C and 820°C), boron distribution evolves remarkably: boron content in solid solution close to the interface and boron interfacial excess decrease similarly with the increase of holding time, while their ratio remains almost constant; at the same time, transient precipitation of M ₂₃(B,C) ₆ particles occurs at the grain boundaries. The experimental results on interfacial excesses at γGB and the solute profiles in the vicinity of the grain boundary agree with the results of a numerical model of coupled diffusion and equilibrium segregation phenomena. These results unambiguously exclude the hypothesis that non-equilibrium segregation controls the kinetics of boron segregation and confirm that boron segregation is only controlled by the equilibrium segregation mechanism.