Interfacial segregation of carbon atoms: the competition between grain boundaries and phase boundaries

The microstructure of a two-phase medium manganese steel is decorated by interfaces whose character is defined by crystallography and the misorientation between adjacent grains, which in turn influences elemental segregation and shapes the resulting decorations. This study investigates how adjacent grain and phase boundaries impact a boundary's segregation behavior, with a focus on the competition for carbon (C) enrichment in a laminated ferrite (α)- austenite (γ) microstructure subjected to a series of heat treatments. It was found that semi-coherent α-γ Kurdjumov-Sachs (KS) phase boundaries show less carbon segregation than general γ grain boundaries. Furthermore, when a γ grain boundary is present at a junction with the phase boundaries, it acts as an extracting agent for C. DFT calculations support these observations, demonstrating that carbon segregation is energetically more favorable at the γ grain boundary compared to the α/γ phase boundary, due to the more negative segregation energy at the former.