On the solute segregation to grain boundaries in SA508 steel

Solute segregation to grain boundaries (GBs) within a high-Ni alloy (SA508 Grade 4N) aged at 427°C was studied to investigate the role of phosphorus in causing non-hardening embrittlement in these steels. Electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) were employed to identify and characterise grain boundaries prior to targeted atom probe tomography (APT) analysis. Boundaries were classified as prior‑austenite grain boundaries (PAGBs) and packet/block (lath) boundaries, enabling segregation behaviour to be directly correlated with crystallographic misorientation and boundary type. Distinct segregation patterns were observed at PAGBs compared to other boundaries, with strong anti‑correlation between P/C and Ni/Mn/Si at PAGB interfaces. In unaged PAGBs, non‑equilibrium segregation driven by vacancy drag promoted Ni enrichment, whereas W‑shaped Ni concentration profiles in aged PAGBs indicate P‑driven reverse vacancy flow. In contrast, solute enrichment at lath boundaries was found to depend strongly on misorientation angle. These results highlight the critical importance of controlling the 5-DoF when assessing segregation‑driven embrittlement mechanisms in bainitic/martensitic steels. They also emphasise the potential of grain boundary engineering as a design strategy for improving resistance to embrittlement in nuclear and other demanding applications.