Solid-state grain-boundary defect-phase formation in the Fe–Zn system

Liquid metal embrittlement (LME) of next generation galvanized steels has recently been shown to involve intermetallic phase formation at preferably high-angle grain-boundaries (HAGB) prior to microcracking. These GB phases emerge already at temperatures much below the melting point of Zn and point towards the importance of solid-state GB mechanisms. To better isolate such early GB phase formation, we pursue here in-situ heating inside a transmission electron microscope on a HAGB of a Fe-Zn solid solution model system. Clear Zn segregation in the HAGB is seen, followed by the formation of an L1 0 FeZn GB phase. Being not part of the equilibrium phase diagram, this phase is considered as a defect phase. Complementing density functional theory calculations demonstrate how the unexpected phase can be energetically favored inside a GB. In concert with bulk synchrotron diffraction data, we propose that this defect phase is an intermediate state towards the higher Zn-containing Fe 3 Zn 10 phase that otherwise is dominating revealed GB intermetallics during the early stages of LME in galvanized steels.