Solid-state intermetallic grain-boundary precipitation as an early stage of liquid-metal embrittlement in advanced high-strength steels

Abstract Liquid-metal embrittlement (LME) in Zn-coated steels is traditionally understood as a consequence of liquid Zn penetration along grain boundaries (GBs) during thermo-mechanical processing. However, recent thermodynamic predictions brought to light a massive Zn segregation transition at Fe GBs that suggest a strong driving force for intermetallic phase formation at substantially lower temperatures than the melting point of Zn. Leveraging bulk mechanical testing, high-energy synchrotron diffraction, and transmission electron microscopy, we demonstrate here that embrittling Fe-rich intermetallic grain-boundary phases emerge in an advanced high-strength steel prior to any melting of Zn. Their formation and increasing presence with temperature correlates with severe mechanical degradation. These findings provide consistent evidence that the solid-state formation of Fe-rich Fe-Zn intermetallic phases constitutes an early contributing step to LME in galvanized high-strength steels.