Impact of Zn penetration on the tensile deformation behavior of Fe symmetric tilt grain boundaries

Zn-induced liquid metal embrittlement (LME) hinders the widespread application of galvanized advanced high-strength steels (AHSS) in the automotive industry. This phenomenon originates from the penetration of molten Zn along Fe grain boundaries (GBs) during thermomechanical processing, where Zn segregation weakens the interatomic bonding and degrades the mechanical integrity of the GBs. This work employed molecular-dynamics simulations to investigate the tensile deformation behavior of three representative Fe symmetric tilt grain boundaries (STGBs), namely, Σ5(31¯0)[001], Σ9(11¯4)[110], and Σ21(3¯12)[111], both with and without Zn penetration. The simulations reveal that Zn alloying intrinsically weakens Fe by reducing the stacking fault energies for both {112}⟨111⟩ twinning and 〈111〉 dislocation slip. Governed by segregation energy, Zn atoms preferentially occupy Fe GB sites with large local free space, and tensile loading further enhances this segregation as STGBs expand. In contrast, {112}⟨111⟩ twin boundaries mitigate Zn segregation at STGBs by serving as alternative segregation sites. While Zn segregation and twin nucleation degrade STGB structural order, dislocations enhance it, thereby modulating dislocation nucleation through GB structural unit transitions. Due to loading orientation effects, the {110}⟨111⟩ and {123}⟨111⟩ slip systems, characterized by low Peierls stresses, exhibit the highest Schmid factors in Σ5(31¯0)[001] while the lowest in Σ9(11¯4)[110], resulting in pronounced dislocation activity in Σ5(31¯0)[001] and hardly any dislocation nucleation in Σ9(11¯4)[110]. These findings underscore the potential of GB engineering guided by elemental segregation energetics, combined with texture design to accommodate multi-mode deformation, as an effective strategy to mitigate LME in AHSS.