Silicon effect on retardation of Fe-Zn alloying behavior: Towards an explanation of liquid zinc embrittlement susceptibility of third generation advanced high strength steels

Silicon (Si) aggravates liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) by inhibiting intermetallic formation between liquid Zn and the AHSS substrate at elevated temperatures. This study performed detailed microstructural and elemental characterization of an LME-susceptible Si-bearing AHSS to reveal the fundamental mechanism by which Si inhibits Fe-Zn intermetallic reactions. Notably, the interaction between Si-alloyed AHSS and liquid Zn at elevated temperatures results in pronounced Si enrichment in a thin region of the substrate in contact with the liquid. This Si enrichment behavior is at the core of the retarded Fe-Zn reactions, and is explained from a fundamental standpoint by analyses of the phase equilibria in the Fe-Zn-Si ternary system and diffusion kinetics using DICTRA®. Specifically, the liquid dissolves the surface layers of the AHSS substrate upon initial contact at elevated temperatures. The liquid phase, however, has no appreciable solubility for Si; hence, almost all of the Si atoms from the dissolved substrate layers are “back diffused” into the substrate leading to Si enrichment. The Si-enrichment of the substrate reduces the thermodynamic driving force for nucleating intermetallic phases at the steel/coating interface, making it difficult to form a “protective” interfacial layer barrier between the liquid and the substrate. Further, Si enrichment results in an increase in the chemical potential of Zn and reduction in the chemical potential of Fe in the substrate; both these factors slow the kinetics of substrate dissolution and Fe-Zn alloying, leading to an increased liquid fraction in the coating available to activate LME.