The role of microstructure in crack growth during liquid metal embrittlement of Zn-galvanised TWIP steel

Improving the reliability of advanced high-strength steels (AHSS) for automotive applications requires a thorough understanding of liquid metal embrittlement (LME) crack propagation micromechanisms. This study investigates how microstructural features govern crack propagation paths in Zn-galvanised twinning-induced plasticty steel. LME was induced via Gleeble hot tensile tests at 800 °C, and a correlative analysis of the fracture surface's transversal plane revealed key crack-microstructure interactions. The results show that LME fracture is predominantly intergranular, preferentially occurring along high-angle, high-energy random grain boundaries (40°–56°). To quantify the effect of tensile stress on grain boundary segments, a normalised grain boundary stress factor was defined, ranging from 0 (no tensile stress, only shear) to 1 (pure tensile stress). Generally, high-angle grain boundaries require a stress factor below 0.2 for LME, while low-angle grain boundaries ( θ <15°) require at least 0.5. Most coincident site lattice boundaries between Σ5 and Σ29 are affected by LME, whereas Σ3 boundaries remain resistant, even at high stress factor. However, cases where Zn penetration was absent despite high misorientation angles and stress factors, or where cracking occurred under the lowest stress factor (parallel to the loading axis), suggest additional, unidentified factors influence LME. These findings highlight the need for advanced three-dimensional modelling to capture the complex interaction between microstructure, stress state, and Zn penetration, not fully resolved in experiments. These insights could guide the development of LME-resistant steels, supporting their safe and reliable use in the automotive industry.