Decoupling early-stage cracking and propagation mechanisms in liquid metal embrittlement of Zn-galvanised TWIP steel

Abstract

Liquid metal embrittlement (LME) presents a major barrier to the widespread adoption of advanced high-strength steels in automotive applications. Despite extensive research, decoupling its early-stage cracking and propagation micromechanisms remains challenging and is a key research gap. Distinguishing these stages is crucial to understanding the conditions and factors that are favourable for LME and to developing mitigation strategies. Moreover, it can improve the accuracy of predictive models through detailed knowledge from initiation to propagation. In this study, this challenge is addressed by performing interrupted Gleeble hot tensile tests on a Zn-galvanised twinning-induced plasticity steel, simulating resistance spot welding conditions. This approach enables tracking LME progression under applied stress and identifying fracture micromechanisms at early and advanced stages of cracking. Additionally, existing theories on LME micromechanisms are often contradictory, highlighting the need for fundamental research in this area. The findings reveal that LME begins with the contact between liquid Zn and the substrate, leading to Zn diffusion into the substrate by diffusion-induced grain boundary migration and dissolution of the substrate by erosion-corrosion. This dissolution generates defects on the substrate and facilitates Fe diffusion into liquid Zn. Subsequently, defects are filled with liquid and the Zn-rich defect tips, connected to grain boundaries, enhance Zn grain boundary diffusion and weaken intergranular cohesion. Under tensile stress, these weakened boundaries decohere and lead to crack nucleation. Newly formed crack surfaces allow fresh Fe-rich liquid Zn to penetrate, continuing the process until fracture. Future work will focus on the influence of microstructure on LME crack growth.