High-temperature phase evolution of the ZnAlMg coating and its effect on mitigating liquid-metal-embrittlement cracking

Ternary zinc-aluminum-magnesium (ZnAlMg) alloy coatings are the focus of significant attention in the automotive and steel industries due to several advantages over traditional Zn-based coatings. Currently, the literature on this type of coating is limited and focuses mainly on their corrosion resistance and room temperature tensile properties. To assess the relevance of ZnAlMg coatings in current manufacturing processes such as hot stamping and welding of advanced high strength steels (AHSS), it is essential to understand their high-temperature performance, particularly their resistance to liquid metal embrittlement (LME) cracking. This study showed that the ZnAlMg coating had complete resistance to LME cracking at a temperature of about 900 °C, which is traditionally recognized as the temperature at which the highest levels of LME susceptibility are observed in the different families of AHSS. Elemental distribution analysis confirmed that due to an increase in the testing temperature, the lamellar eutectic microstructure of the coating dissolved into the Zn-matrix, with the constituent elements, Al and Mg, segregating towards the steel substrate and the coating surface, respectively. This led to the in-situ formation of a uniform α-Fe(Zn, Al) layer at the steel/coating interface which prevented the direct contact of liquid metal with the steel substrate, resulting in complete suppression of LME at high temperature. Numerical calculations of interdiffusion flux were used to investigate the diffusion behavior of the elements of interest at the interface which indicated that the α-Fe(Zn, Al) layer formed due to the high diffusion rate of Al towards the Fe substrate at 900 °C. The effectiveness of the α-Fe(Zn, Al) layer in mitigating LME was evaluated by calculating the work of adhesion, which showed that this layer preserved its integrity under an applied tensile load, successfully mitigating the initiation of LME. The findings of this study offer valuable insights into developing new avenues for advancing LME resistant coatings by utilizing ternary Zn-based alloy systems.