| dc.description.abstract | Liquid metal embrittlement (LME) is a problematic phenomenon that results in the abrupt failure of a ductile metal that is exposed to a reactive liquid metal while simultaneously experiencing a tensile load. Despite extensive research efforts to understand this phenomenon, the underlying mechanisms driving LME remain unclear due to conflicting hypotheses and limited empirical evidence. The lack of fundamental knowledge consequently hinders efforts to investigate the influence of metallurgical factors on the severity of LME, resulting in challenges in devising effective solutions to mitigate or eliminate this catastrophic event. In this thesis, both fundamental and engineering aspects of LME are examined comprehensively in the iron-zinc (Fe-Zn) system by unraveling the underlying mechanisms of LME, exploring metallurgical factors contributing to its susceptibility, and investigating an effective strategy for mitigating LME.
The results showed that LME crack initiation entails several atomic-scale steps where the interdiffusion of Zn atoms into the grain boundaries led to the formation of a stress-induced diffusion wedge that significantly affects the kinetics of interdiffusion, as well as the mechanical integrity of the grain boundary being attacked. The results of a detailed characterization of the LME crack path revealed that stress-induced grain boundary diffusion was the most probable underlying mechanism for LME crack propagation. It was shown that LME crack propagation was strongly affected by the initial microstructural characteristics, in which the ferritic microstructure was more prone to LME crack initiation, while the austenitic microstructure had a significantly higher LME crack propagation rate. This led to the occurrence of a hybrid ductile/brittle failure in the ferritic microstructure but a completely intergranular brittle failure in the austenitic sample. The results showed that the ZnAlMg coating has exceptional resistance to LME cracking at high temperatures. 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 temperatures.
The study presented an integrated perspective on LME crack formation in the Fe-Zn system and used numerical modeling and empirical results to offer fundamental insights that have so far been lacking in the literature. This study proposed a unified mechanism for the occurrence of LME crack, which is able to reconcile conflicting micro- and macro-scale experimental results reported in the literature. The study also facilitated the resolution of the long-standing debate regarding the LME mechanisms proposed in the literature while offering practically relevant knowledge that leads to the design of LME-resistant Fe-Zn couples. | en |
