Investigating zinc-assisted liquid metal embrittlement in ferritic and austenitic steels: Correlation between crack susceptibility and failure mechanism

Liquid metal embrittlement (LME) is a problematic phenomenon that occurs when a reactive liquid metal attacks the grain boundaries of ductile solid metal, leading to brittle failure under the application of tensile stress. Despite considerable research efforts to understand LME, the nuances associated with the influence of initial microstructure on LME susceptibility and failure mechanism for the iron‑zinc (FeZn) system have not been thoroughly explored. Thus far, research activities on the role of microstructure on LME susceptibility have either provided conflicting results or failed to establish any correlation between crack susceptibility and the failure mechanism. In this study, steels with fully ferritic and austenitic microstructures were subjected to the same thermomechanical processing treatment to gain insight into how relevant features of the initial microstructure such as grain boundary distribution and local chemistry influence LME crack susceptibility and failure mechanism. The results showed that both ferritic and austenitic microstructures were sensitive to the LME crack formation. The ferritic microstructure was more prone to LME crack initiation with relatively low LME crack propagation rate resulting in a much higher frequency of smaller cracks observed in the sample. The austenitic microstructure was resistant to crack initiation but had a significantly higher LME crack propagation rate resulting in fewer cracks which were much larger in size. This led to the occurrence of a hybrid ductile/brittle type failure in the ferritic microstructure but a completely intergranular brittle failure in the austenitic sample. The results offer clear evidence of LME crack susceptibility in ferritic and austenitic steels, which can be used to guide microstructural modification strategies when developing novel methods to eliminate Zn-assisted LME cracking in steels.