Resistivity spot welding
Source: BAM
Have you ever thought that a protective metal coating could actually weaken the material it’s meant to save? When zinc coats advanced high-strength steels (AHSS), it usually guards against corrosion, but under the heat of welding or forming, something unexpected happens. In recent studies, we found that, the Zn solute atoms sneak (segregate) into the grain boundaries (GBs) inducing a massive segregation transition (formation of high Zn concentration island at the GB) that reduces the work of separation and the energy barrier for liquid formation at the GB (a.k.a liquid metal embrittlement (LME)).
These consistent and compelling results motivated us to further explore the microstructure-, time- and temperature-dependent evolution of the GB segregation phenomenon in Fe-Zn system. Particularly, we asked: what roles do GB structure and properties, alloy composition, and temperature play in triggering segregation-induced LME?
In our current study, we tackle these questions using an enhanced CALPHAD-integrated density-based phase-field (DPF) model, capable of capturing GB structural transformations, in combination with atomistic simulations. Our findings reveal that Zn segregation is highly sensitive to GB type, temperature, and alloy composition. Interestingly, GBs with lower atomic disorder tend to suppress Zn segregation transitions, whereas for a given GB type, the degree of segregation increases as temperature decreases. Moreover, we observe that the coupled chemo-structural evolution stabilizes the Zn-rich islands, which may later serve as precursors for intermetallic phase formation along the GB. To consolidate these insights, we constructed GB phase diagrams across a broad temperature range for various GB types (see in the Figure), offering a quantitative view of the Zn segregation transition in the Fe-Zn system. These GB phase diagrams offer a practical roadmap for mitigating LME, either by controlling processing times and temperatures to limit Zn segregation below critical thresholds or by deliberately designing the GB structures beneath the welding zone.
Thermodynamics of grain boundary segregation transition and their relevance for liquid metal embrittlement in Fe-Zn system
Yuki Ikeda, Theophilus Wallis, Robert Maaß, Reza Darvishi Kamachali
Acta Materialia, 2025