3rd generation Zinc (Zn) galvanized advanced high-strength steels (3rd Gen. AHSS) are a promising class of materials for the automotive sector because of their excellent strength-ductility balance and their high corrosion resistance. However, during joining processes in which the Zn coating melts, the steel often suffers from liquid-metal embrittlement (LME) via Zn penetration into the grain-boundary (GB) network followed by microcracking. Whilst LME has been investigated for decades, the underlying mechanisms continue to remain unclear especially about how exactly Zn weakens GBs. Investigating uncracked but Zn-containing GBs, we were previously able to demonstrate that the formation of GB intermetallic phases is intimately linked to LME induced during spot welding, and likely a driver for crack propagation [1]. As a continuation of this work, we now investigate the time evolution of the microstructure by observing interrupted welding using scanning/transmission electron microscopy (S/TEM). It is found that high Zn-containing intermetallic GB phases already form early during the weld (Figure 1), which raised the question of how this emerges in an environment of globally low Zn content. To this end, density-based phase field modeling (DPFM) [2] was used, revealing how a segregation assisted transition in the GB allows for the rapid formation of the GB-phase.