Effect of substrate Al content on liquid metal embrittlement susceptibility in quench and partitioned steels

Alloying considerations for 3rd generation advanced high strength steels (3rd Gen AHSS) are being partially driven by mitigation of liquid metal embrittlement (LME) in these Zn-coated steels. This work, therefore, assesses the effect of substrate Al concentration on the LME susceptibility of electrogalvanized (EG) 3rd Gen AHSS with quench and partitioned (Q&P) microstructures. Two alloys were investigated: 0.5Si-0.05Al and 0.5Si-1.38Al. The 1.38Al alloy exhibited lower LME susceptibility in hot tension testing at 800 °C and less Zn enrichment ahead of the LME cracks. To elucidate the mechanisms creating disparate behavior between the two alloys, diffusion and thermodynamic simulations were employed. Due to limited solubility of Al in the liquid Zn coating, diffusion simulations predict an enrichment of Al on the substrate side of the coating-substrate interface. The enrichment of Al would stabilize the Fe-Zn intermetallic Γ (Fe 3 Zn 7 ) phase at the expense of the liquid phase, such that a physical barrier to Zn diffusion into the substrate and the consumption of the liquid phase may mitigate the LME response. The solubility and diffusivity of Zn in the substrate also decreases with increasing Al concentration, resulting in a decrease in the flux of Zn into the substrate. Thus, multiple factors contribute to a decrease in the stress-assisted, solid-state diffusion of Zn ahead of LME cracks, such that the embrittlement of the 1.38Al alloy was lower than that of the 0.05Al alloy. A mechanistic sequence of events occurring during hot tension testing of both alloys is also proposed. • Increasing aluminum in the steel substrate decreased the LME susceptibility. • Thermodynamic simulations predict Al stabilizes Γ-Fe 3 Zn 7 and destabilizes liquid. • Diffusion simulations predict enrichment of Al at coating-substrate interface. • Ternary and higher order systems used to simulate coating and substrate system. • Ductility and strain energy loss used to assess LME in hot tension testing.