Formation of the ζ phase at an interface between an Fe substrate and a molten 0.2 mass% Al-Zn during galvannealing

An interstitial free (IF) steel was dipped in a bath of molten Zn containing 0.2 mass% Al. The asdipped (galvanized) steel was then annealed above the melting point of Zn (galvannealed), say at 773 K, for a period ranging from 1 to 30 s. Evolution of the microstructure of the Zn-containing coating was examined by transmission electron microscopy (TEM) and the chemical composition around the Zn(Al)/Fe interface was analyzed by energy dispersion spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). In the as-dipped (galvanized) steel a continuous layer of Fe₂Al₅ existed at the Zn(Al)/Fe interface, but no Fe-Zn intermetallic compounds were observed. Galvannealing at 773 K for 1 s resulted in the formation of the ζ phase at the Zn(Al)/Fe₂Al₅ interface, which grew at the expense of the Zn(Al) coating during the subsequent galvannealing. Possible mechanisms of nucleation of Fe-Zn intermetallic compounds are discussed.