Study of microstructure and phase evolution of hot-dipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction

Mild steel was coated by hot-dipping into a molten aluminum bath. The microstructure and phase evolution in the aluminide layer during diffusion at 750 °C in static air were analyzed by electron backscatter diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer aluminum topcoat, minor FeAl₃ and major Fe₂Al₅, respectively. Also, Fe₂Al₅ possessed a tongue-like morphology, which caused corresponding serration-like morphology in the steel substrate. A portion of the peaks of serration-like substrate were isolated, after short exposure at 750 °C, and accompanied by the formation of voids, which continued to appear with further exposure at 750 °C. As the aluminum topcoat was consumed, FeAl₃ phase disappeared and left an aluminide layer of Fe₂Al₅ phase. After 60 min of exposure, FeAl₂ and FeAl phases formed at the interface between Fe₂Al₅ and the steel substrate. With increasing exposure time, the voids condensed and the serration-like morphology disappeared, while FeAl₂ and FeAl phases kept growing. After prolonged exposure, the aluminide layer was composed of FeAl₂ and FeAl and possessed a flat interface between FeAl and steel substrate.