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.

Research highlights▶ EBSD provides crystallographic information regardless of the content of materials. ▶ Mapping and EBSPs functions in EBSD provide a reliable phase identification. ▶ The formation mechanism of tongue-like Fe2Al5 layer has been addressed. ▶ Serration-like substrate will be flattened by the growing FeAl2 and FeAl phases.