Observation of high-temperature phase transformation in the Si-modified aluminide coating on mild steel using EBSD

Mild steel was coated by hot-dipping in a molten bath containing Al-10 wt.% Si. The phase transformation in the aluminide layer during diffusion at 750 °C in static air was analyzed by Electron Backscatter Diffraction. The results showed that the aluminide layer of the as-coated specimen consisted of an outer Al-Si eutectic topcoat and the inner Fe-Al-Si and Fe-Al intermetallic layers. The formation of τ5-Al7Fe2Si and τ6-Al4FeSi was observed with increasing exposure time at 750 °C, while the τ1-(Al,Si)5Fe3 phase precipitated into the Fe2Al5 phase. After 60 min of exposure, the τ5-Al7Fe2Si and τ6-Al4FeSi phases disappeared. The FeAl phase not only formed at the interface between Fe2Al5 and the steel substrate, but also transformed from τ1-(Al,Si)5Fe3 after diffusion for 10 h. With prolonged exposure, the growing FeAl phase decreased the thickness of Fe2Al5 and forced the formation of FeAl2 phase. Finally, the aluminide layer comprised only FeAl2 and FeAl.