EBSD characterization of high-temperature phase transformations in an Al–Si coating on Cr–Mo steel

5Cr–0.5Mo 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 show the aluminide layer of the as-coated specimen consisted of an outer Al–Si topcoat, a middle layer formed of scattered τ5(C)-Al7(Fe,Cr)2Si particles and minor plate-shaped τ4-Al4FeSi2 and τ6-Al4FeSi phases in the Al–Si matrix and an inner continuous τ5(H)-Al7Fe2Si layer, respectively from the coating surface to the steel substrate. The formation of FeAl3 and Fe2Al5 with τ1-(Al,Si)5Fe3 precipitates can be observed with increasing exposure time at 750 °C. After 5 h of exposure, the Al–Si topcoat has been consumed, and the aluminide layer consisted of Fe2Al5 and a few τ1-(Al,Si)5Fe3 precipitates. 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 aluminide layer comprised only FeAl2 and FeAl.