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 degrees 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 tau(5(C))-Al-7(Fe,Cr)(2)Si particles and minor plate-shaped tau(4)-Al4FeSi2 and tau(6)-Al4FeSi phases in the Al-Si matrix and an inner continuous tau(5(H))-Al7Fe2Si layer, respectively from the coating surface to the steel substrate. The formation of FeAl3 and Fe2Al5 with tau(1)-(Al,Si)(5)Fe-3 precipitates can be observed with increasing exposure time at 750 degrees C. After 5 h of exposure, the Al-Si topcoat has been consumed, and the aluminide layer consisted of Fe2Al5 and a few tau(1)-(Al,Si)(5)Fe-3 precipitates. The FeAl phase not only formed at the interface between Fe2Al5 and the steel substrate, but also transformed from tau(1)-(Al,Si)(5)Fe-3 after diffusion for 10 h. With prolonged exposure, the aluminide layer comprised only FeAl2 and FeAl. (C) 2011 Elsevier Inc. All rights reserved.