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

摘要

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 T_5(c)-Al_7(Fe,Cr)_2Si particles and minor plate-shaped T_4-Al_4FeSi_2 and T_6-Al_4FeSi phases in the Al-Si matrix and an inner continuous T_5(H)-Al_7Fe_2Si layer, respectively from the coating surface to the steel substrate. The formation of FeAl_3 and Fe_2Al_5 with Ti-(Al,Si)_5Fe_3 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 Fe_2Al_5 and a few τ_1-(Al,Si)_5Fe_3 precipitates. The FeAl phase not only formed at the interface between Fe_2Al_5 and the steel substrate, but also transformed from τ_1-(Al,Si)_5Fe_3 after diffusion for 10 h. With prolonged exposure, the aluminide layer comprised only FeAl_2 and FeAl.