Influence of Si on the intermetallic compound formation in the hot-dipped aluminide medium carbon steel

Medium carbon steels hot-dipped in various Al-Si baths were prepared with Si contents of 0, 2.5, 5, 7.5 and 12.5 wt%. The microstructures of the topcoat and the intermetallic layer were investigated through SEM, EDS, EBSD and TEM analysis. Isothermal sections at 750 °C and 650 °C, as well as the liquidus projection, were calculated for the Al–Fe–Si system using Thermo-Calc software with the framework of the CALPHAD approach in order to better realize the microstructure evolution of the aluminized steels. With the increased addition of Si in the Al bath, ternary Al–Fe–Si intermetallic compounds are generated, and the thicknesses of the Al5Fe2 layer is reduced. τ5 phase with one crystallographic direction and τ6 phase are formed between the Al13Fe4 layer and the topcoat when the Si content is above 5 wt% and 7.5 wt%, respectively. The dispersedly distributed τ1 phase in the Al5Fe2 phase layer is generated with the Si content high than 2.5 wt%, and the proportion increases with the increment of Si. The existence of τ1 will impede the growth of Al5Fe2. Diffusion paths are presented in the isothermal sections to understand the formation of the intermetallic layer. The content of Si in the bath has a great effect on the microstructure of the topcoat, which also varies at different positions. The liquidus projection is utilized to reveal the primary phase formation in the topcoat.