利用EBSD研究矽含量對於鋁化低碳鋼中介金屬相形成之影響

EBSD Study of Effect of Silicon on the Formation of Intermetallic Phases in Aluminide Coating on Mild Steel

本研究採用低碳鋼熱浸於700℃之純鋁、鋁-0.5 wt.%矽、鋁-2.5 wt.%矽、鋁-5 wt.%矽與鋁-10 wt.%矽熔湯中180秒。藉由掃描式電子顯微鏡（scanning electron microscope, SEM）、能量分散光譜儀（energy dispersive X-ray spectroscopy, EDS）與電子背向繞射（electron backscatter diffraction, EBSD）技術，分析鋁湯中不同矽含量對於鋁化層中介金屬相形成之影響。研究結果顯示，熱浸純鋁鋁化層具有最厚之介金屬層，並且在介金屬層與鋼料基材處形成高低起伏之界面。隨著鋁湯中矽含量的提高，介金屬層厚度逐漸減薄，介金屬層與鋼料基材之界面也趨於平坦。在介金屬相的組成中，當鋁湯中矽含量高於2.5 wt.%時，鋁化層中除了具有熱浸純鋁鋁化層中的FeAl3與Fe2Al5相外，也開始在FeAl3相上方形成立方晶τ5(C)-Al7(Fe,M)2Si與散布於Fe2Al5相中的τ1-(Al,Si)5Fe3相。但是當矽含量持續提升，τ5(C)-Al7(Fe,M)2Si相會逐漸被六方晶τ5(H)-Al7Fe2Si與τ6-Al4FeSi相取而代之。

Mild steel samples were coated with pure Al, Al-0.5 wt.%Si, Al-2.5 wt.%Si, Al-5 wt.%Si, and Al-10 wt.%Si, respectively, by hot-dipping in molten baths at 700 ℃ for 180 seconds. The effect of silicon over the microstructure and the formation of intermetallic phases in the samples was investigated by a combination of scanning electron microscope (SEM), energy dispersive Xray spectroscopy (EDS) and electron backscatter diffraction (EBSD). Micrograph observations revealed that the sample coated with pure Al possessed the thickest intermetallic layer and a rough interface between the intermetallic layer and the steel substrate. As the silicon content in the molten bath increased, the thickness of the intermetallic layer decreased significantly and the interface between the intermetallic layer and the steel substrate became flat. On the other hand, the EDS and EBSD results revealed the aluminide layer on the sample hot-dipped in an Al-2.5 wt.%Si molten bath not only possessed FeAl3 and Fe2Al5, as that of the sample hot-dipped in a pure Al molten bath, but also formed cubic τ5(C)-Al7(Fe,M)2Si above FeAl3 and τ1-(Al,Si)5Fe3 scattered in Fe2Al5. However, as the silicon content in the molten bath started to increase, τ5(C)-Al7(Fe,M)2Si was gradually replaced by hexagonal τ5(H)-Al7Fe2Si and τ6-Al4FeSi.