以EBSD觀察低碳鋼熱浸鋁矽塗層於高溫相變化之行為

Observation of High-Temperature Phase Transformation in the Si-modified Aluminide Coating on Mild Steel Using EBSD

本研究採用低碳鋼經熱浸鋁-10 wt.%矽後，於750℃空氣氣氛下進行擴散。藉由電子背向繞射（Electron Backscatter Diffraction, EBSD）技術，分析由Fe與Al-Si合金擴散所形成之鋁化層，以了解鋁化層受高溫所引發之相變化。研究結果顯示，熱浸後之鋁化層，由外層的鋁矽共晶塗層及內層的Fe-Al-Si與Fe-Al介金屬層所組成。隨高溫擴散時間之延長，可發現τ5-Al7Fe2Si及τ6-Al4FeSi相的生成，並在Fe2Al5相中析出τ1-（Al,Si）5Fe3相。於750℃保持60分鐘後，τ5-Al7Fe2Si及τ6-Al4FeSi相將消失。當擴散繼續進行到10小時，FeAl相除了在Fe2Al5與基材界面處生成，也將由τ1-（Al,Si）5Fe3轉變為FeAl相。經長時間擴散後，FeAl相逐漸成長，造成Fe2Al5相的減少，並生成FeAl2相。最終，鋁化層由FeAl2相與FeAl相所組成。

Mild steel was coated by hot-dipping into a molten bath containing Al-10 wt.% Si. The phase transformation in the aluminide layer during diffusion at 750 ℃ in static air was analyzed by Electron Backscatter Diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer Al-Si eutectic topcoat and the inner Fe-Al-Si and Fe-Al intermetallic layers. The formation of τ5-Al7Fe2Si and τ6-Al4FeSi can be observed with increasing exposure time at 750 ℃, while the τ1-(Al,Si)5Fe3 phase precipitated in Fe2Al5. After 60 min of exposure, the τ5-Al7Fe2Si and τ6-Al4FeSi phases disappeared. 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 growing FeAl phase decreased the thickness of Fe2Al5 and forced the formation of FeAl2 phase. Finally, the aluminide layer composed of FeAl2 and FeAl.