以EBSD觀察鉻鉬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 a Cr-Mo Steel using EBSD

本研究採用5Cr-0.5Mo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-Cr與Al-Si合金所形成之鋁化層，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，由外層鋁矽塗層、散布τ5（C）-Al7（Fe,Cr）2Si顆粒、針狀τ6-Al4FeSi及τ4-Al4FeSi2與連續τ5（H）-Al7Fe2Si所組成。隨高溫擴散時間之延長，可在連續τ5（H）-Al7Fe2Si與基材間形成FeAl3與Fe2Al5，並在Fe2Al5相中析出τ1-（Al,Si）5Fe3相。當擴散進行到60分鐘，Fe-Al介金屬層的成長將逐漸取代外側之Fe-Al-Si介金屬相。當鋁矽塗層消耗完，鋁化層由Fe2Al5相與析出之τ1-（Al,Si）5Fe3相所組成。經過10小時擴散，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，鋁化層由FeAl2與FeAl相所組成。

A 5Cr-0.5Mo steel was coated by hot-dipping into a molten bath containing Al-10 wt.% Si. The phase transformation in the aluminide layer during a diffusion treatment 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 topcoat, dispersive τ5(C)-Al7(Fe,Cr)2Si, needle-shaped τ6-Al4FeSi and τ4-Al4FeSi2, and a continuous τ5(H)-Al7Fe2Si layer. The formation of FeAl3 and Fe2Al5 with τ1-(Al,Si)5Fe3 precipitates could be observed with increasing exposure times at 750 ℃. After a 60 min of exposure, the growth of an Fe-Al intermetallic layer gradually replaced the outer Fe-Al-Si intermetallic layer. The Aluminide layer consisted of Fe2Al5 and τ1- (Al,Si)5Fe3 precipitates as the Al-Si topcoat was consumed. The FeAl phase not only formed at the interface between Fe2Al5 and the steel substrate but was also transformed from τ1-(Al,Si)5Fe3 after a diffusion treatment for 10 h. After a prolonged exposure, the aluminide layer comprised only FeAl2 and FeAl.