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On Protective Coatings of Titanium Intermatallics by Pack Ementation
|作者:||黎衍璋||關鍵字:||Pack Cementation;包覆粉浴法;Diffusion Coating;High Temperature Oxidation Resistance;Titanium Aluminide Intermatallics;擴散被覆;抗高溫氧化;鈦鋁介金屬化合物||出版社:||機械工程研究所||摘要:||
(Pack Cementa tion)法，在Ti3A1-Nb介金屬化合物表面形成擴散被覆
Titanium aluminide intermetallic compounds including Ti3Al and Tial were identified as the promising candidate areospace materials for the next generation aviation requirements. However, the disadvantages of their low temperature brittleness and high temperature oxidation still prohibit these advanced materials from commercialization. The purpose of this study is to investigate the reaction mechanism and the influence of pack aluminizing and/or chromizing coatings on the oxidation resistance of Ti3Al-Nb intermetallics.
For pack aluminizing processes, the aluminim enrichment has successfully converted the surface layer Ti3Al substrate into TiAl3, which has higher tendency to grow stable α-Al2O3 at elevated temperatures. Experimental results showed that the 900℃, 24hrs pack revealed better oxidation resistance than the 1000℃, 24hrs and 1050℃, 24hrs packs. Sifnificant reduction in oxidation kinetics was achieved by the pack aluminizing process. The chromizing pack failed to convert the titanium aluminide into chrome-aluminide coatings which resulted in severe spallation from substrate surface. Due to the ordered structure, chromium diffused very inefficiently in the Ti3Al lattice. The simultaneous chrome-aluminizing process showed similar results as a low concentratin aluminizing process. The anticipated effect of Cr as the secondary oxygen getter was never observed.
The reaction mechanism of pack aluminizing is featured in the outward diffusion of Ti to react with Al coming fromt he activated pack to form TiAl3 phase at the solid/gas interface. The Nb distribution served as the reference system. The oxidation of the aluminized substrate started with co-oxidation of Al2O3 and TiO2 via interface control. Subsequently, the growth of Al2O3 took the control and formed a dense barrier layer between the substrate and the transient oxides. Due to the high concentration of Al content at surface, scattered cracks were observed. Within the 120 hrs testing period, most cracks were arrested by lateral oxidation. However, to achieve the optimum mechanical performance, a reduced aluminum concentration by low activity packs or rare earth doping is recommended.
|Appears in Collections:||機械工程學系所|
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