Characterization of Si-aluminide coating and oxide scale microstructure formed on γ-TiAl alloy during long-term oxidation at 950 °C

Abstract

Recently γ-TiAl intermetallics have been successfully applied on low pressure turbine blades of modern jet engines mostly due to their low density, high specific strength and creep resistance, all of which make them excellent alternatives for Ni-based superalloys. However, due to insufficient high temperature oxidation resistance above 800 °C there is the necessity for development of protective coatings that will allow the formation of a thin and slow growing α-alumina oxide scale.The paper presents the results of investigations concerning the behavior and influence of silicon on the substructure of aluminized ɣ-TiAl alloy in the as-deposited state as well as after long-term isothermal high temperature oxidation. The Si-modified aluminide coating was produced by pack cementation method and oxidized isothermally at 950 °C for 3000 h. The detailed microstructural examination was performed using analytical SEM and STEM techniques along with Focused Ion Beam (FIB) technique which was utilized for the preparation of a sample from the metal-scale interface after long-term oxidation test. The addition of silicon to the aluminide coating resulted in the formation of nanometric Ti5Si3 precipitates that were found to bind both Ti and Nb from the alloy. It was demonstrated that the investigated ɣ-TiAl with Si-modified aluminide coating is capable of forming a continuous and uniform α-alumina oxide scale at 950 °C that remains adherent for 3000 h. The oxide scale was found to obey the growth mechanism typical for Ni-based superalloys and consisted of outer equiaxed and inner columnar grains.