Improvement of the High-Temperature Oxidation Resistance of γ-TiAl by Selectively Pre-treated Si-based Coatings

Abstract

In this study the degradation of the oxidation resistance of γ-TiAl alloys caused by the high affinity of titanium for oxygen was mitigated by forming thermodynamically stable titanium silicides on the surface. A Si-25at%Ti-coating was deposited on Ti-45Al-8Nb (at%) substrate by means of magnetron sputtering. To form surface layers of thermodynamically stable titanium silicides an optimized pre-treatment process at 1000°C for 100h at 10^-6mbar was used applying slow heating and cooling rates. This annealing process resulted in the formation of a Ti₅Si₃ phase and a Ti₅Si₄ layer on top, whereas titanium silicides with higher silicon content (Ti₅Si₄, TiSi and TiSi₂) formed when the coating system was pre-treated in air at 750°C for 100h. During latter pre-treatment, a mixed oxide scale grew on top of the coating. The oxidation behavior of the pre-treated coatings on γ-TiAl was studied by cyclic testing at 900°C in air. Phase formation and microstructure evolution of the coated samples were investigated after 10, 100 and 1000 cycles using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. All specimens exhibited a mixed oxide scale on top after cyclic testing and exceeded a lifetime of 1000 cycles at minimum. Uncoated γ-TiAl failed after 520 cycles at 900°C by severe scale spallation.