Microstructure formation and high temperature oxidation behavior of Ti-Al-Cr-Y-Si coatings on TiAl

Highlights: • Closed Hollow Cathode PVD was applied for deposition TiAlCrYSi coatings on TiAl. • The growth mechanism and phase transformations in the coating are characterized. • The TiAlCrYSi coating provides excellent oxidation resistance at 850 °C. • Microstructural proof is provided for the Cr effect on the formation of α-Al₂O₃. • The segregation of yttrium to the grain boundaries of α-Al₂O₃ is evidenced. The paper presents the application of Closed Hollow Cathode Physical Vapor Deposition (CHC-PVD) method for the depostion of Ti-Al-Cr-Y-Si coatings on γ-TiAl 48-2-2 alloy for high temperature oxidation protection. The study concerned the analysis of the coating's growth mechanism, initial microstructure as well as phase transformations investigations using high resolution Scanning Transmission Electron Microscopy (STEM) and high temperature X-ray diffraction (HT-XRD). The coated alloy was subjected to high temperature oxidation test at 850 °C where a fivefold lower mass gain compared to bare 48-2-2 alloy was observed. Detailed microstructural investigations allowed to characterize the thermally grown oxide scale, which was found to be composed of nanometric layers of titania, equiaxed (Al,Cr)₂O₃ and columnar alumina. These investigations provided microstructural evidence for the Cr effect on the formation of Al₂O₃, which was postulated previously. Yttrium was found to segregate to the grain boundaries of alumina oxide scale during high temperature oxidation, indicating the occurence of the reactive element effect.