Oxidation Behavior of Aluminide Coated Ti-Al-Cr-Nb-Zr-Y Alloys at High Temperatures

In this study the oxidation behavior of diffusion aluminide coating containing layers of TiAl3 and TiAl2, develop on a substrate of Zr-Y doped α2-Ti3Al/γ-TiAlCrNb intermetallic alloy using pack aluminizing method, was investigated isothermally at 800°C, 900°C, and 1000°C under atmospheric air pressure. The pack cementation was carried out at 850°C for 25 hours in a pack containing 20%-wt Al, 2%-wt NH4Cl, and 78%-wt Al2O3.The phases in the coatings and oxide layers were examined by optical and scanning electron microscopy as well as X-ray diffraction method, while chemical composition of the oxides and phases were examined with EDS attached on the SEM. The experimental results showed that the addition of Zr and Y increases the oxidation resistance of the coating by formation of complex oxides mainly of Al2O3 at the coating surfaces and sub-surface. Combination of oxidation and interdiffusion process cause transformation of TiAl3 layer to TiAl2 that decrease the oxidation resistance through the formation of TiO2 rod crystals on the junction between TiAl2 and Al2O3 in the outer layer.