Surface protection of TiAl alloys and Ti against high-temperature environmental degradation

Ti and TiAl alloys are lightweight materials that hold great promise for advanced aerospace, automotive and power generation applications. They are, however, limited in applicability by their poor oxidation resistance at elevated temperatures. We have developed viable techniques for enhancing the high-temperature environmental durability of these materials. In the case of TiAl, the process has involved a single step, i.e. plasma immersion implantation (PIII) of fluorine relying on the so-called “halogen effect”. Optimum processing conditions have been established under which the F-implanted alloys acquire a stable, adherent and highly protective alumina scale upon subsequent high-temperature oxidation in air. The extent of oxidation protection has been evaluated by testing F-implanted TiAl laboratory coupons as well as machine components (e.g. turbine blades and turbochargers) at temperatures as high as 1050°C, and for times up to 6000 h under conditions of both isothermal and thermal cyclic oxidation. Analytical methods such as elastic recoil detection, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis have been used for materials characterization. In the case of Ti, an efficient protective coating that serves as a barrier to the oxygen in-diffusion has been developed. Specifically, the coating consists of a γ-phase TiAl formed by magnetron co-sputtering of Ti and Al onto the Ti substrate, and subsequent vacuum annealing. After PIII of F, the coating has proven capable of forming a protective alumina scale upon exposure to air for extended times at temperatures up to 600°C.