Oxidation protection of Ti and TiAl alloys
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 stability of these materials.
In the case of TiAl alloys (Al content of about 45 to 55 at.%), the process has involved a single step, i.e. plasma-based ion implantation of halogens, notably fluorine, relying on the so-called “halogen effect”. Optimum conditions have been established under which the F-implanted TiAl alloys acquire a stable, adherent and highly protective alumina scale upon subsequent high-temperature oxidation in air. The extent of environmental durability has been evaluated by carrying out isothermal and thermal cyclic oxidation tests on both F-implanted TiAl laboratory coupons and TiAl machine components (e.g. turbochargers and turbine blades) at temperatures as high as 1050°C, and for times of up to 6000 hours.
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 PBII of F, the coating is capable of forming a protective alumina scale upon exposure to air for extended times at temperatures up to 600°C.
A γ-phase TiAl coating formed by the same process can similarly be applied to low Al content (typically < 10 at.%) TiAl alloys in which the concentration of Al is insufficient for the halogen effect to be triggered after PBII of F.
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