Formation of an alumina-containing scale for the surface protection of TiAl alloys and Ti against environmental degradation at elevated temperatures


Formation of an alumina-containing scale for the surface protection of TiAl alloys and Ti against environmental degradation at elevated temperatures

Yankov, R. A.; Kolitsch, A.; Munnik, F.; von Borany, J.

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 above 600°C. We have developed a process for enhancing the high-temperature oxidation resistance of these materials. For TiAl, the process involves a single step, i.e. plasma immersion implantation (PIII) of fluorine relying on the so-called "halogen effect". Optimum conditions have been established under which the 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 samples at temperatures up to 1050°C. Results from characterization by elastic recoil detection, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis have proven the possibility of forming a protective alumina scale on both laboratory coupons and components such as turbine blades and turbochargers. In the case of Ti, surface processing involves two steps, i.e. Al enrichment of the Ti near-surface, and introduction of F by PIII. The Ti samples so modified have shown marked environmental stability at temperatures up to 700°C.

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