Surface protection of titanium and titanium-aluminum alloys against environmental degradation at elevated temperatures

Ti and its alloys with Al are a class of lightweight materials, which find extensive use in a number of advanced aerospace, automotive and power generation applications. These materials, however, are limited in applicability by their poor oxidation resistance at elevated temperatures (> 500°C for Ti, and > 750°C for TiAl).
We have developed a technique for protecting the above-mentioned materials against high-temperature environmental degradation (oxidation and embrittlement). In the case of TiAl alloys of an Al content of about 40 to 60 at.% , the technique has involved a single step, i.e. plasma immersion ion implantation (PIII) of fluorine, making use of the so-called “fluorine effect”. Optimum process parameters 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 oxidation protection has been evaluated by testing F-implanted TiAl samples either isothermally or under conditions of thermal cyclic oxidation at temperatures ranging from 900° to 1050°C, and for times as long as 6000 hours. Results from characterization by elastic recoil detection analysis (ERDA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and Rutherford backscattering spectrometry (RBS) have proven the possibility of forming a protective alumina scale on both laboratory coupons and machine components such as jet turbine blades and turbochargers. In the case of Ti and low-Al-content Ti-Al alloys, e.g. Ti3Al, the technique has involved two steps, namely Al enrichment (aluminization) of the material’s near-surface, and introduction of F by PIII to activate the fluorine effect. Under optimized process conditions, the Ti and Ti3Al samples so modified have shown marked environmental stability at temperatures as high as 700°C and for extended oxidation times due to the presence of a protective alumina layer.