Microstructural studies of fluorine-implanted titanium aluminides for enhanced environmental durability

Titanium aluminides based on the gamma-phase (γ-TiAl) are promising materials for advanced power generation, aerospace and automobile applications. Oxidation-resistance problems, however, limit the maximal service temperature of these alloys to about 700°C. A significant improvement in environmental durability of γ-TiAl up to 1050°C can be achieved by ion-implanting fluorine into the alloy subsurface relying on the so-called halogen effect. Plasma immersion ion implantation (PIII) of F has been employed because of the possibility to process components of complex geometry as well as to inject sufficiently high F doses in relatively short times. In this work, characterization of the microstructure of F-implanted γ-TiAl alloys has been undertaken using cross-sectional transmission electron microscopy in conjunction with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. Preliminary studies by elastic recoil detection analysis have revealed anomalously broad, high-concentration (up to 60 at. %) F profiles of either Gaussian-like or plateau-like shape extending to much larger depths than those predicted by theory; a phenomenon which cannot be accounted for by standard ion-solid interaction and F diffusion mechanisms. It has been found that the F implant profiles result from a complex amorphiztation/recrystallization (a/c) process, which occurs via the a/c front progressing toward the bulk and giving rise to anomalous F diffusion. The final F distribution is implantation-temperature dependent, with higher temperatures causing partial dynamic annealing of the amorphized TiAl material and profile shrinkage. Long-term pos-implantation oxidation tests have indicated that enhanced oxidation resistance is always associated with Gaussian-type as-implanted fluorine profiles coupled with optimal fluorine doses while flat-topped implant profiles resulting from the implantation of excessively high F doses produce a poorly oxidation-resistant surface. The results of these analyses have been helpful in understanding the behavior of the implanted F from both a basic scientific and a technological standpoint.