Investigation of the fluorine microalloying effect in the oxidation of TiAl at 900°C in air

High-temperature oxidation resistance of gamma titanium aluminides can be achieved by the formation of a continuous scale of slowly growing Al₂O₃ /1-3/. The formation of such a scale can be stimulated by the addition of small amounts of fluorine. In this study the fluorine is applied in two ways: treatment with diluted HF and ion implantation. Cast γ-TiAl (Ti-50at.%Al) was prepared as coupons of dimensions 8x8x1 mm³ and polished with SiC paper down to 4000 grit. Microstructural investigations show a minor amount of the α₂-Ti₃Al phase (lamellar structure) within the γ-TiAl phase. Ion implantation was done using a 60 keV implanter. Oxidation was carried out at 900°C and air, partly in combination with Thermogravimetric Analysis (TGA). The post oxidation investigations started with optical microscopy followed by the morphologic surface study using SEM. Non destructive Ion Beam Analysis (PIGE/RBS-technique) was applied to determine the fluorine profile within the first 1.3 µm and to measure the elemental profiles of the main elements Ti, Al and O in the oxide layer down to the Al depleted sub-surface region. These PIGE/RBS-investigations were also performed at the samples after HF treatment but before oxidation. Finally a cross-section was prepared in order to study the layer structure by SEM.