Effect of Au diffusion in Au-coated (γ + α2) titanium aluminides

In order to understand the role of α2-Ti3Al precipitates during the oxidation of Au-coated γ-48-2-2-TiAl alloys, the γ-TiAl and α2-Ti3Al microstructures resulting from Au diffusion at 900 °C were investigated. The as-obtained coatings were characterized using scanning electron microscopy (SEM), combined energy and wavelength dispersive X-ray spectroscopies (EDS, WDS), electron backscatter diffraction analysis (EBSD), X-ray diffraction (XRD) and isothermal oxidation tests. Results showed that Au-coated-γ-TiAl alloys presented an upper aluminoformer TiAlAu2 layer and a lower non-protective TiAlAu layer, both with narrow homogeneity range, and with thicknesses that could be estimated with the use of the Prasad model. A more complex layer structure is obtained after Au diffusion in α2-Ti3Al alloys, since eight different phases were identified. In the lower layers, two new ternary tetragonal phases (Ti2AlAu and Ti3Al2Au) were found, and a partial diagram for the Al-Au-Ti 900 °C isotherm is proposed. Absence or depletion of Al was found in the upper layers and, in addition, a low oxidation resistance of the Au-coated-α2-Ti3Al alloy was reported.