STEM investigations of γ-TiAl produced by additive manufacturing after isothermal oxidation

Short-term isothermal oxidation of TiAl 48-2-2 alloy produced by Electron Beam (EBM) was investigated at 750, 800, 850 and 900 degrees C with special focus on the oxidation kinetics and metal-scale interface evolution. It was shown that at the lowest oxidizing temperature a passivating and thin mixture of alumina and titania is formed with subsurface Z-phase formation. At higher temperatures a porous and non-protective oxide scales formed with subsequent nitridation at the metal-scale interface. Moreover, long-term isothermal oxidation at 750 degrees C up to 3000 h in 500 h intervals was performed and related to the initial band microstructure of the alloy resulting from EBM process. Internal oxidation was observed on lamellar alpha(2)/gamma colonies while alpha-alumina mixed with TiN and Ti2AlN was found to form on equiaxed gamma-TiAl regions. Analytical Scanning Electron Microscopy (STEM) aided with Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) were used for investigations of the oxide scales as well as nitride layers formation at different temperatures.