The initial stages in the oxidation of TiAl

The oxidation behaviour of titanium aluminides containing 36 wt.-% Al (Ti36Al) and 35 wt.-% Al plus 5 wt.-% Nb (Ti35Al5Nb) has been investigated by electron microscopic methods with emphasis on transmission electron microscopy (TEM). The oxidation experiments were carried out at 800 to 1000 degrees C in laboratory air for 0.5 h to 4 h. In addition thermogravimetric measurements were made. It has been shown that the shortterm oxidation of TiAl can be divided into two stages. In stage I the preferred formation of aluminium oxide leads to an aluminium depletion of the metal subsurface zone and the subsequent formation of titanium nitrides which enhances the oxidation rate. After consumption of the depletion layer a repeated cycle of aluminium oxide formation, subsequent local depletion of the metal subsurface zone in Al and consumption of the Ti-rich metal phase by nitride formation is observed leading to linear oxidation behaviour (stage Ii). In the niobium containing alloy the dissolution of alumina in titania is decreased and thus the formation of aluminium oxide at the metal/oxide interface is favoured. By electron diffraction it has been found that the aluminium oxide formed at the metal/oxide interface most probably is an aluminium oxynitride Al27O39N. The aluminium depleted metal phase has been analyzed to consist of alpha(2)-Ti3Al and a new cubic phase with a composition between of alpha(2)-Ti3Al and gamma-TiAl.