Niobium titanium aluminides

In the niobium containing alloy which shows a better oxidation resistance the doping of titania with niobium may reduce the dissolution of AlON. By this means a thin layer AlON is formed at the interface leading to a reduced oxidation rate. Thus it is assumed that the oxidation behaviour of titanium aluminides could be improved by stabilizing the aluminium oxide at the metal/oxide interface either by prevention of aluminium depletion of the metal subsurface zone or by reduction of A1203 dissolution in Ti02. 

A great deal of research effort has been directed towards explaining the effect of niobium alloying on the oxidation behaviour of the titanium aluminides y- IiAl and 2-Ti3Al in air. A comprehensive survey is given in [14]. Among the possible mechanisms currently discussed are ... 

At higher niobium levels, the 02 phase evolves to a new ordered orthorhombic structure that is based on the composition Ti2AlNb (O phase), which has been observed in titanium aluminides with compositions finm Ti-25Al-12.5Nb to Ti-25A1-30Nb (Ref 1-4). The crystal structures of the o2 and ordered orthorhombic phases are compared in the accompanying figure, which shows the basal planes and atomic positions in the lanes above and below the plane of sheet. 

Sunderkbtter J D, Gobel M, HaanappelVAC, Perez P and StroosnijderM F (1999), The niobium effect on the oxidation behaviour of TiAl-based alloys An approach to decouple relevant parameters . Gamma Titanium Aluminides 1999, San Diego, TMS. 

---