Early oxidation behaviors of Ti50Al and Ti45Al8Nb alloys at high temperature

The early oxidation behaviors of Ti50Al and Ti45Al8Nb (atomic fraction, %) alloys at 900 degrees C in air have been investigated. The surface morphologies of oxide films are analyzed by SEM. Oxide layers formed for different oxidation times were investigated by XPS. The changes of the layer composition with depth were analyzed by AES. Experimental results revealed that the oxidation resistance of Ti45Al8Nb was remarkably promoted by Nb addition. After oxidization for 90 min the surface of Ti50Al was covered by coarse grained titanium rich oxides, a part of the scale had spalled during cooling, and its inner part is a loose and porous structure rich in titanium, aluminium and oxygen (and probably nitrogen). For Ti45Al8Nb after oxidization for 90 min a thin oxide film with more fined grained Al2O3 was formed on the outer surface. In the inner layer with TiO2 and other oxides Nb substituted for Ti in TiO2 as a cation with valences 5, which decreased oxygen diffusion into inner, suppressed the TiO2 growth and improved the growth of the outer pure Al2O3 layer. Al2O3 layer impeded the inward diffusion of oxygen and the outward diffusion of metallic elements.