Dual surface and bulk control by Nb of the penetration of environmental elements in TiAl intermetallic alloys

X-ray photoelectron spectroscopy (XPS) and micro-hardness measurements were combined to study the effect of Nb doping on the high temperature environmental interactions of α2-Ti3Al intermetallic alloys with pure oxygen and oxygen/nitrogen gas mixtures. The XPS data show a pre-oxidation stage in the first minutes of low pressure exposure to the gaseous environments at 650 deg. C, characterized by the absence of oxidation of the metal constituents of the alloy, the adsorption on the surface, and the sub-surface penetration of atomic oxygen and nitrogen. The XPS measurements of the surface and sub-surface concentrations reveal the effect of niobium on the surface adsorption of oxygen and nitrogen and on sub-surface penetration, showing that nitrogen is disfavored. The micro-hardness data show a bulk effect of niobium that counteracts the near-surface embrittlement caused by the penetrating environmental elements. The surface and bulk effects of niobium combine to mitigate the embrittlement of the material by nitrogen