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 °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.