Effects of high temperature air and vacuum exposures on the room temperature tensile behavior of the (O + B2) titanium aluminide Ti-22Al-23Nb

A new class of titanium alloys, based on the orthorhombic Ti2AlNb phase, have received considerable attention recently as a potential matrix candidate for foil-fiber-foil processed intermetallic matrix composites. In the present study, the microstructure and room temperature deformation of one such an orthorhombic titanium aluminide alloy, Ti-22Al-23Nb (atomic per cent), were investigated. The O + B2 microstructure of the as-processed sheets was found to be quite inhomogeneous with respect to the O phase precipitation. The thermal and environmental responses of the sheet microstructure were investigated at 760 °C for 100 h in air and vacuum. The microstructure was stable but emerged as being very sensitive to oxygen exposure even in a vacuum environment. The effects of high temperature exposure on room temperature deformation were studied by heat treating tensile samples prior to testing at 760 °C for 100 h in air and vacuum. The room temperature ductility and strength were very sensitive to the variations in size and arrangement of the O phase, resulting from processing, and to the presence of surface oxides and oxygen-enriched surface layers, resulting from pre-exposure treatment and specimen surface preparation. This study has clearly shown that improved resistance to oxygen exposure represents a challenge in the development of this type of alloy for use at temperatures near 760 °C.