EFFECTS OF HIGH-TEMPERATURE AIR AND VACUUM EXPOSURES ON THE ROOM-TEMPERATURE TENSILE BEHAVIOR OF THE (O-22AL-23NB(B2) TITANIUM ALUMINIDE TI)

A new class of titanium alloys, based on the orthorhombic Ti2AlNb phas e, have received considerable attention recently as a potential matrix candidate for foil-fiber-foil processed intermetallic matrix composit es. Ln the present study, the microstructure and room temperature defo rmation of one such an orthorhombic titanium aluminide alloy, Ti-22Al- 23Nb (atomic per cent), were investigated. The O + B2 microstructure o f the as-processed sheets was found to be quite inhomogeneous with res pect to the O phase precipitation. The thermal and environmental respo nses of the sheet microstructure were investigated at 760 degrees C fo r 100 h in air and vacuum. The microstructure was stable but emerged a s being very sensitive to oxygen exposure even in a vacuum environment . The effects of high temperature exposure on room temperature deforma tion were studied by heat treating tensile samples prior to testing at 760 degrees C for 100 h in air and vacuum. The room temperature ducti lity and strength were very sensitive to the variations in size and ar rangement of the O phase, resulting from processing, and to the presen ce of surface oxides and oxygen-enriched surface layers, resulting fro m pre-exposure treatment and specimen surface preparation. This study has clearly shown that improved resistance to oxygen exposure represen ts a challenge in the development of this type of alloy for use at tem peratures near 760 degrees C.