Oxidation of orthorhombic titanium aluminide Tl-22AL-25NB in air between 650 and 1000 degrees C

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25N b was studied in air between 650 and 1000 degreesC by isothermal thermograv imetry and postoxidation scanning electron microscopy (SEM), energy dispers ive spectroscopy (EDS), and x-ray diffraction, Microhardness measurements w ere performed after exposure to gage hardening due to nitrogen and oxygen i ngress, The parabolic rate constant of Ti22Al-25Nb was of the same order as conventional titanium alloys and Ti,Al-based titanium aluminides at and be low 750 degreesC. Between 800 and 1000 degreesC, the oxidation resistance o f Ti-22Al-25Nb was as good as that of gamma -TiAl based aluminides; however the growth rate changed from parabolic to linear after several tens of hou rs at 900 and 1000 degreesC. The mixed oxide scale consisted of TiO2, AlNbO 4, and Al2O3, with TiO2 being the dominant oxide phase, Underneath the oxid e scale, a nitride-containing layer formed in the temperature range investi gated, and at 1000 degreesC, internal oxidation was observed below this lay er. In all cases, oxygen diffused deeply into the subsurface zone and cause d severe embrittlement, Microhardness measurements revealed that Ti-22Al-25 Nb was hardened in a zone as far as 300 mum below the oxide scale when expo sed to air at 900 degreesC for 500 h, The peak hardness depended on exposur e time and reached five times the average hardness of the bulk material und er the above conditions.