Oxidation of Orthorhombic Titanium Aluminide TI-22AL-25NB in Air between 650 and 1000 deg C

摘要

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25Nb was studied in air between 650 and 1000 deg C by isothermal thermogravimetry and postoxidation scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti_3Al-based titanium aluminides at and below 750 deg C. Between 800 and 1000 deg C, the oxidation resistance of 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 hours at 900 and 1000 deg C. The mixed oxide scale consisted of TiO_2, AlNbO_4, and Al_2O_3, with TiO_2 being the dominant oxide phase. Underneath the oxide scale, a nitride-containing layer formed in the temperature range investigated, and at 1000 deg C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 #mu#m below the oxide scale when exposed to air at 900 deg C for 500 h. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.