Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C

The environmental behavior of an orthorhombic titanium-aluminide alloy, Ti-22Al-25Nb, was studied in dry and humid air between 650 and 1,000 C by scanning electron microscopy, energy dispersive spectroscopy, 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 Ti3Al-based titanium aluminides at and below 750 C. Between 800 and 1,000 C, the oxidation resistance of Ti-22Al-25Nb was as good as γ-TiAl base aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1,000 C. The mixed oxide scale consisted of TiO2, AlNbO4, and Al2O3 with TiO2 being the dominant oxide phase. Underneath the oxide scale, a nitride layer formed in the temperature range investigated and, at 1,000 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 microm below the oxide scale when exposed to air at 900 C for 500 hr. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions