OXIDATION CHARACTERISTICS OF TI3AL-NB ALLOYS AND IMPROVEMENT IN THE OXIDATION RESISTANCE BY PACK ALUMINIZING

The oxidation behavior of three Ti-3-Al-Nb alloys: Ti-25Al-11 Nb, Ti-2 4Al-20 Nb, and Ti-22Al-20 Nb was investigated in the temperature range of 700-900 degrees C in air. The uncoated alloy Ti-25Al-11 Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessiv e oxide scale spalling. The scale analysis, using XRD, SEM/EDAX, and A ES revealed that the scale was a mixture of TiO2, Al2O3, and Nb2O5 wit h the outer layer rich in TiO2. The effect of variation in Al and Nb c ontent on the oxidation behavior is discussed A decrease in Al content of the alloy adversely affects the oxidation resistance, and it seems that a Nb content as high as 20 at.% is also not beneficial. Hence th ese alloys, especially Ti-24Al-20 Nb and Ti-22Al-20 Nb, should not be used in the as-received condition above 750 degrees C. An attempt was made to improve the oxidation resistance of these alloys by pack alumi nizing which led to the formation of an Al rich TiAl3 surface layer do ped with Nb. The coating process was gaseous-diffusion controlled with a parabolic Al deposition rate. The weight gains for the aluminized a lloy specimens oxidized at 900 degrees C in air were much lower than t hat of the uncoated specimens. The weight gains were further decreased in the case of Si-modified aluminized specimens. The scale analysis r evealed an alumina-rich scale with some amount of titania doped with N b. The improvement in the oxidation resistance of the pack-aluminized alloys at 900 degrees C is attributable to the formation of the alumin a-rich oxide scale. The addition of Si to the aluminizing pack seems t o promote fur ther the growth of an alumina-rich scale by lowering the oxygen partial pressure in the system.