Oxidation characteristics of Ti{sub 3}Al-Nb alloys and improvement in the oxidation resistance by pack aluminizing
- Indian Institute of Technology, Bombay (India)
The oxidation behavior of three Ti{sub 3}-Al-Nb alloys: Ti-25Al-11Nb, Ti-24Al-20Nb, and Ti-22Al-20Nb was investigated in the temperature range of 700 - 900{degrees}C in air. The uncoated alloy Ti-25Al-11Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessive oxide scale spalling. The scale analysis, using XRD, SEM/EDAX, and AES revealed that the scale was a mixture of TiO{sub 2}, Al{sub 2}O{sub 3}, and Nb{sub 2}O{sub 5} with the outer layer rich in TiO{sub 2}. The effect of variation in Al and Nb content 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 these alloys, especially Ti-24Al-20Nb and Ti-22Al-20Nb, 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 aluminizing which led to the formation of an Al rich TiAl{sub 3} surface layer doped with Nb. The coating process was gaseous-diffusion controlled with a parabolic Al deposition rate. The weight gains for the aluminized alloy specimens oxidized at 900{degrees}C in air were much lower than that of the uncoated specimens. The weight gains were further decreased in the case of Si-modified aluminized specimens. The scale analysis revealed an alumina-rich scale with some amount of titania doped with Nb. The improvement in the oxidation resistance of the pack-aluminized alloys at 900{degrees}C is attributable to the formation of the alumina-rich oxide scale. The addition of Si to the aluminizing pack seems to promote further the growth of an alumina-rich scale by lowering the oxygen partial pressure in the system.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 539368
- Journal Information:
- Oxidation of Metals, Vol. 47, Issue 5-6; Other Information: PBD: Jun 1997
- Country of Publication:
- United States
- Language:
- English
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