It is a well known fact that formation of a stable, protective Alpha-Al2O3 oxide takes place above a temperature of 900 °C. During oxidation of newly developed Gamma-TiAl alloys, I observed presence of Alpha-Al2O3 at a temperature of 800 °C. Can any discuss why presence of these phases can still be found at 800 °C. Unfortunately, I was able to find no literature till now discussing the presence of these phases at this temperature.
Hi Muhammad, is it possible that you have some kind of solid solution? Is it really only Al and O? Could you check this by TEM? How did you identify Al2O3?
Dear Dr. Nolze,
This is what I am getting from XRD. TEM measurements are really cumbersome for this study. I will check again through XRD if it is a solid of TixAlyOz or is it really Al2O3 phase. There is a possibility that Theta-Al2O3 can form at this temperature but unfortunately only Alpha-Al2O3 peaks match the peaks available in XRD pattern.
It is possible to observe the Alpha-Al2O3 in the oxidation scale. For example, in our previous work, we observed Alpha-Al2O3 in the oxidized sample at 800oC by XRD and was confirmed by Raman spectroscopy. Please see the attached file.
In my opinion, during initial (early) stage of oxidation, the oxide film formed as TiO2\Al nanocomposite like, and as a result of the solid-state reactions between Al and TiO2 at temperature range from 650 to 800°C, Al3Ti phase formed. In this stage there are two observations. The first shows that the alpha Al2O3 will form during heating Al3Ti phase starting from 800 to 1000 °C. And the second was stated by your results, which indicate that alpha Al2O3 present at 800°C. Again, in my opinion, this little disagreement between the above two observations attributed to the difference in the nanostructure size of the formed Al3Ti phase.
Best regards
Dear Muhammad Naveed.
Supporting your finding, the author(s) of published article on link below used different incident angle (0.2; 0.5; 2.0; 4.0) during XRD characterization of formed scale at oxidation temperature 700C and 800C. They observed presence of Alpha-Al2O3 at a temperature of 800 °C then presented more than one reason for this case. Please go through the discussion part to see that during the first stages of oxidation of titanium aluminides, lower titanium oxides, such as Ti2O,TiO or Ti2O3can be formed, which next transform into TiO2. One of the transient oxides, Ti2O3, is known to have a corundum structure (rhombohedral), and this could explain the favorable nucleation and growth of alpha Al2O3.
Best regards
https://www.researchgate.net/publication/251629977_Scale_composition_and_oxidation_mechanism_of_the_Ti-46Al-8Nb_alloy_in_air_at_700_and_800_C
Article Scale composition and oxidation mechanism of the Ti–46Al–8Nb...
Based on work conducted on FeCrAl alloys, formation of alpha-Al2O3 may not be determined only by temperature. Factors such as the alloy processing route, the formation of transient oxides having isostructural relationships with alpha-Al2O3, may promote alpha-Al2O3 nucleation at relatively lower temperatures.
The temperature is not the only factor you have to consider. Other factors should be taken into your account such as: 1) how much Al content in the alloy and is it high enough to form an external alumina scale. 2) what are the competitor elements to form oxide scale. In your alloy, Titania preferentially forms to alumina. 3) the oxidative atmosphere, Dry, humid or wet and its O2 partial pressure.
Back to your question, Yes, What you could detect is may be Alpha Alumina.
Alpha alumina is formed exclusively on TiAl3 at 700-1200 C. Smialek and Humphrey, Scripta met., 26, 1992, 1763-1768. Google TiAl3 oxidation for more confirmations.
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