Oxidation behavior and transmission electron microscope characterization of Ti-44Al-x Nb-2(Ta,Zr) alloys

Abstract

Quaternary additions of 2 at. pct of Ta or Zr were made to the ternary Ti-44Al-xNb (X ϭ 9 and 11) alloys to study the oxidation behavior at 900°C, 950°C, and 1000°C for a period of 1 week. The Ta addition improves the oxidation resistance, while it is degraded by Zr compared to the ternary alloys. Identification of the oxides formed in the scale has been characterized by energydispersive atomic X-ray (EDAX) in a scanning electron microscope (SEM). The transmission electron microscope (TEM) analysis of the microstructures developed during oxidation has been compared with Ti-44Al-xNb alloys in order to determine the influence of quaternary additions of Ta and Zr on the phase transformations taking place during the extended period of heating. The formation of spotty ␣ 2 in the isolated ␥ grains appears to be associated with the inferior oxidation resistance of xNb2Zr alloys.

Transformation and Precipitation Processes in a Metal Substrate of Oxidized TiAl-Based Alloys

“…TinO 2n−1 oxides are characterized by equilibrium dissociation pressure close to that of Al 2 O 3 . The rate of growth of TiO and Ti 2 O 3 is definitely higher than that of Al 2 O 3 formation [25], and therefore, TiO [24] further course of the oxidation process. As already mentioned, rutile has a defective anion sublattice and inward oxygen diffusion takes place.…”

“…Through the defective anion sublattice, inward oxygen and nitrogen diffusion takes places, while the outward diffusion of metal ions occurs through the defective cationic sublattice. The energy of Al 2 O 3 formation is more negative than the energy of TiO 2 formation; [24] therefore, in the alloy in which the concentration of Ti and Al is the same, the Al 2 O 3 layer "should" be formed (Fig. 10).…”

Thermal Stability and Oxidation Resistance of Pt–10 Al–4Cr Alloy at Super-High Temperatures

“…The calculated Q from the slope of the line in this figure has been found to be 14 kcal/mol for the Pt-10Al-Cr alloy as shown in Table IV. The relatively high Q values for the Pt alloy 6,7 of this study can be considered as an indication for higher oxidation resistance based on the assumption that Q represents the activation energy for oxidation 6,7 and similar oxidation mechanisms are operative in the temperature range of this study. The high activation energy for oxidation of the alloy may be attributed to the formation of Al 2 O 3 in the scale.…”

Effects of High Niobium Addition on the Microstructure and High-Temperature Properties of Ti-40Al-<I>x</I>Nb Alloy

“…In Ti-40Al-10Nb alloy, the presence of Widmanstätten laths as the major precipitates causes the formation of TiO 2 during oxidation. The literature 18) demonstrates that the solubility of oxygen in the phase is around 20 mol%, whereas it is only 500-1000 ppm in the phase, suggesting that oxygen tends to be absorbed preferentially by the phase. Because TiO 2 grows mainly by oxygen diffusion, the circumstance that oxygen tends to be absorbed preferentially by the phase promotes the formation of TiO 2 during oxidation.…”