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8/30/04

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Oxidation Behavior and Transmission Electron Microscope Characterization of Ti-44Al-x Nb-2(Ta,Zr) Alloys JOHN C. WOO, S.K. VARMA, and R.N. MAHAPATRA Quaternary additions of 2 at. pct of Ta or Zr were made to the ternary Ti-44Al-xNb (X
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.

I. INTRODUCTION

CONVENTIONAL Ni-based superalloys are currently facing challenges from newly emerging -TiAl alloys, which can provide comparable mechanical properties for both aerospace and automobile industrial applications. The low density and high creep[1,2,3] resistance offered by these alloys support further developments. However, the poor oxidation resistance and low room-temperature ductility are the properties that prevent them from their use at this time. An Nb addition has been successfully used to provide improvement in the two categories.[4–7] Close proximity of the Gibbs free energies for the formation of oxides of Al and Ti results in the formation of TiO2 and Al2O3 in -TiAl alloys during high-temperature oxidation. The F° values for Al2O3 and TiO2 are 446 and 445 kJ/mole, respectively.[8] Improvements in the oxidation resistance depend entirely on the successful formation of a continuous layer of Al2O3 on the surface. The formation of oxides of Ti and a few other elements results in poor oxidation behavior up to 750 °C.[9–14] It has been determined[15] by thermogravimetric analysis (TGA) that Nb provides the best oxidation resistance when added in the range of 8 to 11 at. pct to Ti-44Al-xNb alloys (x
0, 4, 8, and 11). It supports the experimental results of Leyens et al.[16] for orthorhombic Ti-25Al-11Nb alloys. Thus, an addition of Nb in a range other than 10 to 15 at. pct can result in accelerated oxidation at high temperatures. The Ellingham diagram shown in Figure 1 indicates the relative stability of the oxides of Ti, Al, Nb, Ta, and Zr. Thus, Ta and Zr were added to ternary Ti-44Al-xNb JOHN C. WOO, Student, and S.K. VARMA, Professor, are with the Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968–0520. Contact e-mail: skvarma@ utep.edu R.N. MAHAPATRA is with the Naval Air Warfare Center, Aircraft Division, Patuxent River, MD 20650. This article is based on a presentation made in the s