Interdiffusion in the TiO 2 oxidation product of Ti 3 Al
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INTRODUCTION
THE global interest in the Ti3Al aluminide for hightemperature applications has continued to inspire more fundamental research in the material. Ti3Al is of particular interest because of its high-temperature specific strength, specific modulus, and good environmental properties at high temperatures. The high-temperature performance of Ti3Al derives mainly from its protective oxide formation. Interstitial elements such as oxygen, nitrogen, carbon, and hydrogen are known to substantially influence the performance of titanium alloys used in high-temperature environments. Oxygen is much more soluble in ␣2-Ti3Al than in ␥-TiAl. Schafrik[1] reports that the solubility of oxygen in ␣2-Ti3Al lies between 1500 to 2300 ppm at elevated temperatures. Oxygen is known to stabilize the ␣2 phase. Many applications of metal oxides are dependent on the atomic and electronic properties of the particular oxide. Consequently, considerable information is available in the literature on the transport properties of many oxides. Equally considerable is the disparity of the data on each material. This disparity could arise from the purity of the material, the extent of control on the procedure, or indeed the crystallinity of the specimen used. Numerous investigators[2–6] have studied the diffusion of oxygen in rutile, using single crystals or polycrystals. In this work, oxidation studies were carried out and parabolic rate constants obtained were used to calculate the diffusivity data. Kofstad[7] has reported a similar technique of obtaining diffusivity data using the parabolic rate constant. The inspiration for this work derives from our previous works[8–12] to understand the kinetics of scale formation, adhesion, and spallation in the titanium-based aluminides through the study of oxygen diffusion in the oxide scales that form in service.
I.C.I. OKAFOR, Research Fellow, X. WEN, Graduate Student, and R.G. REDDY, ACIPCO Professor, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. Manuscript submitted November 5, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
II. MATERIALS AND EXPERIMENTAL PROCEDURES A. Materials and Sample Preparation Titanium wires of 99.9 pct purity and aluminum rods with purity of 99.999 pct were used in this study. Samples of Ti3Al alloy were prepared by repeated arc melting of the titanium wires and the aluminum rods. Melting was done in a copper hearth under a flow of purified argon using tungsten electrodes. The resulting ingots were analyzed to have a composition of 67.66 at. pct Ti, 31.97 at. pct Al, and 0.37 at. pct O. The ingots were homogenized at 1273 K for 18 hours in argon atmosphere, polished using No. 600 emery papers, and then washed with acetone.
B. Oxidation Oxidation experiments were conducted using a Thermogravimetric Analysis (TGA) setup, which has been previously described in detail.[13] Platelike Ti3Al samples of thickness between 2 to 3 mm were used in the oxidation study. Each sample was suspended by means of
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