Auger electron analysis of the initial oxidation of titanium aluminides based on Ti-48Al
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I.
INTRODUCTION
TITANIUMaluminides based on the composition Ti48A1 (at. pct) have been subjected to increased scrutiny in recent years owing to their potential application in aircraft engines and structures and in gas turbines. One of the important aspects of this evaluation is the environmental impact of O and C absorption into the bulk of the material. It is of fundamental importance to study the mechanism of adsorption of these elements into the surface layers, which in turn can provide us more information on the reactivity of the constituent elements and/or phases in the alloys. It is well established that a certain critical amount of A1 content has to be exceeded before we have the formation of a continuous A1203 scale on the surface in the 700 ~ to 900 ~ temperature range, lL21 The increased amount of AI in the TiA1 alloys (>52 at. pct) promotes the outward diffusion of A1 at elevated temperature which in turn favors the formation of A1203.t~'2] Other factors that affect the formation of A1203 scale include the type of oxidant (air or pure O), oxidation temperature, alloying elements, and in some cases the preparation of the sample prior to oxidation331 The compact A1203 scale is a good diffusion barrier to C, O, and H. As the temperature is lowered below 600 ~ diffusion of the constituents of the alloy as well as C and O become sluggish, decreasing the thickness of the oxide layers to a few monolayers. This subject has been largely ignored as a result of the difficulty in measurement of weight gain in a typical thermogravimetric setup. The use of surface science techniques like the work function Y.T. PENG, Graduate Student, formerly with the Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019, is with the Engineering Materials Program, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742. P.B. ASWATH, Assistant Professor, is with the Materials Science and Engineering Program, and A.R. KOYMEN, Assistant Professor, is with the Department of Physics, University of Texas at Arlington, Arlington, TX 76019. Manuscript submitted July 12, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
method, electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), e t c . are useful tools to study the affinity and kinetics of formation of monolayers of oxide and adsorbed layers of O and C at room temperature. The surface of an annealed Ti-48 (at. pct) AI alloy was examined by AES at room temperature. Both a contaminated surface and the surface during and after sputtering were studied using depth profiling and scanning Auger mapping (SAM). The Ti-48 A1 alloy was compared to the behavior of pure Ti and AI foils.
II.
EXPERIMENTAL PROCEDURE
The specimen under investigation has a bulk composition of Ti-48 A1 (at. pet) and was supplied by McDonnell Douglas Research Labs (MDRL), St. Louis, MO. The as-received material was produced from a powder metallurgy route using 35 mesh powder produ
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