The Surface Interaction of Oxygen with a Gamma TiAl
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THE SURFACE INTERACTION OF OXYGEN WITH A GAMMA TiAI
T. N. TAYLOR AND M. T. PAFFETT" Chemical and Laser Sciences Division, Los Alamos National Laboratory Los Alamos, NM 87545
ABSTRACT The composition and bonding of incipient and atmospheric oxides grown on a Ti - 47.0 Al - 2.2 Nb alloy were examined using Auger, x-ray, and low-energy ion scattering spectroscopies. The depth distribution of various components was determined with secondary-ion mass spectroscopy (SIMS). An oxide grown on the material in air at room temperature showed advanced Al and Ti oxide bonding states, which had very different stabilities during heating in vacuum up to 600*C. The Ti-oxygen bonding state was markedly reduced while the Al oxide was somewhat enhanced during this process. Further controlled studies at 600'C, involving oxygen adsorption at pressures below - 10-6 Torr, showed an initial Al cation surface enrichment until the detectable Al 2p peak was 90% oxidized. Further oxygen exposure gradually produced a fully oxidized Ti valence state with a pronounced enrichment of the Ti cation at the vacuum-solid interface. The results are discussed in terms of the kinetic and thermodynamic factors of the rate-limiting steps in the oxidation process. INTRODUCTION The thrust of the present research is to employ surface-science probes to measure the chemical properties of a y-TiAl during its interaction with oxygen. This approach is important for understanding the fundamental processes that are involved in bulk hydrogen [ 1] and oxygen [2] environmental embrittlement of these light-weight structural materials because it is the surface oxide coating that is first encountered by the gas-phase species. A number of atmospheric oxidation experiments have shown how the reactivity and oxide composition for a binary titanium aluminide depend on the Ti-Al bulk stoichiometry [2,3]. It has been shown that, as the Al content is increased, a point will eventually be reached where the outward diffusion of Al limits Ti oxide formation and produces a continuous alumina scale, which acts as both an adsorption and diffusion barrier to oxygen and hydrogen. Dopants, such as Nb, are typically added to titanium-aluminum alloys because they enhance this effect, while also inhibiting the net oxide growth [2,4]. A Nb oxide complex is believed to bond with the immiscible oxides of Ti and Al to block oxygen fast diffusion paths. Ti-oxide surface enrichment is one of the major concerns relating to the use of TiAl-based materials at elevated temperatures in a hydrogen environment. The hydrogen reduction of this oxide layer to produce chemically active Ti sites is one potential pathway for bulk hydride embrittlement. By examining the controlled adsorption of oxygen on a well characterized titanium aluminide surface, information is obtained on how the Ti and Al cations compete for the oxygen and how the surface region is reconfigured during the early stages of oxide formation. Understanding these chemical processes as a function of bulk and surface composition is quite import
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