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Session 3264

Oxygen Diffusion into Titanium

Aaron P. Broumas, Nick M. Degnan, Michael L. Meier

Department of Chemical Engineering and Materials Science University of California, Davis

Abstract The oxidation of titanium alloys involves both the growth of a stable oxide and the diffusion of oxygen into titanium. Oxygen, one of the most effective solid solution strengtheners in titanium, will produce a hardness profile similar to the concentration profiles produced by diffusion into the metal. Microhardness testing can be used to measure these profiles and work out the diffusivity of oxygen in titanium. A laboratory experiment has been developed in which commercially pure titanium is annealed in air, the microhardness profiles are obtained, and the results are analyzed to obtain the activation energy for diffusion. The value obtained was close to that for tracer self- diffusion in α-Ti. This inexpensive experiment, based on scientific literature from the 1950s, gives the students hands-on experience with the experimental methods and interpretation of the data used to study this very basic materials behavior.

Introduction Titanium is well known for its excellent resistance to corrosion primarily due to a passive oxide layer at the surface. When exposed to air at elevated temperatures, 300oC or more, this oxide layer will grow and simultaneously oxygen will diffuse into the metal. The oxygen atoms that do diffuse into the metal can occupy both interstitial and substitutional sites, making the material harder, and since the concentration of oxygen at the surface would be greater than concentration deeper into the metal it should be harder near the surface and softer in the interior. The correlation between the concentration profiles and hardness profiles suggests that microhardness testing can be an effective tool in studying the oxygen diffusivity in titanium.

The governing differential equation for diffusion, known as Fick’s second law, is:

∂C ∂ 2C =D (1) ∂t ∂x 2

where D is the diffusivity and C is the concentration of the diffusing species1. The solution to this differential equation, where surface concentrations are held fixed and the diffusion medium is infinitely long, generates the concentration profile described by the equation

“Proceedings of the 2003 American Society for Engineering Education Annual Conference and Exposition 2003, American Society for Engineering Education”

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Degnan, N., & Meier, M., & Broumas, A. (2003, June), Diffusion Of Oxygen In Titanium Paper presented at 2003 Annual Conference, Nashville, Tennessee. 10.18260/1-2--11928