On the determination of diffusion coefficients of oxygen in one-phase Ti (alpha-Ti) and two-phase Ti-4Nb (alpha- and beta-Ti) by micro-hardness measurements

In this study, attention is paid to oxygen dissolution in the metallic matr ix, ie., pure titanium (alpha -Ti) and niobium-alloyed titanium (alpha- and beta -Ti) during exposure at 800 degreesC in dry anti humidified argon/oxy gen and nitrogen/oxygen atmospheres. rn particular, the diffusion coefficie nts and the total amount of oxygen in both materials have been determined b y profiling micro-hardness in the matrix after oxidation. Experimental data showed that even after oxidation in nitrogen-containing atmospheres, the a mount of dissolved nitrogen in pure and niobium-alloyed titanium was signif icantly lower than the amount of oxygen. This justifies that only oxygen sh ould be considered as the dissolved species, rt is shown that micro-hardnes s data are very useful for diffusion studies. Microhardness values, obtaine d from a one-phase material, i.e., pure titanium (alpha -phase), can be "cu rve-fit" by a simple error function. However, concerning a two-phase materi al, i.e., Ti-4Nb (alpha- and beta -titanium) a modification of the simple e rror-function approach is needed, resulting in an adapted function consisti ng of two different error functions combined with a prefactor, indicating t he relative contribution of each phase. From the available data, it was det ermined that the diffusion coefficient of oxygen in a-Ti is about two order s of magnitude higher than in beta -Ti..