Impurity diffusion was studied in the hexagonal close-packed α-phase. Samples were used which had various impurity contents. These included ultra-pure material with extremely low concentrations of interstitial impurities such as Fe, Co and Ni. In-depth profiling by secondary ion mass spectrometry was used for the Al diffusion measurements. Measurements were made both perpendicular to, and parallel to, the c-axis; using single crystals and coarse-grained polycrystals (table 267). The results for the ultra-pure α-phase, perpendicular to the c-axis, could be described by:

D (m2/s) = 6.6 x 10-3 exp[-329(kJ/mol)/RT]

The ratio of the parallel diffusivities to the perpendicular diffusivities was equal to about 0.65. These results were treated as being the intrinsic diffusion properties of α-Ti. They were consistent with the normal diffusion behavior in other hexagonal close-packed metals. It was concluded that substitutional solute diffusion in α-Ti was intrinsically normal and was dominated by the vacancy mechanism. Diffusion in less pure material was more rapid and required a lower activation energy. This was attributed to an enhancement of atomic mobility in the matrix, due to interstitially dissolved fast-diffusing impurities.

M.Köppers, C.Herzig, M.Friesel, Y.Mishin: Acta Materialia, 1997, 45[10], 4181-91