Self-diffusion and Al impurity diffusion were studied in the hexagonal close-packed a-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. Self-diffusion measurements were performed by using a 44Ti radiotracer and the ion-beam sputtering technique. 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 (tables 19 and 20). The results for the ultra-pure a-phase, perpendicular to the c-axis, could be described by:

Ti:     D (m2/s) = 1.35 x 10-3 exp[-303(kJ/mol)/RT]

Al:     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 0.5 for self-diffusion and equal to about 0.65 for Al diffusion. These results were treated as being the intrinsic diffusion properties of a-Ti. They were consistent with the normal diffusion behavior in other hexagonal close-packed metals. It was concluded that both self-diffusion and substitutional solute diffusion in a-Ti were intrinsically normal and were 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.

Intrinsic Self-Diffusion and Substitutional Al Diffusion in a-Ti. M.Köppers, C.Herzig, M.Friesel, Y.Mishin: Acta Materialia, 1997, 45[10], 4181-91 (inadvertently omitted from DDF165-166, and included here for completeness)

Table 19

Diffusivity of Ti in a-Ti Single Crystals

* wrt c-axis

Table 20

Diffusivity of Al in a-Ti Single Crystals

* wrt c-axis