Intrinsic self-diffusion and substitutional Al diffusion in alpha-Ti

Self-diffusion and Al impurity diffusion were studied in the alpha (h.c.p.) phase of Ti. We used four Ti materials with different impurity contents, including ultrapure Ti with extremely small concentrations of interstitial impurities (Fe, Co and Ni). The self-diffusion measurements are performed with the radiotracer Ti-44 and the ion beam sputtering technique. For Al diffusion measurements in-depth profiling by secondary ion mass spectrometry is applied. The measurements are made both perpendicular (perpendicular to) and parallel (parallel to) to the c axis using single crystals and coarse-grained polycrystals. The measurements on ultrapure alpha-Ti yield the Arrhenius parameters D-0 perpendicular to = 1.35 x 10(-3)m(2)/s and Q(perpendicular to) = 303 +/- 2 kJ/mol for self-diffusion and D-0 perpendicular to = 6.6 x 10(-3) m(2)/s and Q(perpendicular to) = 329 +/- 2 kJ/mol for Al diffusion. The anisotropy factor D-parallel to/D-perpendicular to approximate to 0.5 for self-diffusion and approximate to 0.65 for Al diffusion. These results are treated as intrinsic diffusion properties of alpha-Ti. It is demonstrated that they are well consistent with the normal diffusion behaviour in other h.c.p. metals. We conclude that both self-diffusion and substitutional solute diffusion in alpha-Ti are intrinsically normal and dominated by the vacancy mechanism. Diffusion in less pure alpha-Ti occurs faster and with a smaller activation energy. This effect is explained by the enhancement of atomic mobility in the matrix material owing to the interstitially dissolved fast-diffusing impurities. (C) 1997 Acta Metallurgica Inc.