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 c ontents, including ultrapure Ti with extremely small concentrations of interstitial impurities (Fe, Co and Ni). The self-diffusion measureme nts are performed with the radiotracer Ti-44 and the ion beam sputteri ng technique. For Al diffusion measurements in-depth profiling by seco ndary 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 measur ements on ultrapure alpha-Ti yield the Arrhenius parameters D-0 perpen dicular to = 1.35 x 10(-3)m(2)/s and Q(perpendicular to) = 303 +/- 2 k J/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 ani sotropy factor D-parallel to/D-perpendicular to approximate to 0.5 for self-diffusion and approximate to 0.65 for Al diffusion. These result s are treated as intrinsic diffusion properties of alpha-Ti. It is dem onstrated that they are well consistent with the normal diffusion beha viour 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-T i occurs faster and with a smaller activation energy. This effect is e xplained by the enhancement of atomic mobility in the matrix material owing to the interstitially dissolved fast-diffusing impurities. (C) 1 997 Acta Metallurgica Inc.