Self-diffusion and fast cobalt impurity diffusion in the bulk and in grain boundaries of hexagonal titanium

Bulk self-diffusion was measured in α-Ti single crystals between 776 and 1132 K using the radiotracer 44Ti and ion beam sputtering for profile sectioning. The results, which represent diffusion perpendicular to the c axis, follow a straight Arrhenius relation: Self-diffusion is interpreted by a vacancy mechanism. The low vacancy migration enthalpy may be related to the unusually low elastic constant c 66 in α-Ti. Grain-boundary self-diffusion was also measured in α-Ti; compared with corresponding results in other h.c.p. metals it was found to be in line with a vacancy mechanism. Grain-boundary diffusion of Co, which is a fast bulk diffuser, is six to seven orders of magnitude larger than grain-boundary self-diffusion. Even if a large Co grain-boundary segregation is accounted for, the evidence is that Co diffuses interstitially in α-Ti grain boundaries.