Internal friction measurements were performed by using a torsion pendulum which was modified for use in a vacuum. Elastic after-effect experiments were performed by using the same equipment. The wires (0.76mm diameter) were surface-cleaned before testing. It was assumed that diffusion occurred via the jumping of interstitials between adjacent octahedral sites. On this basis, H diffusivities were deduced from the internal friction results. Diffusion coefficients were also calculated by using a tetrahedral model. The 2 sets of data were then fitted to the Arrhenius equation. It was found that the octahedral model gave a better fit than the tetrahedral model; although this did not exclude the possibility of a double site-occupancy mechanism, or of more complicated diffusion paths. The diffusivity (octahedral model) was described by:

D (m2/s) = 5.21 x 10-7 exp[-158.48(kJ/mol)/RT]

H.Kokawa, T.Watanabe, A.Uehara, S.Karashima: Scripta Metallurgica, 1978, 12[4], 357-9

Table 258

Parameters for H and D Diffusion in Ta at 0 to 100C