It was recalled that attention had been focused on diffusion in -Ti because of the unusual features of self-diffusion and impurity diffusion of certain elements. These included non-linear Arrhenius plots and very low values of the apparent frequency factor and the apparent activation energy. Measurements of the impurity diffusion coefficient for Al in -Ti had not been carried out because the radioactive tracer method was unavailable; due mainly to the low specific activity of 26Al, and to the difficulty of coating this isotope onto a Ti surface. Here, measurements were made of interdiffusion coefficients in the  phase of the Ti-Al system at temperatures ranging from 1223 to 1573K (table 268), using Ti/Ti-2.1at%Al couples. After annealing, the average grain sizes of the Ti and Ti-2.1at%Al samples were about 2mm, and their O concentrations were less than 600ppm[wt]. The value of the pre-exponential factor (1.2 x 10-7m2/s) was lower by a factor of 100 than the value for self-diffusion which had been suggested on the basis of a vacancy mechanism, and indicated a negative activation entropy. On the other hand, the activation energy (150kJ/mol) for the impurity diffusion of Al was equal to about 50% of that for self-diffusion which was given by an empirical rule. Furthermore, the activation energy was considerably lower than the value for self-diffusion which was predicted by an empirical relationship between the activation energies for diffusion in cubic metals and their latent heats of melting. The very low values of the pre-exponential factor and the activation energy indicated that the impurity diffusion of Al in -Ti could be regarded as being anomalous.

H.Araki, T.Yamane, Y.Minamino, S.Saji, Y.Hana, S.B.Jung: Metallurgical Transactions A, 1994, 25[4], 874-6

Table 268

Diffusivity of Al in -Ti