The diffusion coefficients were measured in the a-phase of Fe-free binary alloys at temperatures ranging from 830 to 1100K (tables 181 and 182). The diffusivity values for the 2 alloys were essentially indistinguishable and were little different to published diffusion coefficients for Hf in Fe-free polycrystalline samples of a-Zr. 'The temperature dependence of the diffusivity in the alloys was characteristic of that in the bulk of intrinsic a-Zr:

D(m2/s) = 5 x 10-7 exp[-2.7(eV)/kT]

The present Hf diffusivity values were much lower than the corresponding values which were measured in commercial Zr-2.5Nb alloy. This difference was attributed to the effect of Fe in solid solution. Also, the absence of a strong Nb-induced enhancement of Hf diffusion in a-Zr suggested that intrinsic Nb diffusion in a-Zr might not be controlled by a normal vacancy mechanism.

H.Zou, G.M.Hood, J.A.Roy, R.J.Schultz, J.A.Jackman: Philosophical Magazine A, 1995, 71[4], 901-6

Table 181

Diffusion of Hf in Zr-1at%Nb

Table 182

Diffusion of Hf in Zr-2.5at%Nb