Theoretical and experimental data on diffusion along moving and stationary boundaries in metals were reviewed. A model was presented for diffusion in polycrystals which contained both stationary and moving boundaries. The model predicted the occurrence of 3-step concentration depth profiles. The first step was caused by direct volume diffusion, the second was caused by diffusion along moving boundaries, and the third (with the deepest penetration) was caused by diffusion along stationary boundaries. By fitting the model to experimental profiles, the product, sD, for stationary boundaries, and the velocity and fraction of moving boundaries could be determined. The model was applied to experimental studies of Co impurity diffusion in Nb, which had been carried out by using radiotracer sectioning techniques. The value of the product for Co impurity diffusion in Nb was described by:

sD(m3/s) = 4.8 x 10-13exp[-149.5(kJ/mol)/RT]

The product of velocity and temperature could be described by:

VT(mK/s) = 580 exp[-181.9(kJ/mol)/RT]

Due to a high grain boundary mobility, some of the boundaries moved during the experiments.

Y.M.Mishin, M.Köppers, C.Herzig: Solid State Phenomena, 1995, 41, 79-92

Table 133

Grain Boundary Diffusivity of Co in Nb