The diffusion of 57Co along grain boundaries in high-purity material was studied at temperatures ranging from 823 to 1471K (table 133), by using the serial sectioning technique. As it was not possible to stabilize the grain size by using any pre-diffusion annealing process, some of the grain boundaries could migrate during diffusion annealing. The product of the grain-boundary segregation factor, the grain boundary diffusion coefficient and the grain boundary width for stationary boundaries, and the velocities of moving boundaries, were deduced from the diffusion profiles. The values of the product obeyed an Arrhenius dependence:

P (m3/s) = 4.84 x 10-13exp[-149.5(kJ/mol)/RT]

These values were 2 to 4 orders of magnitude higher, in the present temperature range, than those which were expected for grain boundary self-diffusion in Nb. This observation was consistent with the fast lattice diffusion of Co in Nb and provided evidence for a combined vacancy-interstitial grain boundary diffusion mechanism. From the temperature dependence of the boundary velocity, an activation enthalpy of 182kJ/mol was estimated for grain boundary motion.

M.Köppers, Y.Mishin, C.Herzig: Acta Metallurgica et Materialia, 1994, 42[8], 2859-68