Diffusion of the 63Ni radiotracer in a well-compacted nanocrystalline (80 to 100nm) γ-phase 40wt%Ni alloy was measured by using the serial sectioning technique at 610 to 1010K. The microstructure exhibited 2 different length-scales, with nano-sized grains that formed μm-sized clusters (or agglomerates). Three main diffusion paths therefore governed the diffusion behavior. These were the nanocrystalline grain interior, the nanocrystalline grain boundaries and the inter-agglomerate interfaces. The absolute values, and the Arrhenius parameters of Ni grain boundary diffusion,

D (m2/s) = 9.3 x 10-4 exp[-177(kJ/mol)/RT]

in the nano- γ-phase alloy were similar to those for Ni grain boundary diffusivity in coarse-grained polycrystalline γ-Fe. It was therefore concluded that the nanocrystalline grain boundaries had quasi-equilibrium structures. They also exhibited pronounced grain growth (from 30 to 100nm) during preparation. The inter-agglomerate interfaces, which represented the fastest diffusion path,

D (m2/s) = 1.9 x 10-3 exp[-134(kJ/mol)/RT]

were expected to be in a non-equilibrium state.

S.V.Divinski, F.Hisker, Y.S.Kang, J.S.Lee, C.Herzig: Interface Science, 2003, 11[1], 67-80