  We studied tracer self-diffusion in ultrafine grained Ni prepared by high
pressure torsion. Two Ni materials of low (99.6 wt. %) and high (99.99 wt. %)
purity levels were investigated. While the ultrafine grained structure of less
pure Ni remained stable during diffusion annealing, recrystallization and
subsequent grain growth occurred in high purity Ni at the same annealing
conditions. Nevertheless, qualitatively similar ultrafast diffusion rates were
measured in the samples of both purity levels. In high purity Ni, the kinetics
of recrystallization was found to deviate strongly from the predictions of the
Johnson-Mehl-Avrami-Kolmogorov theory. Moreover, the ultrafast diffusion paths
withstood the recrystallization process. A model which accounts for solute
redistribution in front of the moving boundary is suggested. Retaining of
deformation-induced ultrafast diffusion paths in recrystallized Ni is explained
by a specific mechanism of enhanced stability of the residual ultrafine grained
fraction against recrystallization.
