Effect of recrystallization on diffusion in ultrafine-grained Ni

We studied the effect of recrystallization and grain growth on grain boundary 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 compared and the kinetic properties of stationary and mobile grain boundaries were examined in detail. Unusual concentration profiles with characteristic 'kinks' were measured for Ni self-diffusion in 99.99 wt. % pure Ni, which is undergoing recrystallization and grain growth during diffusion annealing treatment. This behavior is found to be related to specific kinetic properties of grain boundaries which encompass the recrystallized areas and consume numerous defects of the ultrafine grained matrix during their motion. We proposed a model of self-diffusion in recrystallizing material which takes into account a hierarchy of stationary and moving fast diffusion paths, and satisfactory explains the observed diffusion behavior.