Ultrafast atomic transport in recrystallizing ultrafine grained Ni

TL;DR

This study reveals ultrafast atomic diffusion in ultrafine grained Ni, demonstrating that such rapid diffusion paths persist through recrystallization, challenging existing theories and suggesting a new model involving solute redistribution.

Contribution

It introduces a model accounting for solute redistribution during recrystallization, explaining the stability of ultrafast diffusion paths in ultrafine grained Ni.

Findings

Ultrafast diffusion rates are similar in Ni of different purities.

Recrystallization kinetics deviate from classical theory in high purity Ni.

Ultrafast diffusion paths survive recrystallization, indicating enhanced stability.

Abstract

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.