Grain boundary diffusion and recrystallization in ultrafine grain copper produced by equal channel angular pressing

The diffusion of ⁶³Ni radiotracer in ultrafine grain (UFG) Cu produced by equal channel angular pressing (ECAP) was studied using the serial-sectioning method. The diffusion annealings were performed in the temperature range of 424-553 K for annealing times at which volume diffusion is negligible and only short-circuit diffusion occurs. Complete or partial recrystallization occurred during all heat treatments, and the explicit expression describing the kinetics of recrystallization was obtained from observations of the microstructure after annealing treatments. The measured radiotracer penetration profiles exhibited two distinct slopes, indicating the co-existence of "slow" and "fast" short-circuit diffusion paths in the system. Based on the results of previous studies, the former were associated with the general high-angle grain boundaries in the non-recrystallized, UFG matrix. A model that considers diffusion in UFG polycrystal undergoing recrystallization was developed. Application of this model enabled us deriving the diffusion coefficients along the grain boundaries in UFG matrix from the experimentally measured radiotracer penetration profiles.