Short-circuit diffusion in an ultrafine-grained copper-zirconium alloy produced by equal channel angular pressing

Many unusual properties of ultrafine grain materials obtained by equal channel angular pressing (ECAP) are commonly attributed to non-equilibrium grain boundaries. Such grain boundaries are expected to exhibit higher values of energy, higher amplitude of strain fields, a larger free volume and a higher diffusivity than their relaxed counterparts. In the present study, the diffusivity of Ni-63 radiotracer in ECAP-processed Cu-0.17 wt.% Zr alloy was measured in the low-temperature range of 150-350 degrees C under conditions at which no bulk diffusion occurs. The microstructure observations after annealing indicate that alloying with Zr is essential for stabilizing the ECAP-processed alloys against grain growth and recrystallization. In all samples studied the experimentally measured diffusion profiles exhibited two distinct slopes, which are associated with "slow" and "fast" short-circuit diffusion paths. The diffusivity of "slow" diffusion paths in the ECAP-processed samples coincides with the diffusivity via relaxed grain boundaries in the coarse grain Cu measured by the same radiotracer method at similar temperatures. We associate the "fast" diffusion paths observed in this study with the non-equilibrium grain boundaries produced by ECAP. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.