Mechanically-induced grain coarsening in gradient nano-grained copper

Gradient nano-grained Cu subjected to tensile tests yields at a stress almost twice of that of the conventional coarse-grained Cu at or below the room temperature. Beyond the yield stress, a uniform plastic strain of larger than 30% can be achieved, accompanied by homogeneous grain coarsening in nano-grained surface layer. The observed grain coarsening may induce certain degree of "strain softening". The measured grain coarsening rates strongly depend on temperature, stress and strain rate, suggesting that the grain coarsening is presumably limited by thermally activated dislocation activities at defective grain boundaries, rather than via diffusional process on atom-by-atom basis. A non-diffusional (source limited) Mott-Turnbull rate equation has been proposed to interpret the observed grain coarsening phenomenon. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.