Coupled effect of specimen size and grain size on the stress relaxation of micron-sized copper wires

The coupled effect of specimen size and grain size on the stress relaxation of micron-sized copper wires is investigated experimentally in this study. Both tension and repeated stress relaxation tests are performed on wires with the variation of diameters at annealing temperatures in the range of 300–800 °C. It is found that the yield strength is affected by the grain size and the ratio of the diameter (D) to the grain size (d) within the ratio below a critical value of 3. The classical Hall–Petch relation holds with the ratio of D/d larger than 3. Otherwise, a deviation from the relation arises. It is observed that the relaxation process is elevated as the specimen size and the grain size decrease. With the decrease of specimen size and grain size, the activation volume decreases. The coupled effect of specimen size and grain size in the activation volume is explained theoretically. The specimen size effect during the relaxation is related to the exhaustion mechanism that mobile dislocations annihilate at the free surface. More rapid exhaustion of mobile dislocations from the free surface arises in a smaller-diameter wire, thus accelerating the relaxation process. It is proposed that the deformation is dominated by the interaction of dislocations with grain boundaries within the grain size of less than about 6 μm. Within the ratio of D/d less than 3, the mechanism becomes the interaction of dislocations with forest dislocations.