Size and stress dependences in the tensile stress relaxation of thin copper wires at room temperature

The tensile stress relaxation behavior of thin copper wires with diameters of 20, 35 and 50 pm is experimentally investigated at room temperature. It is revealed that the stress relaxation can occur at low starting stress below the yield strength at room temperature. Significant size and stress dependences in the stress relaxation are observed. That is at a given stress, a smaller diameter sample exhibits a much faster stress relaxation. At a given diameter, the stress relaxation is accelerated with increasing the starting stress. The value of activation volume increases with increasing the wire diameter, while it increases with decreasing the starting stress. The strain rate elevates as the wire diameter decreases or the starting stress increases. A bilinear relation between the logarithmic strain rate and the logarithmic stress during the relaxation is observed only for the wire sample with a diameter of 20 pm. The intersection of dislocations with grain boundaries is considered to be the main deformation mechanism during the stress relaxation. A rapider exhaustion of mobile dislocations is observed in the wire with smaller diameter in the primary of relaxation. It is attributed to annihilation of dislocations easily escaping from the surface due to larger fraction volume of the near-surface zone. After the primary regime, the higher possibility of dislocations being pinned or immobilized by grain boundaries in a larger-diameter sample is accounted for the size dependence.