The effect of low stresses on creep and surface profiles of thin copper wires

The creep and surface profile changes of wires, of diameters from 25–500 μm of high purity (>99.99%) copper, have been investigated close to their melting temperature under stresses up to 400 kPa. Strain rate always varied linearly with stress. For the thinnest wires the rate was about twice the rate expected from Nabarro–Herring diffusional creep theory and between one and two orders of magnitude larger than expected from Harper–Dorn creep. For 500 μm wire, the measured rate was initially near to Harper–Dorn prediction but became constant only at longer durations at a level about five times lower than this. The surface profile observations provided information of a small contribution of grain boundary sliding to the creep process when grain boundaries are not closely perpendicular to the stress. The wire profiles near the closely perpendicular boundaries showed a smooth local diameter increase or decrease depending respectively on whether the stress was sufficient or insufficient to overcome the effect of the surface tension forces tending to shrink the wire. These profile changes indicated the location of vacancy sources and sinks and the measured steady state creep rates could be accounted for by the direction and magnitude of the vacancy fluxes.