Tensile deformation of silver micro-wires of small thickness-to-grain-size ratios

A recent letter from us has indicated that the tensile proof strength of polycrystalline Ag micro-wires, with thicknesses in the range from 20 to 50 μm, depends strongly on the specimen size (t) and weakly on the grain size (d) when t/d becomes smaller than about 3. In this work, we report further coupled effects of specimen size and grain size in the regime when their ratio is small. At a given grain size from 3 to 40 μm, the tensile elongation was found to decrease as the wire thickness decreases, and as the t/d ratio gets smaller than about 3, the tensile elongation loses its dependence on the grain size. The work-hardening rate was found to scale approximately with the proof strength, and so they should be controlled by the same metallurgical factors. Transmission electron microscopy examination of the dislocation microstructures showed that in the regime where the grain size dominates strength and ductility, the dislocation density rises rapidly on deformation, but in the regime where the specimen thickness dominates strength and ductility, the dislocation density remains on the same order of magnitude as the undeformed state. The easy escape of dislocations from the specimen is thought to be the reason for the observed size effect of strength.