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Yield stress influenced by the ratio of wire diameter to grain size – a competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires

Polycrystalline copper wires with diameters of 25, 30 and 50 µm were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the ‘smaller is softer’ phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface.