A study of the mechanical and fatigue properties of metallic microwires

There is an increasing necessity to record the deformation characteristics of microelements containing freestanding bond wires. The data required are either mechanical or thermal such as Young's moduli, stress-strain values, fatigue- and thermal-strain data, but the nominal strength of a structure changes by scaling its size. Due to this size effect, material data cannot be taken from macrospecimens, thus special testing procedures were introduced. Laser optical sensors based on the speckle correlation technique were applied to determine non-contacting strain values with high strain resolution. For the mechanical properties tensile tests were used. A special ultrasonic resonance fatigue system is described for testing freestanding microwires. In this study the stress-strain and fatigue response of microwires of Cu with a purity of 99.99+% with diameters between 10 and 125 mu m with a typical bamboo structure have been investigated. A size dependence of the yield strength which increased with decreasing diameter was observed, while the fracture elongation showed contrary behaviour. Fatigue life also decreased with reduced diameters. An explanation is presented based on formed oxide layers, strengthening effects by dislocation pile-ups and a pronounced localization of formed slip steps acting as notches being most dominant in the very thin microwires.