The Mechanical Properties of Polycrystalline Cu Microwires Having the Crystal Grains Grown by Joule Heating

Abstract

This paper reports on the relationship between the mechanical properties and the grain size of Cu microwires modified by Joule heating. The increase in yield strength as the grain size of a metal or alloy decreases is known as the Hall-Petch relation. Because the crystal grain size in thin metallic wires is fine, these have higher strength compared to their bulk counterparts. To improve the formability of 25 μm-thick Cu microwires, the wires were heat-treated at various temperatures by Joule heating, and the grain size of the wires was evaluated quantitatively by cross section method. Larger crystal grains grew at higher temperatures, and the wire heat-treated at the highest temperature of 600°C had a bamboo structure, in which the grain boundaries were only in the radial direction of the wire. Small-span, three-point bending tests were performed on the heat-treated Cu microwires to determine their mechanical properties. The Young’s modulus of the wires was found to be independent of grain size, with an average value of 86.4 ± 2.4 GPa. On the other hand, the yield stress of the wires clearly depended on the grain size. The yield stress of a Cu microwire that had not been subjected to Joule heating was 311 MPa, and this decreased to 75 MPa after heat treatment at 600°C. Finally, we confirmed that the Hall-Petch relation was applicable to the Cu microwires, except for those that, due to insufficient heat treatment, had crystal grain structures in which the grains were highly elongated in the axial direction of the wire.