Abstract
From the background of miniaturization and high–density mounting in mobile information communication devices such as smartphones and tablet terminals, micro connectors are getting thinner and narrower. It is therefore, required to improve the strength and electrical conductivity of copper alloys used in the devices. Recently, our group successfully found that the procedure of finish–rolling and then annealing at a low temperature was available to provide a 0.2% proof stress exceeding 1000 MPa and an electrical conductivity exceeding 30% IACS for a Cu–2.4Ni–1.2Co–0.90Si alloy (mass%). In this study, we investigated the strength, electrical conductivity, and microstructural evolution of before and after annealing at a low temperature for the Cu–2.4Ni–1.2Co–0.9Si alloy, by means of XRD, EBSD, FE–SEM, TEM, and three–dimensional atom probe (3DAP). According to the 3DAP analysis, clusters of Ni, Co, and Si with a size of approximately 1 nm were formed after annealing at a low temperature. It is suggested that fine age–induced precipitates, (Ni,Co)2Si, with a size of 5 nm, should be sheared by the finish–rolling procedure and then ultrafine clusters should be formed by annealing at a low temperature. Therefore, the strengthening by annealing at a low temperature must be contributed to ultrafine clusters.
