Microstructural evolution and mechanical properties of heavily cold-rolled and subsequently annealed copper titanium alloys

Abstract

Cold-rolled Cu-Ti alloy sheets are commonly used as connectors for electronic devices, because of their excellent mechanical properties and formability relative to conventional commercial copper alloys. Cu-Ti alloy sheets having even higher strengths are highly desired to meet the demands of continued miniaturization of electronic devices. In order to achieve exceptional mechanical properties, severe plastic deformation (SPD) methods are of great interest, because they can produce ultra-fine grained materials with grain size less than 1 µm. This study investigated changes in microstructure and mechanical properties of Cu-Ti alloy sheets during heavy cold rolling and subsequent annealing to determine optimum process routes and conditions to attain excellent mechanical properties. Cu-Ti alloys with and without Cu-Ti precipitates in the matrix were prepared as the initial materials. Pre-existing Cu-Ti precipitates were plastically deformed and severely elongated in the rolling direction via cold rolling, accelerating the formation of a nano-lamellar structure. A mean lamellar boundary spacing of 20 nm was achieved at an equivalent strain of 6.7. Ultimate tensile strength, yield strength, and Vickers hardness increased with a decrease in the lamellar boundary spacing, following the Hall-Petch relationship. Therefore, the strength of heavily deformed Cu-Ti sheets can be primarily attributed to grain boundary strengthening related to the lamellar boundaries. Subsequent annealing after cold rolling clearly enhanced the mechanical properties, and a maximum Vickers hardness value of 443 HV (4.35 GPa) was attained in the sample with pre-existing precipitates rolled to a strain of 6.7. These results suggest that pre-existing precipitates promote microstructural refinement during heavy cold rolling, and heavily cold-rolled Cu-Ti alloys exhibit anneal hardening behavior, leading to excellent mechanical properties.